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Title
ASTROPHYSICS IN 1992
Permalink
https://escholarship.org/uc/item/37g5m86z
Journal
PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC, 105(683)
ISSN
0004-6280
Author
TRIMBLE, V
Publication Date
1993-01-01
DOI
10.1086/133121
License
CC BY 4.0
Peer reviewed
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University of California
Publications of the Astronomical Society of the Pacific
105: 1-21, 1993 January
Astrophysics in 19921
Virginia Trimble2
Astronomy Department, University of Maryland, College Park, Maryland 20742
and Physics Department, University of California, Irvine, California 92717
Received 1992 October 12; accepted 1992 November 3
ABSTRACT. 1992 was an astrophysically exciting year for most of us. E-mail lines buzzed with news
of the COBE anisotropy, the Gallex solar-neutrino flux, a pulsar with two planets, and the remarkable
isotropy of the gamma-ray burst sources seen by CGRO. Meanwhile, more routine work continued on
problems like star formation that have been around for decades and seem unlikely to be fully resolved
in this millenium. The overview of the year presented here attempts to strike some balance between the
new discoveries and the less spectacular, but equally important, progress that comes from many people
tackling a subject in different ways. Seven self-contained sections discuss the Sun, extra-solar-system
planets, the search for brown dwarfs, triggered star formation, gamma-ray sources, the unified model of
active galaxies, and large-scale lumpiness in the universe. Five sections of shorter items concern ( 1 ) the
solar system and Milky Way, (2) discoveries of things long sought, (3) the converse, and (4) and (5)
various kinds of corrections and updates.
1. INTRODUCTION from nearby objects (the Sun) to distant ones (the Uni-
verse). Shorter items are collected in the final sections.
"Astrophysics in 1991" (cited hereafter as Ap91) ap-
Suggestions of topics for "Astrophysics in 1993" will be
peared in the January 1992 issue of Ρ ASP (104, 1) and
welcome between now and 15 September 1993.
attracted a wide range of comments, from much-
appreciated kind words (some even from colleagues whose
work had not been mentioned), through gentle protests 2. THE SUN FROM BOTTOM TO TOP
that something had been misrepresented, to a bitter criti-
2.1 Gallex Sees the Sun
cism of the language employed as inadequately gender-free.
The most flattering objection came from an astronomer Ap91 promised that, if the solar-neutrino problem were
who worried that such a review might be too influential solved, you wouldn't have to read my review to find out
and so damage workers whose papers happened not to be about it. This has still not happened; but the number of
mentioned. This has not happened to anyone, I am sure. relevant experiments has doubled.
The worrier is, however, cited this year (though his scien- Raymond Davis first reported a worrisome upper limit
tific honors are far beyond my poor power to add or de- of 3 SNU to the neutrino flux captured by his 37C1 exper-
tract). iment in 1968 (Davis et al. 1968). For the next two de-
Well, here we are again, with what are meant to be cades, the situation changed remarkably little, except that
comprehensible, capsule introductions to astrophysics top- more and more of us learned that 1 SNU= 10~36 captures
ics that looked exciting during the year 1991 October to per target nucleus per second, that the Sun ought to be
1992 September. The obvious has not been avoided producing 6-10 of them, and that astrophysicists, nuclear
(Gallex and COBE are here), but I have also tried to pick physicists, and weak-interaction physicists were enthusias-
out a few topics where the interest derives from the cumu- tically blaming each other for the discrepancy.
lative impact of a number of investigations, rather than Confirmation of the deficiency and of the Sun as the
from a single discovery. source (based on direction of arrival) came in 1988 from
A couple of rather arbitrary ground rules have been Kamiokande, the Kamioka Neutrino Detection Experi-
imposed. First, there is no fine tuning of last year's topics, ment (formerly the Kamioka Nucleón Decay Experiment,
though major changes and corrections are noted in Sec. 13 Hirata et al. 1989). The current numbers for these two
("Acta et Retracta"). Second, the focus is on items pub- experiments and corresponding standard solar model pre-
lished in 21 journals available in at least most large astron- dictions are: Homestake records 2.1 ±0.3 SNU where 8±3
omy libraries. Thus conferences are cited for background are expected; and Kamiokande sees 0.46 ±0.08 of its ex-
material, but have not been systematically scanned for in- pected flux (Davis et al. 1990; Hirata et al. 1991; Bahcall
clusion. And preprints are not cited at all, because your and Pinsonneault 1992).
library and our libraries almost certainly do not get the Because Kamioka (a direct scattering experiment) has
same ones. a higher energy threshold than Homestake, the two sets of
The longer sections are ordered, as in Ap91, roughly numbers made sense as an effect of weak-interaction phys-
ics in which some of the electron neutrinos rotate to muon
neutrinos, and then cannot rotate back once they leave the
2Invited review paper.
Beatrice M. Tinsley Visiting Professor, University of Texas, Austin, Sun (MSW effect; Bahcall and Bethe 1990). An implica-
Spring 1992. tion was that experiments with a lower energy threshold,
1 © 1993. Astronomical Society of the Pacific
© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System
2 TRIMBLE
using 71Ga as a detector, would probably see considerably granular scale. Rather, there is structure down to <0'/3-
less than one-third of the standard model flux (1321¾ 0W5 (Keller and von der Lühe 1992, who have pioneered
SNU). speckle polarimetry), with most flux tubes only 50-100 km
Last year, Abazev et al. (1991) weighed in with a null across (Topka et al. 1992), located between the granules
result of 201¾ SNU from SAGE (Soviet-American Gal- (Hanschmeier et al. 1991), and corresponding to bright
lium Experiment; changing the name to Russian-American points in the chromospheric network (Muller et al. 1992).
would improve neither the acronym nor the accuracy, Sunspots of less than 1" diameter (visible as micropores in
since the gallium tank sits in the Caucasus), and some the network) also exist far away from conspicuous active
cautious backpatting occurred, contingent upon momen- regions (Zirin and Wang 1992).
tarily expected numbers from Gallex, an Italian-French- On larger scales, the granules at first glance seem to be
German-Russian-American collaboration located in a tun- randomly distributed over the solar surface (away from
nel under Gran Sasso, Italy. active regions). But Hale's (1892) first spectroheliograms
The numbers arrived in June: 82±27 SNU (Anselmann revealed a meshlike bright network, now called the super-
et al. 1992). The next increment of SAGE data is due any granulation, with a characteristic size scale of 30,000 km.
day now, and rumor suggests an electron-neutrino flux There is also an intermediate, 3-10,000 km scale, me-
closer to the Gallex one than to zero. The combination of sogranular structure. The mesogranules live an hour or
these and the two earlier experiments does not absolutely more (Chou et al. 1992) and the supergranules a day or
rule out a standard Sun and neutrino oscillations (Shi et al. more, also allowing them to move distances of order their
1992). The gallium numbers and rumors of numbers are, own sizes.
however, interestingly close to the 74 -79 SNU that must It has been questioned whether the three regimes are
come from just the basic, energy-producing reaction, really discrete (Strauss et al. 1992). But a three-hour se-
p+p^>d+e^ +ye. This is what you are likely to get if the quence of closely spaced observations from Pic du Midi has
center of the Sun is 30% or so cooler than the standard revealed the full history of mesogranules and clarified the
model, which would also reduce the 37C1 and Kamioka relationship among the three types of convective structures
count rates (the latter nearly to zero). (Muller et al. 1992). In summary, the supergranules carry
The figurative ball (or apple of discord) thus now hov- the mesogranules horizontally along with their flow, while
ers uncertainly between the weak interaction and astro- the mesogranules in turn carry along (advect) the gran-
physics courts (Krauss 1992). Resolution might come ules. Magnetic flux is thereby concentrated into the super-
from modified 71Ga numbers in the next year or two, from granular network as seen. Simon and Weiss (1991) have
the advent of SNO (Sudbury Neutrino Observatory, with proposed that the supergranules generate sinking plumes
sensitivity to high-energy neutrinos of all three flavors) in that penetrate to the base of the solar convection zone, so
about 1995, or from a new idea at just about any time. that no larger-scale structure should be expected.
Notoriously, modern astrophysics still has no proper
theory of convection. But in the solar case we can perhaps
2.2 Granules Large and Small say, at least, that all the pieces of the puzzle have been laid
When energy that has been diffusing for 105 yr from the on the table, right side up.
neutrino-producing zones finally leaves the Sun, it must do
so radiatively (otherwise solar observers would have a very
difficult time). But nearly all the energy transport in the 2.3 Will the Real Solar Iron Abundance Please Stand Up?
outer 0.3 Rq is convective; and the rising and falling cells
leave their imprints on the departing photons, affecting Nearly a generation ago, astronomers breathed a mod-
their numbers, color temperatures, and Doppler shifts over est collective sigh of relief when revised oscillator strengths
the solar surface. of permitted iron lines resolved an order-of-magnitude dis-
Most conspicuous in a white-light image are the —1000 crepancy between the coronal and photospheric iron abun-
km granules separated by darker lanes (Γ'=725 km at 1 dances, which had long stood at 7.8 and 6.5 respectively,
AU), known to the elder Herschel and his contemporaries, on the logarithmic scale where H—12.00 (Smith and
and having lifetimes of 6-10 min. Doppler data indicate Whaling 1973). The coronal had been more nearly correct.
speeds of 0.5-1.0 kms-1, so they travel only about their It is a measure of real progress that the discrepancy now
own sizes before radiating away their excess heat. They resolved is in the next decimal place, between a photo-
are, at least very crudely, the convection cells of standard spheric value about 7.66 and a meteoritic one of 7.51
mixing-length theory. And the biggest ones carry most of ±0.01. Once again, the photosphere loses. New measure-
the flux (Roudier et al. 1991). Recent improvements in ments of / values and of level lifetimes have brought the
angular and temporal resolution have expanded our under- photospheric value, based on both Fe I and Fe II lines,
standing of convective features on the solar surface to both down to 7.51 ±0.03 (Biémont et al. 1991; Holweger et al.
smaller and larger scales. 1991; Hannaford et al. 1992). Until the unified value has
On the finest scales, speckle imaging has revealed bright fully pervaded the literature, readers (and writers) con-
lanes between the granules and intergranular boundaries. cerned about abundances of iron in other objects at the
These could be upwelling or shocks (de Boer et al. 1992). 0.1-0.2 dex level must keep an eye on the particular value
Magnetic flux is not uniformly distributed, even on the to which a given [Fe/H] has been normalized.
ASTROPHYSICS IN 1992 3
3. EXTRA-SOLAR-SYSTEM PLANETS: WIN SOME, shaw (1992), who also joined Rasio et al. (1992a) in re-
LOSE SOME proposing objects like 1957 + 20 as progenitors. Postnov
and Prokhorov (1992) believe that survival of planets be-
The section of Ap91 on binary pulsars and X-ray bina- longing to a single, presupernova star cannot be ruled out.
ries ended by declining to discuss 1829—10, the first re- None of the papers that have made it into print seem to
ported pulsar with a planetary-mass companion. This turns have advocated accretion-induced collapse of a white
out to have been a good choice—there was planetary mo- dwarf, whose companion then sacrifices its last few
tion involved in the data stream for this pulsar all right; X 1028 g to provide a "protoplanetary" disk for the new
unfortunately, it was that of the Earth (Lyne and Bailes neutron star. This therefore constitutes my addition to the
1992). A correction for ellipticity of the Earth's orbit, already voluminous literature of the subject.
needed when the initial pulsar position is poorly known, The best bet for choosing among these ten or so alter-
had been omitted from the analysis. But the same AAS natives would seem to be discovery of additional pulsar/
meeting (January 1992, Atlanta, GA) that heard this un- planet systems, and comparison of them with 1257+12.
happy news also heard the announcement from Wolszczan The white-dwarf merger scenario predicts that some mas-
and Frail (1992) that a 6.2-ms pulsar, 1257+12, has not sive single white dwarfs, as well as single pulsars, should
just one, but two planets, with orbit periods of 66.6 and have planets (Livio et al. 1992). Unfortunately, the veloc-
98.2 days (neither a simple submultiple of 365.24). Data ity amplitude will be no larger than it is for the pulsar case,
from the NRAO 140-foot dish independently confirm both about 30 ms-1, and white dwarfs do not seem to carry
periods (Backer et al. 1992). Because the current orbits are accurate clocks around with them the way pulsars do.
in a 3:2 resonance, the periods should change in a predict- The discerning reader will have noticed that, according
able and detectable way over the next few years, constitut- to most of the models proposed, 1257+12 is orbited only
ing a definitive test of orbiting objects as the cause of the by very small lumps of dense stellar material, not by real
velocity variation (Rasio et al. 1992; Malhotra et al. 1992). planets. One of those has also come and gone. The radial-
This curious sequence of events was a happy one for velocity variations of γ Cephei, previously credited to a
theorists—the models proposed to explain 1829 — 10 ac- Jupiter-mass companion, now seem much more likely to
quired the status of predictions for 1257+12, which, in result from intrinsic stellar variability (Walker et al.
turn, generated its own set of explanations. Many of the 1992).
models involved binary-star processes of one sort or an- Candidates for protoplanetary disks, on the other hand,
other. And since 1257+12 (but not 1829—10) is a milli- have proliferated. About one-fifth of main-sequence A
second pulsar—the kind generally believed to have been stars display colors indicative of dust disks like that of
spun up by binary mass transfer—it is a much likelier sort Vega (Cheng et al. 1992). Disks revealed by infrared col-
of planetary host (Stevens et al. 1992). In general, publi- ors and/or molecular line emission are even commoner
cations in 1991 were meant to apply to the first, those in around Τ Tauri and related stars (Ohashi et al. 1991).
1992 to the second pulsar-plus-planet system. Detailed fitting of the infrared spectra of a few reveals gaps
Podsiadlowski et al. (1991) led off the procession with in the disks at 0.5-2.0 AU that could be zones cleared out
two proposals: (a) a merger of two white dwarfs, leading by a low-mass companion or "Jupiter" in the process of
to a neutron star plus disk, in which planets could form, or formation (Marsh and Mahoney 1992).
(b) collision between a neutron star and a main-sequence If by "planets" you mean "groups of small objects or-
star with a planetary family, in which the neutron star biting stars and coeval with them," then, for all we know,
cannibalizes the other star and keeps its planets. Fabian the solar system may still be the only game in town.
and Podsiadlowski (1991) considered the possibility of
planet formation in a disk of material transferred to a neu-
tron star while it was part of a Be X-ray binary. A second
explosion in the system then removes the Be star. Lin et al. 4. BROWN DWARFS—THE SEARCH CONTINUES
( 1991 ) suggested the first single-star model, in which plan- Brown dwarfs are semi-mythical gas spheres of normal
ets might form from material that fell back toward the stellar composition but masses too low ( ^0.085 Mq for
neutron star into a disk, after being incompletely expelled Population I composition) to initiate hydrogen fusion re-
by a supernova. Krolik's (1991) first choice was further actions. They could be important, first, as a significant con-
ablation of a companion like the 0.01 Λ/q one belonging to tributor to non-luminous mass (dark matter) in the Milky
PSR 1957+20. Nakamura and Piran (1991) closed Way and elsewhere (Ashman 1992), and, second, as a
out the year by using a supernova in a binary both to probe of our understanding of star formation in general,
liberate the second star and to ablate enough material from since there is no obvious, a priori reason for the process
it to form a disk and, eventually, planets. that determines the stellar mass spectrum to know about
Wijers et al. (1992) also favored a normal, massive nuclear-reaction cross sections.
close binary as the initial state, but gave one of the stars The first problem in studying brown dwarfs is intrinsic
planets from the beginning, and used a common-envelope faintness (even if all the galactic halo dark matter were
binary phase to eliminate the companion and to spiral the BDs, you would find only about one per HST WF/PC
planets into their current locations. Descent from a low- field). The second is confusion with normal red (late M)
mass X-ray binary was proposed by Tavani and Brook- dwarfs in the absence of independent age information [be-
4 TRIMBLE
cause cooling curves for the two overlap for the first billion 1992). Now that we know the answer, what was the ques-
years or so (D'Antona and Mazzitelli 1986)]. tion, what does the answer mean, and what is the evidence
An unweighted vote of the interested astronomical com- for it?
munity at present would probably yield a sizable plurality The traditional topic of investigation was the formation
for the view "brown dwarfs exist, but are not common of the solar system. Historians of science have divided a
enough to be dynamically important in star clusters or plethora of suggested mechanisms into dualistic and mo-
galaxies/' This is true for Milky Way field stars even if the nistic, meaning that either some other object was involved,
stellar mass distribution continues to rise well past the last or it wasn't. A close encounter of two stars, with material
mass point (0.14 Mq) for which it has a reasonably secure dragged out of one or both, is the canonical dualistic mech-
value (Richer and Fahlman 1992). It will not be true anism; Kant's nebular hypothesis, the canonical monistic
for those globular clusters for which N{M) is rising as one (Brush 1990 and references therein). Within that
M~2'5 or steeper down to 0.1 Mq and beyond (Richer framework, triggered or stimulated star formation is a du-
et al. 1991). The low-mass stars in these clusters are less alistic idea; spontaneous star formation a monistic one.
centrally condensed than the more massive ones, and so When I first met the generic problem 15 or so years ago,
could contribute quite a lot of mass without much affecting the underlying idea was that the free-fall collapse time of
the measured central velocity dispersion (Richer 1992, pri- giant molecular clouds was only 107 yr, but the Milky Way
vate communication). had not yet turned all its gas into stars. Thus something
Contexts where there is evidence against even modest must stabilize the clouds (turbulence, magnetic fields, ...)
numbers of BDs include the globular cluster NGC 5053, and something else be needed to start them collapsing—
whose mass function stops at 0.35 Mq (Fahlman et al. cloud-cloud collisions, expanding supernova remnants or
1991) and a couple of binaries where advertized BD com- OB wind shells, a passing spiral-density wave, or whatever.
ponents are probably really M dwarfs (Henry et al. 1992 There is, of course, an alternative in which clouds do in-
on Wolf 424 AB; Cochran et al. 1991 on HD 114762). deed start to form stars within 107 yr, but the first few stars
Finally, the commonest case of "we find a few likely formed disrupt the rest of the cloud. The essential time
candidates, but not many" obtains in: (1) the Hyades, scale is then the (longer) one on which clouds reform. Star
whose luminosity distribution shows a broad peak around formation in this picture is spontaneous and self-
Mv= + 12, but significant downturn toward the BD re- regulating.
gime and about seven candidates (Reid 1992; Bryja et al. Triggering specifically for the solar system has been in
1992); (2) a CORAVEL sample of spectroscopic binaries and out of favor several times in recent years, owing to the
with 8% ± 6% of the secondary masses in the brown-dwarf discovery of extinct radioactivities that could plausibly be
range (Duquennoy and Mayor 1991); (3) the a Persei attributed (a) to a previous passage of a spiral arm, 108 yr
cluster with one member near 0.07 Mq (Rebolo et al. before the solar system formed and (b) to a much more
1992); (4) images found in the Automated Plate Measur- recent supernova, feeding short-lived radio isotopes di-
ing (APM) facility quasar search, which included one star rectly into the protosolar cloud itself (Reeves 1972; Cam-
like vB 10 (L~10_4 Lq; Irwin et al. 1991); (5) searches eron and Truran 1977), followed by the recognition that
for infrared-emitting companions to white dwarfs, which the radionuclides could well have been trapped in dust
have uncovered one likely BD (Giclas 29-38, confirmed as grains before decaying and the grains in turn have been
a binary by radial-velocity data), but no others among 400 incorporated into the meteorites long after. In the latter
examined (Barnbaum and Zuckerman 1992); and (6) the case, there would be no evidence for triggering of the for-
Pleiades, according to a proper-motion survey (Hambly et mation of the solar system (but also, of course, no evidence
al. 1991) that identified at most ten candidates, but not against it).
according to an infrared survey (Simons and Becklin What Elmegreen's answer means is that, in virialized
1992) that found 22 ±10 BD-like objects in a particular systems dominated by gravity, condensation of the densest
field and would imply 200 Mq of brown dwarfs in the bits will occur spontaneously, perhaps on a time scale con-
whole cluster, down to 0.04 Mq per star, comparable with trolled by magnetic fields (Mouschovias 1990). But in
the mass of the rest of the cluster. non-virialized (typically larger) contexts, where pressure,
If your interest in brown dwarfs is primarily as a halo gas streaming, and turbulence are important, the initial
dark-matter component, it is worth keeping in mind that phases of cloud collapse will result from gas being pushed
these searches (even in globular clusters) may not be en- around. Evans (1991), in a review just outside our refer-
tirely relevant. The star-formation process while halos ence year, gives considerably less credit to stimulated star
were acquiring their identities need not have been much formation, suggesting that it is restricted to interacting gal-
like current star formation in spiral disks. Cooling flows, axies, ring galaxies, and a minority of OB associations.
which are widely regarded as favoring low-mass stars, The observational evidence is perhaps clearest for
could be a closer prototype, but that is not guaranteed triggering of star bursts in interacting galaxies, including
either. mergers and companions. For instance, 60% of the
brightest (L>1012 Lq) IRAS galaxies, but only 10% of
5. TRIGGERED STAR FORMATION
the faintest (L<1010 Lq) appear to be interacting in the
On the largest scales, most star formation is triggered; Palomar Observatory Sky Survey images (Zou et al.
while on the smallest scales, most of it is not (Elmegreen 1991). A sample of non-Seyfert Markarian galaxies also
includes a large excess of close pairs (Keel and van Soest establishing physical connection. In this context, the stellar
1992). In these pairs, the triggering apparently does not groups of various ages in the Orion Trapezium region are
affect the properties of star formation, which, apart from of interest; the youngest shows up to 40% higher metallic-
the rate, is indistinguishable from that in single galaxies. ity than the older subgroups. In other words, independent
Radio emission from interacting galaxies is another symp- of whether supernovae in the older star-formation regions
tom of vigorous star formation (Batuski et al. 1992). actually caused formation in the younger one, they have at
For IRAS interacting pairs, both galaxies typically show least contributed newly synthesized heavy elements to it
the high Lfir/Lb ratio and infrared colors that are indic- (Cunha and Lambert 1992). In fairness, one should point
ative of rapid star formation (Lutz 1992). The same ap- out that there are equally clear cases of a possible trigger
plies to interacting pairs identified by Arp and Madore that can be shown not to be physically connected to nearby
(Sekiguchi and Wolstencroft 1992). The implication is regions where star formation is in progress (Serabyn et al.
that the detailed geometry of the encounter is not critical 1992, on the Sgr A East region).
to triggering; though the morphology of the pair NGC
7714/7715 (Arp 284) suggests that off-center encounters 6. GAMMA, GAMMA, GAMMA
will trigger only core, not ring, star bursts (Smith and
Wallin 1992). This is not an attempt to found a new sorority, but an
Other interesting specific cases include (a) the SMC, expression of pleasure at the wide-ranging results that have
whose youngest stars are mostly found on the side toward come to us from the Compton Gamma Ray Observatory
the LMC (Gardiner and Hatzidimitriou 1992), (b) an (CGRO hereafter) in its first 18 months of operation. In
interacting pair (CG 692 and 693) in the Bootes void, order fully to appreciate the situation, you probably have
consisting of one starburst and one Seyfert 1 galaxy (Weis- to be old enough to remember when the number of gamma-
ray theorists exceeded the number of detected photons.
trop et al. 1992, who also discuss other examples of simul-
Items I would like to highlight include (a) the status of
taneous triggering of both kinds of activity), (c) a whole,
Geminga, (b) gamma-ray detection of pulsars and X-ray
new, star-forming, dwarf irregular galaxy arguably pro-
binaries, (c) the 57Co line from SN 1987A, (d) gamma-ray
duced during the NGC 4038—4039 ("the Antennae") en-
quasars (etc.), and (e) the confused conditions of gamma-
counter (Mirabel et al. 1992a), and a similar case attrib-
ray bursters.
uted to cannibalism by Cen A (Thomson 1992), and (d)
It is possible for normally sensible people to argue about
the two companions of M81, with about the same amount
the distinction between X rays and gamma rays—the peer-
of gas, but very different star-formation rates (Thronson et
review panel that considered the instrument package for
al. 1991).
CGRO did (after all, money was at stake!). You can draw
Turning from whole galaxies to individual star-
the line at some particular energy (e.g., 100 keV), or be-
formation regions, we find during the year examples of
tween photons emitted by electronic processes (X rays)
each of the traditional sorts of trigger, and perhaps one or
and nuclear processes (gamma rays). I here assume that X
two new ones. Sievers et al. (1992) blame recent passage rays are detected by X-ray satellites like ROSAT, while
through a spiral arm for the star-formation rates (deduced gamma rays are detected by gamma-ray satellites like
from dust data) in W49 A and W51 A, which are 100 CGRO. Sigma/Granat sees both.
times as great as those in clouds of similar masses in inter-
arm regions. Two proposed cases of triggering by super-
6.1 Geminga Finally Exists
nova shells and OB star winds are the LMC OB association
LH 10 (Parker et al. 1992), with LH 9 as a likely source, Geminga is both the Gemini gamma-ray source and Mi-
and IRAS point sources in a CO shell around the SNR lanese dialect for "is not there" or "does not exist/' reflect-
G54.4—0.3 (Junkes et al. 1992). Grondinet et al. (1992) ing its prolonged resistance to identification with a source
suggest that cloud-cloud collisions are the dominant mech- at any other wavelength except X ray, which implied ratios
anism in the LMC/SMC bridge of young stars, with its flat of optical and radio to X-ray and gamma-ray luminosities
initial mass function. Expanding shells of shocked H I can well below the normal ranges for anything. The X-ray
be responsible for star bursts that last as long as the disk- source IE 0630+178, finally revealed a coherent period of
crossing time (Jog and Solomon 1992). Finally (and a new 0.237 s to ROSAT (Halpern and Holt 1992). The profile is
one to me) Habe et al. ( 1992) propose that the first cloud a single broad peak, and the position that of a 25th-
to collapse in a magnetized complex will drive Alfvén magnitude star previously suggested as the identification
waves along the field lines to trigger additional cloud col- (see Bignami et al. 1988 for the earlier, checkered history).
lapses. Knowledge of the period permitted its recovery in both
Different kinds of stimulants should produce different EGRET (Bertsch et al. 1992) and Cos Β (Bignami and
patterns of star formation. Cloud-cloud collisions should Caraveo 1992) data. Above 50 MeV, the pulse profile is
put the massive stars on the fringes of clouds (Greaves and double-peaked (at phases 0.25 and 0.75 relative to the
White 1991a), while stellar-wind triggers, as in 30 Dora- X-ray signal). The long baseline between Cos Β and other
dus, will eventually result in the youngest stars being con- period determinations leads to good values for the slowing-
centrated toward the core (Hyland et al. 1992). down rate (/*= 1.1 X10-14 ss_1) and therefore the mag-
Most evidence for triggering involves current morphol- netic field (1.5-1.6X 1012 G), slowing-down age (3.2-3.7
ogies and gradients of stellar ages, with no way of definitely X105 yr), and total luminosity (3.2-3.5X1034 ergs-1.
6 TRIMBLE
assuming the standard magnetic-dipole formulae). The 5576, 5580, 5584). Included are familiar objects like the
source cannot be more than 40 pc from us if this luminos- galactic-center black-hole candidate GX 339—4; massive
ity is to account for the observed flux, and the optical X-ray binaries like OAO 1657—415 (phase modulation of
absolute magnitude is then > + 22! whose 37.7-s rotation period has revealed a 10.4-day orbit
Geminga thus moves from the category "sui generis" to period in the BATSE data stream) and Vela X-l; and re-
the lesser distinction "nearest gamma-ray pulsar" (next current X-ray transients (e.g., 0834—430 in its fifth ap-
section). It is, however, not quite time to declare that all is pearance). New objects have also appeared, conspicuously
well. The absence of detectable radio emission (<1 mJy, a hard X-ray transient, GRO J0422 + 32, for which optical
I AU Circular 5532) requires very narrow beaming and/or and radio identifications were quickly reported (IAUC
local absorption. And the standard model for gamma-ray 5588, 5589, 5593), as well as QPOs (IAUC 5592). It is
production by pulsars in general (evolved to account for being widely advertized as a black-hole candidate, owing to
Vela and the Crab) was not expected to operate at periods its similarities to Nova Muscae and V404 Cyg (see Sec.
longer than about 0.15 s (Cheng et al. 1986; Cheng and 10.7).
Ruderman 1989). The accretion-powered sources do not all, even to first
order, display the same hard X- to gamma-ray temporal or
spectral behavior, but all do, so far, at least seem to fall
6.2 Gamma-Ray Pulsars and X-Ray Binaries within previously known categories of X-ray binary. Op-
At the outset of fiscal year 1992, we knew of only two portunities for modeling abound!
rotationally powered pulsars whose emission extended into
the gamma-ray regime, the Crab and Vela. An expectation 6.3 The 57Co Line from SN 1987A
that short-period pulsars ought to be gamma sources had, The post-maximum light curve of SN 1987A persuaded
nevertheless, motivated the heroic search of Cos Β error virtually everybody that it was powered by ~0.07 Mq of
boxes (Boriakov et al. 1983) that led to the discovery of 56
Ni->56Co->56Fe (Amett et al. 1988). About 800
the second (recycled) millisecond pulsar, 1953 + 29. days after the explosion, the declining light curve leveled
That expectation has now been confirmed, and the num- off, with the object brighter than 56Co alone could account
ber of pulsed, rotationally powered, gamma-ray sources for. This had long been predicted by proponents of an
has at least doubled, with the CGRO detections of early, bright pulsar (Bandiera et al. 1988), but in the event
1509-58 (Wilson et al. 1992; Ρ=0.150 s) and 1706-44 was more widely blamed on additional radioactivities, es-
(Kniffen et al. 1992: 0.102 s). The latter retrospectively pecially 57Co (Kumagai et al. 1991). About five times the
pulses also in Cos Β data. These two are among the ten or solar ratio of 57Fe/56Fe in isotope 57 is required to account
so expected in advance to be detectable by CGRO (Bhatia for the light curve. CGRO has now seen the gamma-ray
et al. 1992; Bailes and Kniifen 1992), being simply the line radiated after 57Co decays to 57Fe, leaving it in an
fastest, closest pulsars. Geminga arguably makes five, and excited state (Kurfess et al. 1992). The detection is some-
a handful more may turn up. In addition, some unidenti- what marginal, but leads at any rate to a 3σ limit firmly
fied point sources of gamma rays are likely to be faint or below this 5-to-l overabundance. Other energy sources
misaligned pulsars, for which deep radio searches might be must be contributing to the radiation now coming to us
profitable (Bailes and Kniffen 1992). from the supernova of the century.
The objects are not transparently all doing the same
things. Of the new pulsars, 1509 — 58 shows a single peak 6.4 Gamma-ray Active Galaxies
per pulse period in both X and gamma rays, while 1706—
44 is single-peaked in EGRET and Cos Β data, but un- AGNs seen above about 100 keV numbered three prior
pulsed down to the 0.4% level in the ROSAT soft X-ray to fall 1991 (Gehrels and Cheung 1992). Because the in-
band (Becker et al. 1992). The Crab and Vela sources, on ventory included one radio galaxy (Cen A), one Seyfert
the other hand, are both still there in OSSE/CGRO and (NGC 4151), and one quasar (3C 273), and no two had
Sigma/Granat data (IAU Circulars 5557 and 5481) and the same spectrum or variability, generalizations were hard
continue to show two gamma-ray peaks per period. to come by. Nevertheless, something like 14 papers pre-
The high sensitivity and extensive sky coverage of sented at an October, 1991 meeting on AGNs (Holt et al.
BATSE/CGRO and Sigma/Granat have also resulted in a 1992) dealt with gamma-ray emission mechanisms.
major increase in the inventory of accretion-powered X-ray EGRET revealed 3C 279 as not only the brightest active
binary systems with spectra known to extend to or beyond galaxy around ( 1048 erg s_1 above 60 MeV for Η =15) but
100 keV. Cyg X-l, the "best-bet" black-hole X-ray binary, also as ten times brighter than its own Cos Β and SAS-2
was also the best-known hard source. It remains among the upper limit (Kanbach et al. 1991; Hartmann et al. 1992).
brightest and has revealed QPOs (quasi-periodic oscilla- The object was unusually bright at radio, optical, and
tions) in both BATSE and SIGMA data (IAUC 5576), X-ray wavelengths at more or less the same time, the fire-
showing that the phenomenon cannot be closely tied to works having had a time scale of about 2 weeks and vari-
anything requiring a solid surface. ability down to about 2 days.
As this is being written, about a dozen other systems The first model, a proton-initiated cascade (Mannaheim
have been seen (IAUC 5352, 5364, 5370, 5377, 5378, 5394, and Biermann 1992), followed hot on the heels of the flare.
5395, 5430, 5433, 5454, 5481, 5504, 5510, 5535, 5557, Dermer et al. (1992) focused on a significant common
feature of 3C 273 and 279—both have strongly superlumi- star connection was strongly suggested by short time scales
nal radio sources, implying a beam or jet aimed nearly at (seconds down to milliseconds or less) and by 20-60 keV
us, and so relativistic amplification of the emission in our spectral features, generally interpreted as cyclotron reso-
direction. Maraschi et al. (1992) also emphasize the im- nances in magnetic fields near 1012 G (Yoshida et al.
portance of beaming. 1992). We had to be seeing only nearby neutron stars to
Remarkably, the number of sources (in fall 1992) seems account for the isotropy of the bursts over the sky, with no
to exceed the number of models. Since 3C 279, the EGRET concentration to galactic center or plane. And we had to be
team has reported probable detections, and in some cases seeing old neutron stars for there to be enough potential
variability, of gamma-ray sources associated with 4C sources within a few hundred parsecs. The stronger bursts
38.41, PKS 0528 + 134, and PKS 0208-512 (IAUC that dominated early samples (e.g., 225 events between
5431), QSO 0836+710 (IAUC 5460), Markarian 421, 1978 and 1988 recorded by Pioneer Venus Orbiter; Chuang
PKS 0537-441, and QSO 0716 + 714 (IAUC 5470), 3C et al. 1992) also displayed the number-brightness relation
454.3 and CTA 102 (IAUC 5477), PKS 0202+149, 0235 expected for a homogeneous, isotropic population, dN/
+ 64, and 0420-014 (IAUC 5519), and PKS 0454-463 dPo:P~2A±0·01, where Ρ is peak count rate and the theo-
and 1606+106 (IAUC 5594). retical index is —2.5.
Some of these are known superluminal radio sources, Other samples already hinted that something was pecu-
and all show some kind of Blazar-like behavior (large op- liar, though. A statistic called V/Vmax (Schmidt 1968) is
tical polarization, rapid optical variability, compact, flat- 0.5 for homogeneous source distributions, less than 0.5 for
spectrum radio emission, etc.; Dermer and Schlickeiser things that are mostly close to us (like planets), and more
1992). than 0.5 for things preferentially far away (like quasars).
Most notably, Mkn 421 is (at ζ=0.0308) the closest of Venera data with F/Fmax=0.412 ±0.019 (Mitrofanov
the superluminal sources. As a result, not only is X/r1 in et al. 1991) and Ginga data with F/Fmax=0.35 ±0.035
our favor, but the probability of photons being wiped out (Ogasaki et al. 1991) and yet no sign of anisotropy were
by gamma-gamma interactions and pair production en not entirely consistent with the hypothesized galactic-disk
route to us is much reduced. Thus it is no surprise that the population.
BL Lac Mkn 421 is the first, and so far only, of the 100 It was nevertheless widely expected that the fainter
MeV AGNs also to be recorded in the TeV range by the bursts to be seen by CGRO/BATSE would show both an-
Whipple Observatory (Weekes et al. 1992). The detected isotropy on the sky (correlated with the galaxy) and evi-
flux is roughly one-quarter that of the Crab Nebula, im- dence for an edge to the distribution from the number-
plying a TeV (1 TeV =1012 eV) luminosity in excess of brightness relation. This is not what happened (Meegan et
2xl043 /z_2ergs_1 for isotropic emission. If the gamma al. 1992a,b).
rays are strongly beamed, Mkn 421 is not so bright, but The population of gamma-ray bursts seen by CGRO is,
objects otherwise similar, but aimed away from us, must be instead, as close to isotropic as can be determined from
fairly common to put one correctly oriented one this close about 200 events (however they are weighted), with a di-
to us. pole moment (cos 0) =0.008 ±0.035 and a quadrupole
How these galactic nuclei do what they do and what it moment (sin2/?) =0.31 ±0.02, where the isotropic values
means for radiation backgrounds, models of AGNs, and so are 0.0 and 1/3. But the distribution is not homogeneous in
forth should be described as "work in progress." space for any reasonable luminosity function: F/Fmax
=0.33 ±0.02, and the index of the integrated number-
6.5 Gamma-Ray Bursters brightness relation is —0.8, where homogeneity in three
dimensions requires —1.5. Whatever the sources are, we
Twenty years after gamma-ray bursts were announced are very near the center, and we are seeing the edge.
(Klebesadel et al. 1973), we still do not understand their It continues to be true that the range of spectral and
basic physics. This is not a record. Solar coronal lines were temporal behavior is enormous (Golenetskii et al. 1991)
recorded in 1869 by C. A. Young and identified with for- and that there are no persuasive identifications of the burst-
bidden transitions of highly ionized atoms only in 1942 by ers with either optical flashes or with steady sources at any
B. Edlén (Swings 1943). Cosmic rays (1900-35), the de- wavelength (Varady and Hudec 1992). And while the ad-
ficiency of solar neutrinos ( 1968-present), and the perihe- vertized cyclotron features continue to be found in events
lion advance of Mercury ( 1859-1915) are other contenders detected by Granat and Ginga, none has been reported
for longest-duration mystery. But the pattern is not one we from CGRO. A minor oddity has, however, disappeared. A
like or are much used to (compare pulsars, 1968-68; X-ray small, soft-spectrum subset of the bursters had recurred at
binaries, 1962-67; X-ray bursters, 1976-76; and supernovae the same positions in the 1970^ and early 1980^. It looked
1933-33; references to the discoveries and explanations are for a while as if all these soft repeaters turned off by 1984
given by Trimble 1992). (Kouveliotou et al. 1992a); but one has now turned on
The burster situation is the more frustrating in that, a again (Kouveliotou et al. 1992b).
couple of years ago, nearly everybody had agreed on what The theoretical situation is in considerable disorder. I
we were seeing: explosive events on the surfaces of nearby count 34 burst-model papers in my notes from the refer-
( ^200 pc) neutron stars (old pulsars), drawing on inter- ence year, at most half a dozen of which can be simulta-
nal and/or accretion energy (Ho et al. 1992). The neutron- neously correct. A good many deal with details of produc-
tion on nearby neutron stars and were already in press Μθ) black hole, accreting material from a disk that is, in
when the BATSE data burst upon us (e.g., Blaes and Ra- turn, fed by gas and stars from the surrounding galaxy, a
jagopal 1992). Others attempted to characterize the core hot corona around the thermal disk, a relativistic jet flow-
radius of a galactic population that could evade the ing out perpendicular to the disk and collimated by it, and
isotropy limits, yielding results as disparate as 5, 18, and various other components that participate in extracting en-
26-79 kpc (Brainerd 1992a; Mao and Paczyñski 1992; ergy and radiating it in the observed forms of continuous
Wasserman 1992). radio-to-gamma-ray flux and emission lines at many wave-
Finally, because the observable Universe is the most lengths (Holt et al. 1992).
obvious example of a structure whose edge we see isotro- Within this framework, a canonical unified model asso-
pically in all directions, many papers attempted to identify ciates differences in the observed object with different ori-
events that could occur very rarely, but produce enough entations of the jet to our line of sight, though the mass and
gamma rays to be seen out to redshifts of order unity. A accretion rate of the black hole are also allowed to vary.
somewhat-prejudiced subset includes (1) mergers of two For instance, Blandford and Rees (1977; also Blandford
neutron stars or neutron star plus black hole, with neutrino and Königl 1979) identified BL Lacs, OVVs, and intrinsic
pairs typically involved in the radiation process [Paczyñski low-frequency radio variability with jets pointed very
1991; Narayan et al. 1992; Mészáros and Rees 1992 and nearly toward us, misaligned counterparts looking like
Rees and Mészáros 1992, who also have considered the M87. Scheuer and Readhead (1979) proposed that orien-
effects of beaming (you need less luminosity per source, tation distinguishes radio-loud from radio-quiet AGNs.
but more sources) and collisions of the expanding fireball Kapahi and Saikia (1982) identified the ratio of compact
with surrounding interstellar gas (you get back as radia- to extended radio flux as a signature of orientation and
tion much of the energy that had gone into bulk flow of suggested other properties that should be correlated with
baryons)]; (2) accretion-induced collapse of a white dwarf it. And Orr and Browne (1982) associated orientation
to a neutron star, again with neutrino pairs as a typical with flat-versus-steep radio spectra. Properties expected to
intermediary (Dar et al. 1992; Woosley and Baron 1992; result from a jet seen roughly end-on include: a preponder-
and Usov 1992, who also gives the newborn neutron stars ance of compact, flat-spectrum radio emission; large, vari-
magnetic fields of —1015 G); and (3) models associated able optical polarization; rapid variability; narrow emission
with processes we already invoke to explain active galactic lines (if the gas moves mostly in the collimated disk);
nuclei (Carter 1992; Brainerd 1992b). There is, as you superluminal radio motion and sharp bending of jets; and
might expect, a certain correlation between the proposed strong asymmetry (due to Doppler effects) between jets on
models and the kinds of physical systems the proposers opposite sides of the source, with the degree of asymmetry
have worked on before, in other contexts. predictable from superluminal velocity etc.
It is easy to think of specific signatures for cosmological Closely related is the idea (Rowan-Robinson 1977) that
bursters in terms of correlations of spectral hardness (color Seyfert 1 galaxies (with broad permitted and narrow for-
temperature, average photon energy) and time scales with bidden emission lines, higher total luminosity, and higher
apparent brightness (that is, average redshift). Presence of Lx/Lovi)f when seen through the obscuration of their ac-
such correlations in existing data has been rumored, but cretion disks will be classified as Seyfert 2^ (with only
the highly uniform CGRO sample probably has the best narrow lines, lower Lx, and, on average, brighter compact
chance of either demonstrating them or ruling them out. radio emission; no members of either class are large double
Thus, once again, theory waits upon observation (and all radio sources). The reality of this possibility was first dem-
of us upon the rare skills of our colleagues who can build onstrated for the prototypical Seyfert 2, NGC 1068, by
things and make them work, often without getting much Antonucci and Miller (1985).
credit for it). Given the range of ideas comprehended in "the" unified
model, it is perhaps not surprising that new observations
and correlations of old ones published in 1992 should di-
7. MODELS OF ACTIVE GALACTIC NUCLEI: vide rather equally between ones that support or further
PRESERVING THE UNION clarify the importance of orientation effects and those that
The first unified model belonged to Shklovski (1964), cut the other way. A third comparable body of papers
who suggested that the quasar 3C 273 between outbursts correlated properties in ways that I could not see affected
would look like the radio galaxy Cygnus A. Active galaxies this particular issue and so are not mentioned here.
today include not only quasars (with both compact and
extended radio emission and both flat and steep radio spec- 7.1 Are There "Real" Seyfert 2's?
tra) and radio galaxies, but also radio-quiet quasistellar
objects (QSOs), BL Lacertids and optically violent vari- Additional objects with obscured nuclei like NGC 1068
ables (OVVs, together Blazars), Seyfert galaxies (of types continue to turn up. NGC 4507 (Awaki et al. 1991 ) would
1, 2, and various intermediate numbers), and, at the quiet have Lx=3xl043 ergs-1 if seen face on, and so be
end, liners, warmers, and their ilk, though many of these counted as a Seyf 1. NGC 4388 probably has a hidden
latter are undoubtedly galaxies with rapid central star for- ultraviolet (ionizing) nucleus in the Seyf 1 range (Colina
mation, rather than anything more exotic. The paradig- 1992). Similarly, the infrared and ionizing luminosities of
matic AGN model consists of a central, massive (108-10 NGC 3271, if unobscured, would be bright enough to
count as type 1, but there is absolutely no evidence for a nal motion also show the expected correlations of
broad-line component in the scattered, polarized, or infra- (Doppler-boosted) luminosity with the relativity parame-
red light (Storchi-Bergmann et al. 1992). In this sense, ter, 7, required to interpret their motions (Cruz-Gonzalez
3271 counts as a "real" type 2. and Carrillo 1992).
Statistical data suggest that there must be many such. Additional ammunition for the view of BL Lacs as pri-
For type Ts and 2^ separately, linewidths are correlated marily in the eye of the beholder comes from ruling out
with ionization potential and maximum electron density several other explanations of their properties, including
(above which collisional deexcitation dominates) in ways gravitational lensing, unusual environments, and evolu-
that make physical sense. But the two correlations are dif- tionary connection with flat-spectrum radio quasars
ferent in a way that would require mostly radial motion of (Gabuzda et al. 1992; McHardy et al. 1992; Padovani
the emitting clouds if orientation were the primary cause of 1992). But the radio-loud/quiet distinction seems to have
the difference (Busko and Steiner 1992). In addition, if all another sort of underlying physical cause, as suggested by
type 2^ were radiating at the average Seyfert 1 X-ray level, the complete absence of broad absorption lines in the
they would somewhat overproduce the known background strong radio objects, and conversely (Stocke et al. 1992).
(Mulchaey et al. 1992). It is only fair to conclude this section by returning to
Cyg A, for which Ward et al. (1991) draw an analogy to
7.2 Disunity? the Seyfert 1/2 dichotomy, describing it as a quasar ob-
scured rather than as Shklovski's quasar out to lunch.
Perhaps the largest surprise to a devout unifier was the
report (Hough et al. 1992) of six quasars with superlumi-
nal motion (expected only for jets oriented at small angles 8. THE LUMPY UNIVERSE
to our line of sight) and lobe-dominated radio emission 8.1 The COBE Microwave Background Anisotropy
(expected for jets oriented at large angles). Also awkward
is the presence of superluminal motion on only one side of The sky, as measured, is exceedingly lumpy at all wave-
a two-sided jet, whose brightness ratio suggests an orien- lengths, including the far-infrared, submillimeter, millime-
tation angle of about 40° (Hummel et al. 1992). Finally, ter, and microwave ranges, where the 3 Κ cosmic back-
there seems to be little correlation of other orientation sig- ground is an appreciable fraction of the total flux. But most
nals (projected radio size, amount of jet curvature, asym- of the lumpiness is due to discrete sources and fluctuations
metry of lobe arms) with the ratio of core to lobe emission in diffuse emission and absorption by ecliptic plane mate-
in a sample of 30 quasars with VLBI data (Hooimeyer et rial and the Milky Way. Understanding all these well
al. 1992). enough to subtract them off and see residual fluctuations of
Also a surprise is the presence of at least two disk gal- cosmic significance is exceedingly difficult.
axies among the resolved hosts of 23 BL Lac objects Great, therefore, was the rejoicing when the COBE/
(Abraham et al. 1992). The necessary degree of jet colli- DMR (Diffuse Microwave Radiometer) team announced
mation was expected to occur only in ellipticals (Blandford at the April APS meeting that they had finally succeeded
and Rees 1977). and could report that the pregalactic universe was no
longer perfectly smooth. The numbers as published
7.3 Unity? (Smoot et al. 1992; Wright et al. 1992; Bennett et al. 1992)
are consistent with a scale-invariant Gaussian with Δ TV Γ
The star of this show is the discovery of a counterjet to = 6x 10" 6 and spectral index n= — \.5±0.5 down to an
the famous, lumpy M87 optical jet (Stiavelli et al. 1992; angular scale of 10° and a quadrupole moment ΔΤ/Τ=5
Sparks et al. 1992). The fine print, however, reveals that X10-6. Pundits ( "self-proclaimed experts of dubious cre-
the jet itself has not actually been seen, only the hot spot dentials" according to W. L. W. Sargent) immediately
where it feeds into the radio lobe on that side. Given the rushed to tell us what we ought to think about it all, from
likely beaming angle, the two jet sides must actually be "the holy grail has been found," "the big bang proved,"
somewhat unequal in intrinsic luminosity, as is true for "most important scientific discovery of the century," and
some other objects (Fraix-Burnet 1992). The radio coun- "proves (or disproves) the cold-dark-matter model of gal-
terjet of 3C 273 remains undetected at a flux ratio 5300:1 axy formation" to "it is, at any rate, probably the best
(Davis et al. 1991 ), but does not absolutely require intrin- available upper limit on the fluctuations."
sic asymmetry. The first round of commentary in the scientific literature
The general scheme in which orientation relates BL (e.g.. Silk 1992) was already considerably more restrained,
Lacs to other kinds of AGNs can be refined to associate pointing out that the data were indeed consistent with the
them with Fanaroff-Riley I radio galaxies (the fainter, predictions of inflation and CDM, but that the angular
plume-shaped ones) and radio-loud quasars with Fanaroff- scale was a factor of 10 or more larger than the one on
Riley II radio galaxies (the brighter, edge-brightened ones; which we expect to see the fluctuations most directly asso-
Urry and Padovani 1992; Padovani and Urry 1992). ciated with galaxy and cluster formation. Further consid-
Cen A is then a specific example of a misdirected BL Lac eration (e.g.. Partridge 1992; note that the page number,
(Morganti et al. 1992) and the radio galaxy 3C 109 of a 188, given in the magazine's table of contents is wrong)
misdirected radio-loud quasar (Allen and Fabian 1992). suggests that, while the rejoicing is entirely appropriate,
The subsets of radio galaxies and quasars with superlumi- our picture of galaxy formation will not sharpen up much
10 TRIMBLE
until those smaller scales have been probed with equal sen- we are, implying a mass concentration (the Great Attrac-
sitivity (also Wright et al. 1992) and that this should hap- tor) on the far side of them, but not as distant as the
pen quite soon. Shapley concentration on beyond at 14,000 km s-1. It was
It is probably true in the light of COBE data that no expected that enlarged samples in the several surveys in
relatively simple model of the form CDM plus cosmolog- progress would eventually all show the same structures
ical constant or CDM plus extra large-scale power can with roughly the same degree of prominance.
simultaneously fit both large- and small-scale galaxy clus- This has not happened. Some surveys have continued to
tering as well as observed streaming velocities (Efstathiou show a concentration of galaxies in the vicinity of the
et al. 1992). But this was also true before COBE. Great Attractor (Scharf et al. 1992; Strauss et al. 1992a)
During this year, a number of other groups reported or velocity streaming toward it from our side (Han 1992;
results on angular scales of Γ-60 obtained from ground or Rauzy et al. 1992). Others have found indeterminate re-
balloon (Andreani et al. 1991; Boughn et al. 1992; Watson sults (Mould et al. 1991; Bothun et al. 1992). And some
et al. 1992; Alsop et al. 1992; de Bernardis et al. 1992). Ail have found the Great Attractor unnecessary (Landy and
are upper limits; but most are within striking distance (an- Szalay 1992; Tully et al. 1992), even for the Seven Samu-
other flight; another year's data) of confirming (or casting rai's own data (Weigelt and Kates 1991).
doubts on) the COBE numbers. Yet other investigations have concluded that the Great
Attractor is actually forbidden, in the sense that galaxies in
8.2 The X-Ray Background that part of the sky are not clumped at the GA redshift
(Visvanathan and van den Bergh 1992) and that galaxies
This is also quite smooth, though apparently because on the far side of the GA are not falling back into it toward
many distant sources contribute to it, not because the emis- us (Mathewson et al. 1992). Mathewson et al. (1992) re-
sion is inherently diffuse. Marginal lumpiness has been report instead that about 600 km s_1 of bulk flow extends
ported several times, most recently by Martin-Mirones et over a region 13,000 km s"1 ( 130 /z_1 Mpc) long, reaching
al. ( 1991 ), who reprocessed HEAO-1 A-2 experiment data from the GA region across us and on to the Perseus-Pisces
to find 3σ fluctuations on a scale of 3°, about what would be region, and perhaps beyond (see also Han and Mould
expected if most of the contributing sources are active ga- 1992).
lactic nuclei. The secondary literature has concentrated heavily on
the "demise of the Great Attractor" aspect of these devel-
8.3 Whither the Great Attractor? opments (Lindley 1992; Finkbeiner 1992). Those of us
Between the well-defined lumpiness of galaxies and clus- who are in the happy position of being paid to ask ques-
tions rather than to provide instant answers do not, how-
ters and the newly found gentle waves of the COBE back-
ever, have to vote on the issue until we are ready.
ground lies a considerable terra infirma called "very-large-
scale structure and streaming." General recognition of this
as an important cosmological issue came relatively re-
cently, perhaps about the time of a 1985 conference includ- 8.4 The Correct Value of the Hubble Parameter
ing "deviations from Universal expansion" in its title (Ma-
dore and Tully 1986), whose proceedings incidentally Lumps connected with Η§ seem to be found primarily
include the first published report of what is now called the on the ears and noses of contending investigators. Turner
Great Attractor, by the Seven Samurai (Burstein et al. et al. ( 1992) point out, however, that existence of lumps in
1986), though neither entity yet had the name. the sense of large-scale streaming means that a local mea-
The general phenomenon of ^300 kms-1 deviations surement of H0=80 km s-1 Mpc-1 (based on data out to
from Hubble flow with coherence lengths of at least 10,000 3000 km s"1) goes with a global value whose 95% confi-
km s-1 was widely doubted when first proposed (Rubin et dence level error bars extend from 50 to 128 kms-1
al. 1976), but is now generally accepted. A couple of years Mpc" ^ The size of the uncertainty and the volume you
ago, it even looked like we were on the verge of agreement need to probe both increase if there are systematic flows
on the important issues of ( 1 ) which galaxies, clusters, and over regions larger than 30 /*_1 Mpc.
larger structures are moving which way relative to the Given that caveat, it seemed nevertheless instructive to
mean flow, and (2) what concentrations of mass are re- identify as many firmly quoted values of ff0 as possible,
sponsible for inducing these peculiar velocities? (See, e.g., determined from specific methods (Cepheids, supernovae
papers in Holt et al. 1991.) Incidentally, measuring the of type II or la, planetary nebulae, gravitational lensing,
actual velocity of a galaxy is easy; the hard part is getting Sunyaev-Zeldovich effect, ...) applied to specific data sam-
its distance accurately enough to know what its smooth ples. Of 12 such papers located, six gave high values (>75
Hubble velocity ought to be. kms-1 Mpc-1), and six gave low values (<75). Huchra
On beyond the local, Virgo, supercluster, recognized (1992) discussed the merits of the two scales, and Jacoby
structures included major visible concentrations of galaxies et al. (1992) reviewed a number of methods but vote for
in the Perseus-Pisces and Hydra-Centaurus regions, in #=85. My strong Mt. Wilson-Palomar childhood feelings
roughly opposite directions in the sky, at distances of in favor of H= 50 were only partially eroded by spending
3-5000 kms-1. The surprise was that the Hydra- 10 weeks this spring in the office next to G. de Vaucouleurs
Centaurus galaxies were moving in the same direction as at the University of Texas, Austin.
9. ODD FACTS AT HOME ones. They include: CCO in the Taurus molecular cloud
No. 1 (Ohishi et al.); Η3θ+ confirmed by a second line
This section contains an assortment of short items about
(Wootten et al. 1991); H^ in Jupiter (Kim et al. 1991;
the solar system and Milky Way. Readers will notice the
Baron et al. 1991); a number of complex organics in Orion
absence of (a) the candidate for a Kuiper-cloud comet and
molecular cloud No. 1 (Greaves and White 1991b);
(b) arguments for and against dark matter in the galactic
CH2DCH in TMC1 (Gerin et al. 1992a); isomers HCCNC
disk. They are missing because I find it difficult to under-
stand just what they mean. and HNCCC (Kawaguchi et al. 1992ab); CH2DCN (deu-
terated methyl cyanide) in Orion (Guerin et al. 1992b);
SiN in IRC +10216 (Turner 1992); and SO+ in a clump
9.1 The Solar System in the supernova remnant IC 443 (Turner et al. 1992).
Pluto and the Earth apparently have something in com- PAHs (polycyclic aromatic hydrocarbons) are, how-
mon (and different from all the other planets), namely ever, undoubtedly the molecules of the year. "Aromatic"
atmospheres in which the dominant constituent is N2 means "containing at least one benzene ring" rather than
(Owen et al. 1992). Among the other Plutonian gases are "smelly," though many are both. Leger and Puget (1984)
CH4 and CO. The primary terrestrial contaminant is O2, first attributed interstellar absorption and emission features
and a good thing, too. to this class, of which naphthalene is a familiar example.
Mercury may have something in common with the Exploring their contributions has required a complex mar-
Earth and Mars, to wit a polar icecap. This is the interpre- riage of astrophysical and laboratory data (Brenner et al.
tation of recent radar reflectivity data suggested by Martin 1992; Mennella et al. 1992; Ehrenfreund 1992; Colangeli et
Slade and colleagues at JPL (Slade et al. 1991). al. 1992; Joblin et al. 1992). Debate persists on whether
Jupiter has something in common with the Sun— they are responsible for the classical diffuse interstellar
detectable /7-mode oscillations {p means that pressure is the bands. Salama and Allamandola (1992a) think pyrene
restoring force), seen in Doppler shifts of the Na I D lines (C16HJo) is a likely candidate for the best-known band at
of reflected sunlight (Mosser et al. 1991). Twenty-four- /14330, while LeBertre and Lequeux (1992) believe that
hour coverage is badly needed to confirm the mode iden- the absence of correlation of the diffuse absorption with IR
tifications, but the preliminary version suggests a solid core emission bands makes the identification unlikely. But the
of about 0.1 Rj (a property not shared by the Sun). presence of PAHs in pre-main-sequence stars (Roche et al.
Pluto, Mercury, and all the planets in between have in 1991), in planetary nebulae (Bergman et al. 1992), carbon
common chaotic orbits (Wisdom and Holman 1991; Suss- stars (Cherchneff and Barker 1992), and the general inter-
man and Wisdom 1992). "Chaotic" in this context doesn't stellar medium (Shan et al. 1991; Siebenmorgen and Krue-
mean quite the sort of total disorder the everyday word gel 1992; Salama and Allamandola 1992b) seems to be well
conjures up. Rather, it means that small changes in initial established.
conditions lead eventually to very different outcomes (so Moving groups are relics of star-formation regions no
that long-range calculations, forward or backward in time, longer gravitationally bound. Most consist of intermediate-
become rather indeterminate). The characteristic time age disk stars (like the prototypical Hyades). But a few
scale for this to happen in the outer solar system is about have recently been identified among OB stars on the basis
4χ106 yr. Orbital chaos need not always mean that the of proper motions (Pavlovskaya and Filippova 1992). And
potential divergent outcomes sprawl all over parameter four blue horizontal-branch stars in the halo, about 30 kpc
space. For instance, the asteroid 522 Helga (which is in a from us, have similar enough radial velocities to indicate
7:12 period resonance with Jupiter) is chaotic with a char- that they are probably the remnants of a dispersed globular
acteristic time scale of 6900 yr, but calculations show that, cluster (Arnold and Gilmore 1992).
after 7X 106 yr, it still isn't very far from its present posi- The local hole: Contrary to the most extreme Coperni-
tion (Milani and Nobili 1992). Similar stable chaos prob- can view, not everything about our surroundings is abso-
ably applies also to Pluto and the inner solar system (Mur- lutely average. A smoothed galactic disk would have a gas
ray 1992). density of about 0.5 H cm-3. But a local bubble of both
Comets may or may not have something in common low density and high ionization, so that iVH^0.005 cm-3,
with the Earth, in the form of chiral molecules, that is, extends out to 60-90 pc in some directions (Welsh et al.
complex organic molecules whose left (or right) handed 1991; Barstow et al. 1992). Beyond this distance 7VH rises
form is much more abundant than its mirror image. Ac- rapidly (Bates et al. 1991 ). The situation is a fortunate one
cording to the proposers (Greenberg et al. 1991), Earth- for extreme ultraviolet astronomy. Without the local hole,
impacting comets could even have been the primary source Voyager 2 would not have seen white dwarfs (Barstow et
of pre-biological terrestrial chirality. al. 1992), and the launch of EUVE in the spring of 1992
would have been much less exciting an event.
9.2 The Milky Way The age of the disk: The oldest open star clusters in the
Milky Way disk (M67, NGC 188 and 6791, etc.) are sig-
Molecules new and old: the golden era of molecular dis- nificantly younger than the oldest halo globular clusters.
covery is over (or at least in abeyance). Most of the mol- But there is no guarantee that they date back to the origins
ecules added to the galactic inventory in 1992 are isomers, of the disk. Two alternative chronometers provide some
isotopic variants, or ionized versions of previously known evidence that they do, so that the oldest open clusters, the
12 TRIMBLE
disk population of globulars, and the oldest field stars are 10.4 Strong Field Test of General Relativity
all something like 8-10 Gyr old. In the latest round on
Deflection of light and the other solar-system measure-
each of these alternatives, first, the faintest white dwarfs
ments (advance of the perihelion of Mercury, delay of ra-
have been cooling for 7.5-11 Gyr, assuming conventional
dio waves passing the Sun) test GR only in the weak-field
physics (Wood 1992). Gravitational settling and
limit of terms first order in GM/Rc1. Neutron stars provide
[O/Fe] >0 could make them somewhat younger, while
an opportunity to look at the effects of much deeper grav-
phase separations of oxygen, iron, and neon relative to
itational potentials. Enough data have now been collected
carbon could age them to 13 Gyr (Xu and Van Horn
on four binary pulsars (1913 + 16, 1534+12, 1953+09,
1992). Second, the radioactive chronometer coming from
and 1855+09) to permit separation of the timing param-
the Nd/Th ratio in stellar spectra can yield dates over an
eters into ones dominated by emission of gravitational ra-
equally wide range, owing to overlapping of the thorium
diation and ones dominated by strong field effects (Taylor
line by at least two others (Morell et al. 1992). There is, in
et al. 1992). The separation is not, in general, possible for
other words, at the moment no strong evidence either for
a single binary system. The authors adopt a parametrized
an age gap between the youngest halo and oldest disk stars
post-Keplerian formation (analogous to the parametrized
or for disk star formation before the formation of the oldest
post-Newtonian formalism used in weak-field tests).
open clusters.
Within that framework, GR once again comes out at the
head of the pack, providing a better fit to the data even
10. EUREKA than some alternative theories devised to permit unification
with other forces.
These dozen or so items are, roughly, phenomena, ob-
jects, or radiation that somebody has looked for before and 10.5 A New Member of the Local Group
that have now been found or clarified. Contrary cases of
things that have gone away appear in Sec. 11. Lavery and Mighell (1992) have found a dwarf sphe-
roidal (dE5) in Tucana with a distance modulus of 24.75
mag, placing it at less than 1 Mpc and so within the Local
10.1 Polarization of Starlight
Group. Van den Bergh ( 1992) has admitted it to the canon
Hall (1949) and Hiltner (1949) discovered interstellar as member number 27. Anyone who doubts the impressive-
polarization of starlight as a result of a series of observa- ness of the achievement is invited to examine Palomar sky-
tions prompted by Chandrasekhar's ( 1946) prediction that survey charts at the position of any of the recognized dSph
electron scattering in stellar atmospheres should result in galaxies!
intrinsic polarization. The stellar effect was finally seen in
1982 (Kemp et al. 1982), a decade too early for this survey 10.6 Even Young Galaxies have Old Stars
(though rather late by the standards of prediction and
Ap91 noted the identification of an old, spheroidal star
discovery in astronomy). But Collins and Cranmer ( 1991 )
population (CH stars and long-period variables) in the
have now predicted that a somewhat similar process
Large Magellanic Cloud. But the LMC, while the proto-
should give rise to polarization across rotationally broad-
type of irregular galaxies, is "really" an Sm with recogniz-
ened pure absorption lines. The effect, most likely to be
able arms. IC 1613, on the other hand, is a true dlrr. Saha
detectable in the ultraviolet, could provide a firm distinc-
et al. ( 1992) have found its RR Lyrae variables, establish-
tion between rotation and other line broadening mecha-
ing the existence of an old stellar population there. They
nisms. We hope they won't have to wait until 2027 for it to
find also that the distance modulus implied by the RR
be seen.
Lyraes is 0.3 mag smaller than the one coming from Cep-
heids, and suggest that the problem may be metallicity
10.2 The Gravitational Deflection of Stellar Radiation dependence of the intrinsic brightness of RR Lyraes.
You thought Eddington saw this in 1919? Ah, but the
deflector in this case is Jupiter, and the radiation came 10.7 One is a Discovery, Two a Confirmation, and Three a
from pulsar 0201 + 113. Naturally, the results are not the Weil-Known Class of Astrophysical Object
most sensitive test of general relativity, but at least the sign On the basis of this criterion, we must now admit to the
was right (Treuhaft and Lowe 1991). "well-known" class low-mass X-ray binaries with black-
hole accretors. Ap91 briefly mentioned A 0620—00, whose
10.3 Radio Sources in M31 1990 radial-velocity curve required a mass for the compact
component well above the maximum possible for neutron
Two of these have long been among the missing—a cen- stars (Haswell and Shafter 1990). It has been joined by
tral, unresolved source analogous to our Sgr A* and emis- GS 2023 + 33=V404 Cyg=Nova Cygni 1938 at more than
sion from the site of SN 1885A. The former has at last 6 Mq (Casares et al. 1992) and by Nova Muscae 1991
turned up, at about one-fifth the total radio power of Sgr with a mass function of 3.1 Mq and so minimum accretor
A*. The latter remains undetected, down to a limit about mass of about 4 Mq for any reasonable donor (Mc-
one-fifteenth the power of the Tycho supernova remnant Clintock et al. 1992). Much more can be said about these
(Crane et al. 1992). systems, especially concerning the ongoing search for de-
finitive X-ray signatures of black holes (Kitamoto et al. bright and faint maxima) could be understood if the dou-
1992; Sunyaev et al. 1991). I am particularly intrigued by ble period was the fundamental, but with two shocks prop-
an analogy between their X-ray light curves and the optical agating through the star each period. Evidence for such a
light curves of the dwarf novae that display superhumps pair of shocks (and pulsation as a running wave) has now
(Mineshige et al. 1992). In DNe, the ratio of superhump been found in phase-resolved spectra of R Scuti (Lebre and
to orbit period is well correlated with mass ratio (Molnar Gillet 1992). Admittedly, the confirmation seems a bit less
and Kobulnicky 1992). If the underlying physics is similar, exciting when we hear that β Cephei variables (Mathias
then Nova Muscae (GS 1124+68) is independently re- et al. 1992) and even RR Lyraes (Fokin 1992) probably
vealed as a black hole and GS 2000+25 joins the ranks. also have two shocks per period.
10.8 Evidence for Continuing Circularization of Binary- 10.11 Dwarf Carbon Stars
Star Orbits Classical carbon stars are giants whose atmospheres
That short-period (close) binaries nearly always have have (presumably) been polluted by products of their own
circular orbits and long-period (wide) ones do not was helium burning. The newly discovered dwarf class (Green
probably known to Kuiper (though the standard "histori- et al. 1992) must represent dirtying of stellar surfaces by
cal" reference seems to be Koch and Hrivnak 1981). The the products of nuclear reactions in close binary compan-
relevant tidal mechanisms can be traced back to George ions (now probably invisible as white dwarfs, as in the case
Darwin and James Jeans, and are closely related to those of Ball stars), since helium burning cannot yet have
that bring stellar rotation periods into synchronization started in stars this close to the main sequence.
with the orbit periods of close systems. An important re- Four dwarf carbon stars have been reported. Because of
sidual issue has been whether circularization happens pri- their faintness, the implied space density is probably com-
marily during the pre-main-sequence phase (Zahn and parable with that of classical carbon stars. I feel that the
Bouchet 1989) or persists throughout stellar lifetimes new class ought also to shed some light on the set of carbon
(Mathieu and Mazeh 1988). In the latter case, the maxi- stars (now eight in number) with silicate features (imply-
mum orbit period to which circularization extends could ing oxygen-rich gas) in their circumstellar shells (LeVan
be an independent age indicator for clusters or other stellar et al. 1992). These have been interpreted either as close
populations, once the physics was fully understood. binaries or as stars in process of vigorous mass loss, so that
I am sure that I do not understand the several mecha- carbon-rich layers have just been uncovered. It is not to-
nisms currently under debate (e.g., Rietord 1992 versus tally clear which point of view is strengthened by the ex-
Tassoul and Tassoul 1992) well enough to summarize the istence of the dwarfs.
issues. But the observational side seems to have taken a
quantum step forward. A sample of Population II halo 10.12 Radio Emission from SN 1987A
field binaries with metallicity [Fe/H]<—1.9, and ages After a faint, few-day burst of optically thick
therefore in excess of 10 Gyr, displays only circular orbits bremsstrahlung (Turtle et al. 1987), our favorite super-
up to a period of 19 days (Latham et al. 1992), in com-
nova kept its radio mouth shut for several years. Emission
parison with cuts at 12.4 days in M67 (an old open cluster
turned back on about day 1200 (July 1990) and gradually
of 5-7 Gyr) and 8.5 days among pre-main-sequence sys- increased thereafter at 0.8-8 GHz (Staveley-Smith et al.
tems (Mathieu et al. 1992). 1992). The emission comes from inside the ring of [O III]
emission resolved by HST, and is ascribed by the discov-
10.9 Thermal X-Rays from (Single) Neutron Stars erers to optically thin synchrotron radiation from shocks
X-ray emission from the Crab pulsar, 0540—69 in the where the blast wave hits clumpy circumstellar material.
LMC, and 1509 — 58 in MSH 15-52 is dominated by the They predict the onset of soft X-ray emission in early 1993,
pulsed emission itself. Upper limits to the unpulsed ther- when the wave reaches the [O ill] shell itself. This is
mal radiation are not particularly constraining of neutron- roughly consistent with predictions made by early model-
star cooling models. ROSAT observers (Finley et al. 1992) ers of the progenitor. But 1993 does not sound as far away
have now seen what they believe is the thermal emission at as it did in 1987.
3-9X105 Κ from PSR 0652+14 (/>=0.385 s, slowing-
down age=l.lxl05 yr). Because of the wide possible 11. GONE BUT NOT FORGOTTEN
range of temperatures (associated with considerable dis- It is a little difficult to describe exactly what these half
tance uncertainty), it is still not possible to decide if an dozen items have in common, except that if roughly the
honest neutron star will cool quickly enough or if pion opposite had been reported, they would have belonged un-
condensate, strange quark matter, or some other exotic der "eureka."
constituent needs to be invoked (Alcock 1987).
11.1 The Seo X-l Radio Source(s)
10.10 RV Tauri Behavior
The radio emission is still there, with two lobes roughly
Schwarzschild (1952) proposed that the light curves of symmetric around a radio core and X-ray and optical point
RV Tauri stars (long-period variables with alternating sources. But only the core source shares the 0^0148 yr-1
14 TRIMBLE
proper motion of the visible star, and the lobes are almost the problem is apparently a somewhat different one from
certainly extragalactic and unrelated to the X-ray binary what we had thought.
(Fomalont and Geldzahler 1991).
11.3 Binary Wolf-Rayet Stars?
Ever since Paczyñski (1967) explained Wolf-Rayets by
11.2 TeV and PeV Sources mass transfer in close binaries (a point of view vigorously
opposed by Underbill 1991, then and now), one has rather
Particles striking the Earth's upper atmosphere at TeV expected them to have companions—if not conspicuous
(Tera=1012) or more energies produce flashes of Ceren- OB stars, then less conspicuous neutron stars or black
kov light; while those of PeV (Peta= 1015) or more initiate holes, left from the initially more massive star. Moffat
extensive air showers of secondary particles. Both can be (1992) has looked long and hard at candidates for the
detected from ground, and relevant detectors exist on every latter sort of system. He finds no new ones and regards
continent, including Antarctica. Photon initiators are ex- many of the old ones as dubious, concluding that at most
pected to yield events with a somewhat different angular 10%-15% of superficially single WRs can have these com-
pattern and a smaller fraction of muon secondaries than pact companions.
are hadronic initiators (cosmic rays). Report of TeV/PeV
events from X-ray binaries and pulsars, beginning with 11.4 Where are the Faint Blue Galaxies of Yesteryear?
Cyg X-3, go back more than a decade (cf. Barnard 1987).
Other reported sources include Her X-l, Vela X-l, LMC Fuzzy visible objects in the sky at Bj^lA are mostly
X-4, 4U 0115 + 53, 4U 1822-37, Sco X-l, and the pulsars faint blue galaxies (Tyson 1988). A combination of mea-
0532 and 1509-58. sured redshifts of the brighter ones (e.g., Cowie et al.
Recent positive reports from long-running experiments 1991) and lensing of the fainter ones by rather distant
include those of Rawat et al. ( 1991 ) on Her X-l, Ton war clusters of galaxies (Tyson et al. 1990) requires them to
et al. (1991) on Sco X-l, Tonwar et al. (1992) on Cyg have redshifts 1. But most of the fuzzy visible objects in
X-3, Meyhandan et al. ( 1992) on LMC X-4, and Acharyu the sky at brighter magnitudes and smaller redshifts are
et al. (1991) on the Crab. Because the detectors have not faint blue galaxies; they are uncommon by Bj=21
rather poor angular resolution, evidence for source identi- (Colless et al. 1991).
fication typically includes phasing with a previously known Only three sorts of answers seem possible to the ques-
orbit or rotation period. The year witnessed about as many tion, where have they gone? First, they have (mostly)
new model papers as new observations, the chief difficulties ceased to be blue and are now other kinds of faint galaxies.
being (a) to get anything up to the required energy (ap- Second, they have ceased to be faint (by merging into the
parently soluable in several ways) and (b) to account for larger galaxies now seen). Or, third, they have faded com-
excess muons (hadronlike behavior) in the showers asso- pletely out of local samples. All three have been proposed
ciated with the X-ray binaries (not soluable without new in some form over the past year. The temptation to write
physics). FBGs is irresistible, and I apologize to those who share my
A new Cerenkov installation at the Whipple Observa- dislike of papers that are constantly sending you backward
tory in Arizona and a new air-shower array in New Mexico to find out what is meant by BART, NSAID, and
have cast doubts on many of the earlier data. Whipple UNESCO.
unquestionably sees the TeV Crab [though the report (Va- It is generally agreed that FBGs are not as strongly
canti et al. 1991) falls outside our period] as a 20σ source clustered as z=0 bright galaxies (Efstathiou et al. 1991,
of unpulsed photons. But six years of Her X-l pointings who also believe that gravitational effects cannot produce
yielded only a few short stretches of possible (3σ) detec- the present degree of clustering if you start with FBGs).
tion, and the image shapes are wrong for photons (Rey- Melott ( 1992) conversely believes that gravitational forces
nolds et al. 1991). Cyg X-3 is limited even more firmly, can produce enough additional clustering to match present
with five straight years at less than 10% of the previously correlations functions and so forth.
claimed flux, whether steady or pulsed at the orbit or ro- Mergers of FBGs to the present population are favored
tation period, and whether in photon-like or handron-like by Guideroni and Rocca-Volmerange (1991) on the basis
events (O'Flaherty et al. 1992). of comparison of blue and infrared counts and also by
The Cygnus array near Los Alamos National Lab has Broadhurst et al. (1992), who predict that the mergers
pushed air-shower capability down to 40 TeV (Alexan- should be accompanied by high star-formation rates at
dreas et al. 1991). The first few years of data revealed no moderate redshift.
steady sources (or time-averaged pulsed sources) down to In contrast, Lacey and Silk (1991) propose (within a
limits of 0.9-5.5 X 10-13 ycm~2s~1 at the positions of context of interaction-triggered star formation) that the
northern pulsars. X-ray binaries, active galaxies, or cata- modern forms will be stripped, low-surface-brightness gal-
clysmic variables. The Crab limit does not preclude a axies in dense clusters and gas-rich galaxies enjoying "re-
power-law continuation of the TeV Whipple spectrum; but tarded" star formation in the field.
there are contradictions with some other positive reports. Babul and Rees (1992) propose nearly the opposite
One cannot quite say that the TeV/PeV sources have combination. Following a first star burst near z=l, the
gone away, or even that the problem has gone away. But faint blue galaxies in low-density environments expel their
gas and so fade away, while those in clusters with high ate amounts of winding and amplification. Possible sources
ambient gas pressure are unable to get rid of their gas. for the necessary seed fields are vorticity in the early uni-
Thus they continue star formation somewhat longer, ap- verse (Rebhan 1992) and inflation (Ratra 1992). I would
pearing now as the very extensive population of dwarf el- probably find these more persuasive if I understood them
liptical galaxies characteristic of rich clusters (Binggeli and better.
Cameron 1991). It is a prediction of this scenario that the
survivors should have M/L ratios proportional to L_l, 12. I THOUGHT THAT I WOULD NEVER SEE
which is at least roughly so for most of the dwarf spheroi- 12.1 A Leading Arm Spiral
dals in the Local Group. If Giraud (1992) is right that
there are three physically different kinds of objects among These went out of fashion about the time astronomers
the FBGs, then three different end points is not implausi- stopped believing van Maanen's measurements of rota-
ble! tional proper motion in face-on disks. But NGC 4622 has
A closely related issue, whether the majority of elliptical three arms, two going one way, and one the other; so at
galaxies can be merger products, has also seen much activ- least one must be a leading arm. The additional peculiarity
ity in our reference year. Space permits notice of only two of a ring suggests the morphology may result from a ret-
points—a possible test for merger products [misalignment rograde encounter (Buta et al. 1992).
of minor and rotation axes (Barnes 1992)], and a way
around the objection coming from Es having a larger num- 12.2 A Binary RR Lyrae Star
ber of globular clusters per unit luminosity than do Ss [the There is nothing physically implausible about these, but
merger triggers formation of additional ones (Ashman and they do seem to be rather thin on the sky. Kinman and
Zepf 1991)]. Carretta (1992), however, suggest, on the basis of colors,
that BB Virginis and possibly AR Herculis, may have sec-
11.5 Extraterrestrial Intelligence ond horizontal-branch stars as companions.
No, it hasn't been found. But a recent 1421 MHz survey
sets an upper limit of 108 for N, the number of communi- 12.3 A Pulsar with a Real (Non-degenerate) Stellar
cating civilizations currently present in the Milky Way Companion
(Blair et al. 1992). We learned as children that this was impossible, because
even as weak a wind as the Sun puts out would put pulsar
11.6 Decaying Dynamos radiation below the local plasma frequency and prevent the
radiation from propagating. But Kaspi et al. (1992) have
In the absence of magnetic monopoles, astrophysical found a 47-ms pulsar, 1259 — 63, at the position of the Be
magnetic fields must either be generated by the planets, star SS 2883, and, since the pulsar is eclipsed, it is a bit
stars, and galaxies that have them or have been frozen in difficult to claim superposition rather than physical asso-
from earlier existence in precursor gas. Either mode ap- ciation! X-ray emission could possibly turn on when the
peared sufficiently unlikely that Fritz Zwicky claimed the system passes through periastron.
correct text of Genesis 1:3 to be "Dixitque Deus: fiat lux
campusque magneticus." ( Vayomer Adonai, Y'hi or v'sadeh 12.4 Diffraction-Limited Optical and Infrared
magneti, in the original). Observation from the Ground
At least for normal spiral galaxies, in situ generation by
a self-excited α-ω dynamo has been the standard model for With the rapid changes brought about by recently de-
some time (Beck et al. 1990). Doubt has been cast on this classified military technology, I dare not try to say any-
conventional wisdom on both observational and theoretical thing about how this is done (which end of the screwdriver
grounds. are you supposed to look through?). But surely there is no
Observationally, galaxies at redshifts as large as z=2 harm in noting some of the very impressive scientific
have fields about as strong as that of the Milky Way achievements of the year. These include: (1) imaging of
(Wolfe et al. 1992), meaning either that the field is largely the lo volcanic hotspots at 0''18 (McLeod et al. 1991), (2)
primordial or that a dynamo operated very rapidly. On the separating the components of μ Cassiopeiae to get an esti-
theoretical side, recent discussions suggest (a) that the mate of primordial helium abundance (Haywood et al.
most probable dynamo field geometry is not that seen most 1992), (3) tracing the size and shape of Mira and changes
often in real galaxies (Elstner et al. 1992), (b) that most of therein with time scales of months (Quirrenbach et al.
the power quickly cascades down to small spatial scales, 1992), due either to infall or to nonradial pulsations, (4)
leaving no significant large-scale field (Kulsrud and resolving the first two spectroscopic binaries, a Equulei
Anderson 1992), while observations indicates roughly and γ Andromedae, of 10-30 that should be reachable with
equal power on large and small scales (Jones et al. 1992), the Mark III Mt. Wilson/NRL system (Armstrong et al.
and (c) that galactic dynamos saturate at fields much 1992; Pan et al. 1992), (5) finding a whole flock of Τ Tau
smaller than the observed ones (Vainshtein and Cattaneo and other pre-main-sequence stars resolved into binaries
1992). and shelled single stars by interferometry and lunar occu-
The alternative is a primordial field of sufficient strength lation (Leinert et al. 1991; Koresko et al. 1991; Simon et
that the observed ones can be produced from it by moder- al. 1991; Maihara and Kataza 1991), (6) seeing individual
16 TRIMBLE
flux tubes on the solar surface (Keller 1992, see also Sec. Fresh crust rises at the centers of the coronae, which pre-
2.2); it seems that the smaller ones (<200 km) are bright sumably overlie mantle plumes. This is a real scientific
and the larger ones (>300 km) are dark, and (7) spotting advance. It remains true that the surface of Venus shows
the Neptunian satellite Proteus (Colas and Buil 1992). no firm signs of lateral motion or plate tectonics.
12.5 Through an Opaque Disk

13.3 Decaying Dark Matter
Because we are used to looking fairly easily from within
the galactic plane out on both sides, most astronomers A neutrino with a mass of 28 eV and a lifetime of 1023
were surprised when Valentijn (1990) proposed that most s indeed cannot dominate the dark-matter content of rich
spiral disks were so opaque that the absorbed luminosity clusters, if Coma, with its absence of a 14 eV line, is typi-
was sufficient to adjust the real M/L ratios to values re- cal. Sciama et al. (1992) have, however, proposed a mod-
quiring no significant dark matter. The current situation is ified picture in which the decaying neutrino is the primary
somewhat confused. Our own vantage point continues to dark matter in isolated galaxies like the Milky Way (and
perhaps in the Universe as a whole), while rich clusters are
be quite transparent—the model used for the HIPPARCOS
input catalog has ^=0.1 mag at ¿>=60ο-90ο (Arenou et dominated by dark baryons, accounting for the small core
radii of the underlying gravitational potentials.
al. 1992), and ^ is only 0.16 mag to Andromeda (Jab-
lonka et al. 1992). But many spirals seem to have very
opaque central bulges (Huizinga and van Albada 1992; 13.4 Blue Stragglers
Cunow 1992). And while the space-averaged disks may be
fairly transparent [e.g., AB=0A6 mag full thickness (Pe- The remark in Ap91 that the discovery that some
letier and Willner 1992)], the fact that both luminosity and globular-cluster blue stragglers are close binaries had not
dust are concentrated in spiral arms may mean that the really solved the problem because the individual stars sep-
effective extinction may be more like a whole magnitude arately were stragglers is subject to misinterpretation. I
[t=1 (White and Keel 1992)]. I remain puzzled, though, meant simply that duplicity per se was not the solution,
because the absorbed visible light must come back out as and did not intend to imply that no conceivable binary
infrared emission (Phillips et al. 1991) and most spiral process was relevant. Encounters between two close binary
galaxies are not more luminous as IR sources than as op- systems can produce a final system of more than twice the
tical ones. turnoff mass, and similar encounters may be responsible
for extreme blue stragglers in rich open clusters like M67
13. ACTA ET RETRACTA and NGC 188 (Leonard and Linnell 1992).
Several items mentioned in Ap91 require correction or

expansion for several different kinds of reasons. References 13.5 Galactic-Center X-ray Binaries (etc.)
to the items being corrected are in Ap91. In addition, some There is indeed a radio source within the error circle of
of the events of the year come perilously close to forcing the "Great Annihilator" IE 1740.2—2942, but it is not the
the author to admit that some of her cherished early work one mentioned in Ap91 (Prince and Skinner 1991; Skinner
was WRONG. et al. 1991). The incorrect identification apparently re-
sulted apparently resulted from the use of a much-
13.1 Extragalactic Recurrent Novae rounded-off position (G. K. Skinner 1992, private commu-
Nova 1990 No. 2=Nova 1968 in the Large Magellanic nication).
Cloud was probably not the first extragalactic RN. S. van Other kinds of vigorous activity continue in the galactic-
den Bergh ( 1992, private communication) has pointed out center region. The (literal) ups and downs of the X- and
that novae 38 (1963) and 79 (1968) in the M31 nova gamma-ray binary sources in the region are best followed
catalogue of Rosino ( 1973) were almost certainly the same by regular reading of IAU Circulars (5398, 5472, 5474,
object, as the catalog itself says. They peaked at 5475, et seq.). The central radio source, Sgr A*, has finally
M v= —6.9 and —7.5, respectively. This error is my fault been resolved by a combination of VLBI and interstellar
for taking the 1991 authors' word without checking. scintillation, placing its size between 8 and 15 AU and its
brightness temperature below the synchrotron-self-
Compton limit (Gwinn et al. 1991; Marcaide et al. 1991;
13.2 Cytherean Subduction
Bââth et al. 1991). A couple of near-infrared objects have
As Ap91 went to press, it was known that the average been imaged within 1 " of Sgr A* and have been suggested
cratering age of the surface of Venus was about 109 yr, so as identifications (Rosa et al. 1992; Eckart et al. 1992).
that crustal reprocessing must occur. The mechanism had Supporters of a massive (1-2X106 Mq) black hole as
not, however, been identified, and there did not seem to be the best explanation of galactic-center phenomena (Melia
any extended subduction zones like the trenches adjacent et al. 1992; Lacy et al. 1991) continue to outnumber op-
to terrestrial island arcs. Sandwell and Schubert (1992) ponents (Krabbe et al. 1991) about 2 to 1. And the radio
have now found evidence that subduction is occurring morphology at VLA scales, with a compact radio core and
around the margins of large coronae (approximately cir- two-sided jet, suggests close analogy with quasars in min-
cular surface features, including Artemis and Latona). iature (Mirabel et al. 1992b).
13.6 The Average Mass of White Dwarfs 14. AND ALL THE REST
"Astrophysics in 1992" is somewhat longer than "As-
Ap91 focused on massive white dwarfs [and two more
trophysics in 1991.^ It was, after all, a longer year (owing
of 1.2-1.3 M& PG 0136 + 251 and PG 1648 + 441, have
to 29 February), in which more papers were published (by
turned up among magnetic WDs in the interim (Schmidt
5% or so). Nevertheless, a number of interesting topics
et al. 1992)]. But this was sheer prejudice. The author's
have been omitted. These include: globular clusters (both
involvement with the first measurements of gravitational
extragalactic systems and details of stellar composition in
redshifts in white dwarfs other than Sirius Β (Greenstein
Milky Way ones); quasar absorption lines (especially the
and Trimble 1967; Trimble and Greenstein 1972) left her
local Lyman-alpha forest); alignments (of galaxies and
with a strong predisposition in favor of average masses
clusters with their neighbors as well as of radio and optical
near 0.75 Mq. A voluminous literature deriving an average
structures); the efficiency of star formation; an assortment
near 0.58 Mq from fits of model atmospheres to broad
of "firsts," "mosts," and "largests"; central black holes in
band colors (e.g., Weidemann 1977) did remarkably little nonactive galaxies; and Type la supernova mechanisms.
to disabuse her of this conviction.
The neatest idea that I am consciously aware of not having
The same sorts of models of line profiles that picked out
cited came from Rutten ( 1991) and the best-written paper
a few massive stars have, however, now been applied to from Barsony et al. (1991).
large enough samples to demonstrate the average (based
on the spectroscopic logg and radii for stars with good As in 1991, first thanks go to those who maintain the
temperatures and parallaxes) is about 0.56 Mq (Bergeron astronomy journal collections at the University of Califor-
et al. 1991). Procyon Β, with a newly improved binary- nia, Irvine, the University of Maryland, and the University
orbit mass of 0.62±0.02 Mq (Irwin 1992) is thus exceed- of Texas, Austin. A number of colleagues offered ideas,
ingly average. preprints, corrections, background material, and other as-
There is apparently some non-gravitational component sistance with this review. First and foremost, the contribu-
to redshifts of Balmer lines in DA white dwarfs. Green- tions of Peter J. T. Leonard are much appreciated (we are
stein (1984) very sensibly switched sides some years ago. thinking about a collaboration for next year). Thanks also
For such a stand to be abandoned while some of its pro- to Sidney van den Bergh, Joachim Wambsganss, Dennis
pounders are alive is relatively unusual in the sciences! Sciama, Eli Dwek, David Soderblom, Martin Rees, Boh-
dan Paczyñski, and Gerald Skinner. Bruce Elmegreen,
Katya Cunha, and David Latham are thanked for permis-
sion to quote items not published at the time of writing
13.7 The Initial Distribution of Binary-System Mass (but firmly expected by time of publication). I am deeply
Ratios grateful to the University of Texas, Austin, its astronomy
department and chairman Gregory Shields, and its Curtis
If the components of close binaries are a random sample T. Vaughan fund for hospitality and support during my
from the single-star initial mass function, then the distri- first-ever sabbatical as Beatrice M. Tinsley Visitng Profes-
bution of mass ratios will directly reflect the shape of that sor there (1992 April 1-June 15).
IMF (whether a power law rising to small values or a
Gaussian). Distinguished astronomers from Kuiper
( 1935) to Jaschek ( 1976) have found such reflections. The
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12.5 Through an Opaque Disk

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