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Volume 719

Number 1, 2010 August 10

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L1

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We present direct numerical simulations of the equations of compressible magnetohydrodynamics in a wedge-shaped spherical shell, without shear, but with random helical forcing which has negative (positive) helicity in the northern (southern) hemisphere. We find a large-scale magnetic field that is nearly uniform in the azimuthal direction and approximately antisymmetric about the equator. Furthermore, the large-scale field in each hemisphere oscillates on nearly dynamical timescales with reversals of polarity and equatorward migration. Corresponding mean-field models also show similar migratory oscillations with a frequency that is nearly independent of the magnetic Reynolds number. This mechanism may be relevant for understanding equatorward migration seen in the solar dynamo.

L5

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Recent observational studies of type Ia supernovae (SNeIa) suggest correlations between the peak brightness of an event and the age of the progenitor stellar population. This trend likely follows from properties of the progenitor white dwarf (WD), such as central density, that follow from properties of the host stellar population. We present a statistically well-controlled, systematic study utilizing a suite of multi-dimensional SNeIa simulations investigating the influence of central density of the progenitor WD on the production of Fe-group material, particularly radioactive 56Ni, which powers the light curve. We find that on average, as the progenitor's central density increases, production of Fe-group material does not change but production of 56Ni decreases. We attribute this result to a higher rate of neutronization at higher density. The central density of the progenitor is determined by the mass of the WD and the cooling time prior to the onset of mass transfer from the companion, as well as the subsequent accretion heating and neutrino losses. The dependence of this density on cooling time, combined with the result of our central density study, offers an explanation for the observed age–luminosity correlation: a longer cooling time raises the central density at ignition thereby producing less 56Ni and thus a dimmer event. While our ensemble of results demonstrates a significant trend, we find considerable variation between realizations, indicating the necessity for averaging over an ensemble of simulations to demonstrate a statistically significant result.

L10

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We imaged the position of the naked-eye burst, GRB080319B, before, during, and after its gamma-ray activity with sub-second temporal resolution using the TORTORA wide-field camera. The burst optical prompt emission, which reached 5.3 mag, has been detected, and its periodic optical variability has been discovered in the form of four equidistant flashes with a duration of several seconds. We also detected a strong correlation (r ≈ 0.82) between optical and gamma-ray light curves with a 2 s delay of the optical emission with respect to the gamma-ray emission. The revealed temporal structure of the optical light curve in comparison with the gamma-ray light curve can be interpreted in the framework of the model of shell collisions in the ejecta containing a significant neutron component. All observed emission features reflect the non-stationary behavior of the burst internal engine—supposedly, a hyperaccreting solar-mass black hole formed in the collapse of a massive stellar core.

L15

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We present high-resolution interferometric Submillimeter Array imaging at 890 μm (∼2'' resolution) of two millimeter selected galaxies—MMJ100015+021549 and MMJ100047+021021—discovered with the Max-Planck Millimeter Bolometer (MAMBO) on the IRAM 30 m telescope and also detected with Bolocam on the CSO, in the COSMOS field. The first source is significantly detected at the ∼11σ level, while the second source is tentatively detected at the ∼4σ level, leading to a positional accuracy of ∼0farcs2–0farcs3. MM100015+021549 is identified with a faint radio and K-band source. MMJ100047+021021 shows no radio emission and is tentatively identified with a very faint K-band peak which lies at ∼1farcs2 from a clumpy optical source. The submillimeter-to-radio flux ratio for MM100015+021549 yields a redshift of ∼4.8, consistent with the redshift implied by the UV-to-submillimeter photometry, z ∼ 3.0–5.0. We find evidence for warm dust in this source with an infrared luminosity in the range ∼(0.9–2.5) × 1013L, supporting the increasing evidence for a population of luminous submillimeter galaxies at z > 3. Finally, the lack of photometric data for MMJ100047+021021 does not allow us to investigate its properties in detail; however, its submillimeter-to-radio flux ratio implies z > 3.5.

L20

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High-quality spectra acquired at the European Southern Observatory enabled us to discover a very weak spectral feature of the OH+ molecule, near 3584 Å. The species likely shares environments with another molecular ion, CH+. Its abundance is by a factor of 30 lower than that of neutral OH.

L23

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We use Hubble Space Telescope imaging to measure the absolute proper motion of the hypervelocity star (HVS) HE 0437−5439, a short-lived B star located in the direction of the Large Magellanic Cloud (LMC). We observe (μα, μδ) = (+0.53 ± 0.25(stat) ± 0.33(sys), + 0.09 ± 0.21(stat) ± 0.48(sys)) mas yr−1. The velocity vector points directly away from the center of the Milky Way; an origin from the center of the LMC is ruled out at the 3σ level. The flight time of the HVS from the Milky Way exceeds its main-sequence lifetime, thus its stellar nature requires it to be a blue straggler. The large space velocity rules out a Galactic-disk ejection. Combining the HVS's observed trajectory, stellar nature, and required initial velocity, we conclude that HE 0437−5439 was most likely a compact binary ejected by the Milky Way's central black hole.

L28

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We have developed a new method for calculating common envelope (CE) events based on explicit consideration of the donor star's structural response to adiabatic mass loss. In contrast to existing CE prescriptions, which specify a priori the donor's remnant mass, we determine this quantity self-consistently and find that it depends on binary and CE parameters. This aspect of our model is particularly important to realistic modeling for upper main-sequence star donors without strongly degenerate cores (and hence without a clear core/envelope boundary). We illustrate the central features of our method by considering CE events involving 10 M donors on or before their red giant branch. For such donors, the remnant core mass can be as much as 30% larger than the star's He-core mass. Applied across a population of such binaries, our methodology results in a significantly broader remnant mass and final orbital separation distribution and a 20% increase in CE survival rates as compared to previous prescriptions for the CE phase.

L32

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Up to now, iron had not been found in any hydrogen-deficient (pre-) white dwarf of spectral type PG1159, despite intense searching in a number of objects. Consequently, an iron deficiency was claimed, in some cases at least 1 dex. The primary indicators were UV lines of Fe vii. Therefore, the search was confined to relatively cool PG1159 stars (Teff ≲150,000 K), otherwise Fe is too strongly ionized for a significant population of Fe vii. In this Letter, we announce the discovery of iron in the very hottest PG1159 stars (Teff = 150,000–200,000 K; RX J2117.1+3412, K 1−16, Longmore 4, NGC 246, H1504+65), based on the identification of the Fe x λλ979.3, 1022.9 lines in spectra obtained with the Far Ultraviolet Spectroscopic Explorer. Surprisingly, our analysis results in a solar iron abundance for these stars. It is conspicuous that they are among the most massive PG1159 stars (0.71–0.82 M), in contrast to those objects for which strongest Fe deficiency was claimed (0.53–0.56 M).

L36

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We describe and use novel techniques to analyze a striking and distinct solar wind event observed by two spacecraft. We show that the event is consistent with an interpretation as a torsional Alfvén wave embedded within a small, nearly radially aligned, magnetic flux rope of total width ∼106 km. It seems likely that the torsional wave was generated by distortions produced within a pre-existing flux rope that erupted from the Sun. Our examination of many events previously identified as flux ropes in the solar wind indicates that torsional Alfvén waves are extremely rare in such events.

L41

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We report observations of weak, circularly polarized, structureless type III bursts from the solar corona in the absence of Hα/X-ray flares and other related activity, during the minimum between the sunspot cycles 23 and 24. The spectral information about the event obtained with the CALLISTO spectrograph at Mauritius revealed that the drift rate of the burst is ≈−30 MHz s−1 is in the range 50–120 MHz. Two-dimensional imaging observations of the burst at 77 MHz obtained with the Gauribidanur radioheliograph indicate that the emission region was located at a radial distance of ≈1.5 R in the solar atmosphere. The estimated peak brightness temperature of the burst at 77 MHz is ∼108 K. We derived the average magnetic field at the aforementioned location of the burst using the one-dimensional (east–west) Gauribidanur radio polarimeter at 77 MHz, and the value is ≈2.5 ± 0.2 G. We also estimated the total energy of the non-thermal electrons responsible for the observed burst as ≈1.1 × 1024 erg. This is low compared to the energy of the weakest hard X-ray microflares reported in the literature, which is about ∼1026 erg. The present result shows that non-thermal energy releases that correspond to the nanoflare category (energy ∼1024 erg) are taking place in the solar corona, and the nature of such small-scale energy releases has not yet been explored.

L45

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Water-rich super-Earth exoplanets are expected to be common. We explore the effect of late giant impacts on the final bulk abundance of water in such planets. We present the results from smoothed particle hydrodynamics simulations of impacts between differentiated water(ice)-rock planets with masses between 0.5 and 5 M and projectile to target mass ratios from 1:1 to 1:4. We find that giant impacts between bodies of similar composition never decrease the bulk density of the target planet. If the commonly assumed maximum water fraction of 75 wt% for bodies forming beyond the snow line is correct, giant impacts between similar composition bodies cannot serve as a mechanism for increasing the water fraction. Target planets either accrete materials in the same proportion, leaving the water fraction unchanged, or lose material from the water mantle, decreasing the water fraction. The criteria for catastrophic disruption of water-rock planets are similar to those found in previous work on super-Earths of terrestrial composition. Changes in bulk composition for giant impacts onto differentiated bodies of any composition (water rock or rock iron) are described by the same equations. These general laws can be incorporated into future N-body calculations of planet formation to track changes in composition from giant impacts.

L50

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In early 2008, the young low-mass star EX Lupi, the prototype of the EXor class of eruptive variables, optically brightened by over 5 mag for a period of seven months. The previous time a change of such amplitude had been observed in EX Lup was over 50 years ago. In this Letter, we present new optical and near-IR high-resolution spectroscopy of EX Lup during the 2008 outburst. We investigate the physical characteristics of the outburst both soon after it began and some four months later, and consider the observed energetics and kinematics. Emission line strengths, widths, and profiles significantly changed between the two observations. Also, modeling of the 2.2935 μm CO overtone band head emission suggests that an inner gap in the circumstellar gas disk around the star may be present and that it is from the inner edge of the gas disk that the CO overtone emission probably arises. We derive a mass accretion luminosity and rate during the extreme outburst of ∼2 ± 0.5 L and ∼(2 ± 0.5) × 10−7M yr−1, respectively, which suggests that this outburst was indeed one of the strongest witnessed in EX Lup, yet not as strong as those observed in FU Orionis stars.

L56

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We present a striking filament pattern in the nonlinear force-free (NLFF) chromospheric magnetic field of the active region NOAA 10956. The NLFF chromospheric field is extrapolated from the Hinode high-resolution photospheric vector magnetogram using the weighted optimization method. The modeled structure is characterized by a highly sheared field with strong horizontal magnetic components and has a virtually identical shape and location as the filament seen in Hα. The modeled field strength agrees with the recent He i 10830 Å observations by Kuckein et al.. The unequivocal resemblance between the NLFF extrapolation and the Hα observation not only demonstrates the ability of the NLFF field to reproduce chromospheric features, but also provides a valuable diagnostic tool for the filament magnetic fields.

L60

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The Galactic globular cluster ω Centauri is a prime candidate for hosting an intermediate-mass black hole. Recent measurements lead to contradictory conclusions on this issue. We use VLT-FLAMES to obtain new integrated spectra for the central region of ω Centauri. We combine these data with existing measurements of the radial velocity dispersion profile taking into account a new derived center from kinematics and two different centers from the literature. The data support previous measurements performed for a smaller field of view and show a discrepancy with the results from a large proper motion data set. We see a rise in the radial velocity dispersion in the central region to 22.8 ± 1.2 km s−1, which provides a strong sign for a central black hole. Isotropic dynamical models for ω Centauri imply black hole masses ranging from 3.0 × 104 to 5.2 × 104M depending on the center. The best-fitted mass is (4.7 ± 1.0) × 104M.

L65

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BP Psc is a remarkable emission-line field star that is orbited by a dusty disk and drives a parsec-scale system of jets. We report the detection by the Chandra X-ray Observatory of a weak X-ray point source coincident with the centroids of optical/IR and submillimeter continuum emission at BP Psc. As the star's photosphere is obscured throughout the visible and near-infrared, the Chandra X-ray source likely represents the first detection of BP Psc itself. The X-rays most likely originate with magnetic activity at BP Psc and hence can be attributed either to a stellar corona or to star–disk interactions. The log of the ratio of X-ray to bolometric luminosity, log(LX/Lbol), lies in the range −5.8 to −4.2. This is smaller than log(LX/Lbol) ratios typical of low-mass, pre-main sequence stars, but is well within the log(LX/Lbol) range observed for rapidly rotating (FK Com-type) G giant stars. Hence, the Chandra results favor an exotic model wherein the disk/jet system of BP Psc is the result of its very recently engulfing a companion star or a giant planet, as the primary star ascended the giant branch.

L69

, , , , , , , , , et al

We report the discovery of very high energy (VHE) gamma-ray emission from the direction of the SNR G54.1+0.3 using the VERITAS ground-based gamma-ray observatory. The TeV signal has an overall significance of 6.8σ and appears pointlike given the resolution of the instrument. The integral flux above 1 TeV is 2.5% of the Crab Nebula flux and significant emission is measured between 250 GeV and 4 TeV, well described by a power-law energy spectrum dN/dE ∼ E−Γ with a photon index Γ = 2.39 ± 0.23stat ± 0.30sys. We find no evidence of time variability among observations spanning almost two years. Based on the location, the morphology, the measured spectrum, the lack of variability, and a comparison with similar systems previously detected in the TeV band, the most likely counterpart of this new VHE gamma-ray source is the pulsar wind nebula (PWN) in the SNR G54.1+0.3. The measured X-ray to VHE gamma-ray luminosity ratio is the lowest among all the nebulae supposedly driven by young rotation-powered pulsars, which could indicate a particle-dominated PWN.

L74

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Cold fronts (CFs)—density and temperature plasma discontinuities—are ubiquitous in cool cores of galaxy clusters, where they appear as X-ray brightness edges in the intracluster medium, nearly concentric with the cluster center. We analyze the thermodynamic profiles deprojected across core CFs found in the literature. While the pressure appears continuous across these CFs, we find that all of them require significant centripetal acceleration beneath the front. This is naturally explained by a tangential, nearly sonic bulk flow just below the CF, and a tangential shear flow involving a fair fraction of the plasma beneath the front. Such shear should generate near-equipartition magnetic fields on scales ≲50pc from the front and could magnetize the entire core. Such fields would explain the apparent stability of cool core CFs and the recently reported CF–radio minihalo association.

L79

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In black hole (BH) X-ray binaries (XRBs), a misalignment between the spin axis of the BH and the orbital angular momentum can occur during the supernova explosion that forms the compact object. In this Letter, we present population synthesis (PS) models of Galactic BH XRBs and study the probability density function of the misalignment angle and its dependence on our model parameters. In our modeling, we also take into account the evolution of the misalignment angle due to accretion of material onto the BH during the XRB phase. The major factor that sets the misalignment angle for XRBs is the natal kick that the BH may receive at its formation. However, large kicks tend to disrupt binaries, while small kicks allow the formation of XRBs and naturally select systems with small misalignment angles. Our calculations predict that the majority (>67%) of Galactic field BH XRBs have rather small (≲10°) misalignment angles, while some systems may reach misalignment angles as high as ∼90° and even higher. These results are robust among all PS models. The assumption of small misalignment angles is extensively used to observationally estimate BH spin magnitudes, and for the first time we are able to confirm this assumption using detailed PS calculations.

L84

, , , , , , , , , et al

We report the detection of 15 X-ray bursts with RXTE and Swift observations of the peculiar X-ray binary Circinus X-1 (Cir X-1) during its 2010 May X-ray re-brightening. These are the first X-ray bursts observed from the source after the initial discovery by Tennant and collaborators, 25 years ago. By studying their spectral evolution, we firmly identify nine of the bursts as type I (thermonuclear) X-ray bursts. We obtain an arcsecond location of the bursts that confirms once and for all the identification of Cir X-1 as a type I X-ray burst source, and therefore as a low magnetic field accreting neutron star. The first five bursts observed by RXTE are weak and show approximately symmetric light curves, without detectable signs of cooling along the burst decay. We discuss their possible nature. Finally, we explore a scenario to explain why Cir X-1 shows thermonuclear bursts now but not in the past, when it was extensively observed and accreting at a similar rate.

L90

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We report on the results of the first deep, wide-field, near-infrared methane imaging survey of the ρ Ophiuchi cloud core to search for T dwarfs. Among the 6587 objects detected, 22 were identified as T dwarf candidates. Brown dwarf models indicate that at the age and distance of the ρ Ophiuchi cloud, these T dwarf candidates have masses between 1 and 2 Jupiter masses. If confirmed as genuine T dwarfs, these objects would be the youngest, lowest mass, and lowest gravity free-floating objects ever directly observed. The existence of these candidates suggests that the initial mass function of the ρ Ophiuchi cloud extends well into the regime of planetary mass objects. A large fraction (59% ± 16%) of our T dwarf candidates appear to be surrounded by circumstellar disks, and thus represents the lowest mass objects yet found to harbor circumstellar disks.

L95

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A filament footpoint rooted in an active region (NOAA 11032) was well observed for about 78 minutes with the 1.6 m New Solar Telescope at the Big Bear Solar Observatory on 2009 November 18 in Hα ±0.75 Å. This data set had high cadence (∼15 s) and high spatial resolution (∼0farcs1) and offered a unique opportunity to study filament dynamics. As in previous findings from space observations, several dark intermittent upflows were identified, and they behave in groups at isolated locations along the filament. However, we have two new findings. First, we find that the dark upflows propagating along the filament channel are strongly associated with the intensity oscillations on the solar surface around the filament footpoints. The upflows start at the same time as the peak in the oscillations, illustrating that the upflow velocities are well correlated with the oscillations. Second, the intensity of one of the seven upflows detected in our data set exhibits a clear periodicity when the upflow propagates along the filament. The periods gradually vary from ∼10 to ∼5 minutes. Our results give observational clues on the driving mechanism of the upflows in the filament.

L99

, , , and

NanoSIMS isotopic measurements of Li, Be, and B in individual hibonite grains extracted from the Murchison meteorite revealed that 10B excesses correlate with the 9Be/11B ratios in 26Al-free PLAty hibonite Crystals. From these data, an initial 10Be/9Be = (5.5 ± 1.6) × 10−4 (2σ) and 10B/11B = 0.2508 ± 0.0015 can be inferred. On the other hand, chondritic boron isotopic compositions were found in 26Al-bearing Spinel-HIBonite spherules, most likely due to contamination with normal boron. No 7Li excesses due to 7Be decay were observed. When combined with previously reported data, the new data yield the best defined 10Be/9Be = (5.3 ± 1.0) × 10−4 (2σ) and 10B/11B = 0.2513 ± 0.0012 for PLACs. A comparison of this value and the best constrained 10Be/9Be = (8.8 ± 0.6) × 10−4 in CV Ca–Al-rich inclusions supports a heterogeneous distribution of 10Be and its protosolar irradiation origin. We consider two possible irradiation scenarios that could potentially lead to the observed Li–Be–B isotopic compositions in PLACs. Although in situ irradiation of solids with hibonite chemistry seems to provide the simplest explanation, more high quality data will be needed for quantitatively constraining the irradiation history.

L104

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We report on the analysis of Rossi X-Ray Timing Explorer Proportional Counter Array (PCA) monitoring observations of the γ-ray binary system LS I +61° 303, covering 35 full cycles of its orbital motion. This constitutes the largest continuous X-ray-monitoring data set analyzed to date for this source. Such an extended analysis allows us to report (1) the discovery of variability in the orbital profiles of the X-ray emission, (2) the existence of a few (recent) short flares on top of the overall behavior typical of the source, which, given the PCA field of view, may or may not be associated with LS I +61° 303, and (3) the determination of the orbital periodicity using soft X-ray data alone.