Galaxies

21 pages, 1509 KiB

Open AccessArticle

Discovering New B[e] Supergiants and Candidate Luminous Blue Variables in Nearby Galaxies

by Grigoris Maravelias, Stephan de Wit, Alceste Z. Bonanos, Frank Tramper, Gonzalo Munoz-Sanchez and Evangelia Christodoulou

Galaxies 2023, 11(3), 79; https://doi.org/10.3390/galaxies11030079 - 19 Jun 2023

Cited by 2 | Viewed by 1551

Abstract

Mass loss is one of the key parameters that determine stellar evolution. Despite the progress we have achieved over the last decades we still cannot match the observational derived values with theoretical predictions. Even worse, there are certain phases, such as the B[e] [...] Read more.

Mass loss is one of the key parameters that determine stellar evolution. Despite the progress we have achieved over the last decades we still cannot match the observational derived values with theoretical predictions. Even worse, there are certain phases, such as the B[e] supergiants (B[e]SGs) and the Luminous Blue Variables (LBVs), where significant mass is lost through episodic or outburst activity. This leads to various structures forming around them that permit dust formation, making these objects bright IR sources. The ASSESS project aims to determine the role of episodic mass in the evolution of massive stars, by examining large numbers of cool and hot objects (such as B[e]SGs/LBVs). For this purpose, we initiated a large observation campaign to obtain spectroscopic data for ∼1000 IR-selected sources in 27 nearby galaxies. Within this project we successfully identified seven B[e] supergiants (one candidate) and four Luminous Blue Variables of which six and two, respectively, are new discoveries. We used spectroscopic, photometric, and light curve information to better constrain the nature of the reported objects. We particularly noted the presence of B[e]SGs at metallicity environments as low as 0.14 Z

_{⊙}

. Full article

(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)

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22 pages, 33967 KiB

Open AccessArticle

On the Evolution of, and Hot Gas in, Wind-Blown Bubbles around Massive Stars - Wind Bubbles Are Not Energy-Conserving

by Vikram V. Dwarkadas

Galaxies 2023, 11(3), 78; https://doi.org/10.3390/galaxies11030078 - 19 Jun 2023

Cited by 4 | Viewed by 1590

Abstract

The structure and evolution of wind-blown bubbles (WBBs) around massive stars has primarily been investigated using an energy-conserving model of wind-blown bubbles. While this model is useful in explaining the general properties of the evolution, several problems remain, including inconsistencies between observed wind [...] Read more.

The structure and evolution of wind-blown bubbles (WBBs) around massive stars has primarily been investigated using an energy-conserving model of wind-blown bubbles. While this model is useful in explaining the general properties of the evolution, several problems remain, including inconsistencies between observed wind luminosities and those derived using this formulation. Major difficulties include the low X-ray temperature and X-ray luminosity, compared to the model. In this paper, we re-examine the evolution, dynamics, and kinematics of WBBs around massive stars, using published ionization gasdynamic simulations of wind-blown bubbles. We show that WBBs can cool efficiently due to the presence of various instabilities and turbulence within the bubble. The expansion of WBBs is more consistent with a momentum-conserving solution, rather than an energy-conserving solution. This compares well with the dynamics and kinematics of observed wind bubbles. Despite the cooling of the bubble, the shocked wind temperature is not reduced to the observed values. We argue that the X-ray emission arise mainly from clumps and filaments within the hot shocked wind region, with temperatures just above 10⁶ K. The remainder of the plasma can contribute to a lesser extent. Full article

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Figure 1
Snapshots of the number density (in cm<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>−</mo> <mn>3</mn> </mrow> </msup> </semantics></math>) with time. From the calculation of the evolution of a wind bubble around a 40 <math display="inline"><semantics> <msub> <mi>M</mi> <mo>⊙</mo> </msub> </semantics></math> star (600 × 400 zones), computed using an expanding grid ([<a href="#B30-galaxies-11-00078" class="html-bibr">30</a>,<a href="#B31-galaxies-11-00078" class="html-bibr">31</a>]). Time increases from left to right and top to bottom. The time in years is listed at the top of each panel. The scale shows the log of the number density. The dense swept-up shell, the ionized HII region, and the shocked wind region are all marked in the figure. The top two panels depict the MS phase. In the bottom left one, the RSG wind can be seen expanding near the star. It does not go too far given its low velocity. The bottom right panel depicts the clumps and filaments in the shocked wind region during the W-R phase. Full article ">Figure 2
The evolution of the outer shell radius over time (shown in brown) from the simulation described herein. Overplotted is a cyan curve with Radius ∝ Time<math display="inline"><semantics> <msup> <mrow/> <mrow> <mn>0.48</mn> </mrow> </msup> </semantics></math>. The good fit clearly shows that the radius is not consistent with that predicted by the Weaver et al. [<a href="#B10-galaxies-11-00078" class="html-bibr">10</a>] model, but is consistent with a momentum-conserving bubble. Full article ">Figure 3
Snapshots showing the velocity vectors (in blue) plotted over the density contours (in dark red). The formation of vortices in the hot shocked wind region is clearly seen. Time increases from left to right and is listed in years at the top of each panel. The vector in the top right corner of each panel denotes a velocity of 500 km s<math display="inline"><semantics> <msup> <mrow/> <mrow> <mo>−</mo> <mn>1</mn> </mrow> </msup> </semantics></math>. The panels all correspond to the MS phase; the last one is at the transition between MS and RSG phases. Full article ">Figure 4
Snapshots showing the pressure within the wind-blown bubble in various phases. Note the variations in pressure within the shocked wind region, as opposed to the uniform pressure expected in the Weaver et al. [<a href="#B10-galaxies-11-00078" class="html-bibr">10</a>] model. Time (in years) increases from left to right and is listed at the top of each panel. The first panel corresponds to the MS phase, the second to the RSG phase, and the third to the W-R phase. Full article ">Figure 5
Snapshots showing the temperature of the wind-blown bubble in various phases. Note the variations in temperature in the hot shocked wind, with low temperatures particularly at the sites of instabilities, and mixing of cool ionized material with the hot interior. Time (in years) increases from left to right and is listed at the top of each panel. The first panel corresponds to the MS phase, the second to the RSG phase, and the third to the W-R phase. Full article ">Figure 6
Panels showing the ionization fraction within the wind-blown bubble over time. Time (in years) increases from left to right, and is listed at the top of each panel. The first panel corresponds to the MS phase, the second to the RSG phase, and the third to the W-R phase. The ionization fraction decreases in the RSG phase, then steadily increases in the W-R phase. Full article ">Figure 7
Evolution of the X-ray spectra within the W-R phase of the bubble. Time increases from left to right. The absorption column due to the nebular material decreases with time. Adapted from Figure 2 in [<a href="#B30-galaxies-11-00078" class="html-bibr">30</a>]. Full article ">

11 pages, 1186 KiB

Open AccessArticle

Vertical Structure of the Milky Way Disk with Gaia DR3

by Katherine Vieira, Vladimir Korchagin, Giovanni Carraro and Artem Lutsenko

Galaxies 2023, 11(3), 77; https://doi.org/10.3390/galaxies11030077 - 16 Jun 2023

Cited by 4 | Viewed by 1988

Abstract

Using a complete sample of about 330,000 dwarf stars, well measured by Gaia DR3, limited to the galactic north and south solid angles

| b | < 75^{°}

and up to a vertical distance of 2 kpc, we analyze the vertical structure [...] Read more.

Using a complete sample of about 330,000 dwarf stars, well measured by Gaia DR3, limited to the galactic north and south solid angles

| b | < 75^{°}

and up to a vertical distance of 2 kpc, we analyze the vertical structure of the Milky Way stellar disks, based on projected tangential velocities. From selected subsamples dominated by their corresponding population, we obtain the thin and thick disk scale heights as

h_{Z} = 279.76 \pm 12.49

pc and

H_{Z} = 797.23 \pm 12.34

pc, respectively. Then from the simultaneous fitting of the sum of two populations over the whole sample, assuming these scale heights, we estimate the thick-to-thin disk number density ratio at the galactic plane to be

ρ_{T} / ρ_{t} = 0.750 \pm 0.049

, which is consistent with a previous result by the authors: in the galactic plane there is a significant number of thick disk stars, possibly as many as thin disk ones, which also points to the existence of more thick disk stars than generally thought. The overall fit does not closely follow the data for

| Z | > 700

pc and points to the presence of more stars beyond the thin disk that cannot be accounted for by the two-disk model. Full article

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23 pages, 968 KiB

Open AccessArticle

Dense Molecular Environments of B[e] Supergiants and Yellow Hypergiants

by Michaela Kraus, Michalis Kourniotis, María Laura Arias, Andrea F. Torres and Dieter H. Nickeler

Galaxies 2023, 11(3), 76; https://doi.org/10.3390/galaxies11030076 - 16 Jun 2023

Cited by 3 | Viewed by 1422

Abstract

Massive stars expel large amounts of mass during their late evolutionary phases. We aim to unveil the physical conditions within the warm molecular environments of B[e] supergiants (B[e]SGs) and yellow hypergiants (YHGs), which are known to be embedded in circumstellar shells and disks. [...] Read more.

Massive stars expel large amounts of mass during their late evolutionary phases. We aim to unveil the physical conditions within the warm molecular environments of B[e] supergiants (B[e]SGs) and yellow hypergiants (YHGs), which are known to be embedded in circumstellar shells and disks. We present K-band spectra of two B[e]SGs from the Large Magellanic Cloud and four Galactic YHGs. The CO band emission detected from the B[e]SGs LHA 120-S 12 and LHA 120-S 134 suggests that these stars are surrounded by stable rotating molecular rings. The spectra of the YHGs display a rather diverse appearance. The objects 6 Cas and V509 Cas lack any molecular features. The star [FMR2006] 15 displays blue-shifted CO bands in emission, which might be explained by a possible close to pole-on oriented bipolar outflow. In contrast, HD 179821 shows blue-shifted CO bands in absorption. While the star itself is too hot to form molecules in its outer atmosphere, we propose that it might have experienced a recent outburst. We speculate that we currently can only see the approaching part of the expelled matter because the star itself might still block the receding parts of a (possibly) expanding gas shell. Full article

(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)

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Figure 1
HR diagram with evolutionary tracks for LMC (<math display="inline"><semantics> <mrow> <mi>Z</mi> <mo>=</mo> <mn>0.006</mn> </mrow> </semantics></math>, [<a href="#B76-galaxies-11-00076" class="html-bibr">76</a>], left panel) and solar (<math display="inline"><semantics> <mrow> <mi>Z</mi> <mo>=</mo> <mn>0.014</mn> </mrow> </semantics></math>, [<a href="#B42-galaxies-11-00076" class="html-bibr">42</a>], right panel) metallicity for models of rotating stars (<math display="inline"><semantics> <mrow> <mi>v</mi> <mo>/</mo> <msub> <mi>v</mi> <mi>crit</mi> </msub> <mo>=</mo> <mn>0.4</mn> </mrow> </semantics></math>) with initial masses from <math display="inline"><semantics> <mrow> <mn>20</mn> <mrow> <mo>−</mo> </mrow> <mn>40</mn> </mrow> </semantics></math> M<math display="inline"><semantics> <msub> <mrow/> <mo>⊙</mo> </msub> </semantics></math>. Shown are the positions of LMC (blue symbols) and Galactic (red symbols) objects. Only B[e]SGs with hot circumstellar molecular CO gas are shown. These populate similar evolutionary tracks as the YHGs. The minimum and maximum temperature values (where known) of the YHGs are connected by dashed lines. YHGs with reported (at least once) CO band emission are shown with triangles. The stars of the current study are labeled. Full article ">Figure 2
Best fitting model (red) to the normalized Phoenix spectra (black) of LHA 120-S 12 (top) and LHA 120-S 134 (bottom). For each star we display the entire fit to the total spectrum (top panels) and the zoom to the band heads (bottom panels). Emission from the Pfund series (blue dashed line), detected in the spectrum of LHA 120-S 134, is included in the total fit. Full article ">Figure 3
Normalized medium-resolution K-band spectra of the YHGs taken with GNIRS. For better visualization, the spectra have been offset along the flux axis. Positions of the CO band heads and of the lines from the Pfund series are marked by ticks. The lines of Br<math display="inline"><semantics> <mi>γ</mi> </semantics></math> and of the Na i doublet are labeled as well. Full article ">Figure 4
Best fitting model (red) to the observed (black) K-band spectrum of [FMR2006] 15 for the model including a rotational component (top) and a pure Gaussian broadening (bottom). Full article ">Figure 5
Residual spectrum of [FMR2006] 15 after subtraction of the blue-shifted CO band emission. For illustration purposes, a synthetic model spectrum of a cool supergiant is shown as well (shifted down along the flux axis for better visualization) with parameters similar to those determined for [FMR2006] 15. Full article ">Figure 6
Comparison of the K-band spectrum of HD 179821 (top) with synthetic spectra for effective temperatures of 5400 K (middle) and 6600 K (bottom). For illustration purposes, the synthetic model spectra are included in this figure and shifted down along the flux axis for better visualization. Full article ">

31 pages, 2008 KiB

Open AccessReview

A Review of the Mixing Length Theory of Convection in 1D Stellar Modeling

by Meridith Joyce and Jamie Tayar

Galaxies 2023, 11(3), 75; https://doi.org/10.3390/galaxies11030075 - 16 Jun 2023

Cited by 36 | Viewed by 3070

Abstract

We review the application of the one-dimensional Mixing Length Theory (MLT) model of convection in stellar interiors and low-mass stellar evolution. We summarize the history of MLT, present a derivation of MLT in the context of 1D stellar structure equations, and discuss the [...] Read more.

We review the application of the one-dimensional Mixing Length Theory (MLT) model of convection in stellar interiors and low-mass stellar evolution. We summarize the history of MLT, present a derivation of MLT in the context of 1D stellar structure equations, and discuss the physical regimes in which MLT is relevant. We review attempts to improve and extend the formalism, including to higher dimensions. We discuss the interactions of MLT with other modeling physics, and demonstrate the impact of introducing variations in the convective mixing length,

α_{MLT}

, on stellar tracks and isochrones. We summarize the process of performing a solar calibration of

α_{MLT}

and state-of-the-art on calibrations to non-solar targets. We discuss the scientific implications of changing the mixing length, using recent analyses for demonstration. We review the most prominent successes of MLT, and the remaining challenges, and we conclude by speculating on the future of this treatment of convection. Full article

(This article belongs to the Special Issue The Structure and Evolution of Stars)

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43 pages, 7503 KiB

Open AccessReview

The Dynamics and Energetics of Remnant and Restarting RLAGN

by Vijay H. Mahatma

Galaxies 2023, 11(3), 74; https://doi.org/10.3390/galaxies11030074 - 13 Jun 2023

Cited by 3 | Viewed by 1606

Abstract

In this article, I review past, current, and future advances on the study of radio-loud AGN (RLAGN; radio-loud quasars and radio galaxies) lifecycles exclusively in the remnant and restarting phases. I focus on their dynamics and energetics as inferred from radio observations while [...] Read more.

In this article, I review past, current, and future advances on the study of radio-loud AGN (RLAGN; radio-loud quasars and radio galaxies) lifecycles exclusively in the remnant and restarting phases. I focus on their dynamics and energetics as inferred from radio observations while discussing their radiative lifetimes, population statistics, and trends in their physical characteristics. I briefly summarise multi-wavelength observations, particularly X-rays, that have enabled studies of the large-scale environments of RLAGN in order to understand their role in feedback. Furthermore, I discuss analytic and numerical simulations that predict key properties of remnant and restarting sources as found in wide-area surveys, and discuss the prospects of future surveys that may shed further light on these elusive subpopulations of RLAGN. Full article

(This article belongs to the Special Issue New Perspectives on Radio Galaxy Dynamics)

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46 pages, 5709 KiB

Open AccessReview

Recent Progress in Modeling the Macro- and Micro-Physics of Radio Jet Feedback in Galaxy Clusters

by Martin A. Bourne and Hsiang-Yi Karen Yang

Galaxies 2023, 11(3), 73; https://doi.org/10.3390/galaxies11030073 - 13 Jun 2023

Cited by 15 | Viewed by 2048

Abstract

Radio jets and the lobes they inflate are common in cool-core clusters and are known to play a critical role in regulating the heating and cooling of the intracluster medium (ICM). This is an inherently multi-scale problem, and much effort has been made [...] Read more.

Radio jets and the lobes they inflate are common in cool-core clusters and are known to play a critical role in regulating the heating and cooling of the intracluster medium (ICM). This is an inherently multi-scale problem, and much effort has been made to understand the processes governing the inflation of lobes and their impact on the cluster, as well as the impact of the environment on the jet–ICM interaction, on both macro- and microphysical scales. The developments of new numerical techniques and improving computational resources have seen simulations of jet feedback in galaxy clusters become ever more sophisticated. This ranges from modeling ICM plasma physics processes such as the effects of magnetic fields, cosmic rays, and viscosity to including jet feedback in cosmologically evolved cluster environments in which the ICM thermal and dynamic properties are shaped by large-scale structure formation. In this review, we discuss the progress made over the last ∼decade in capturing both the macro- and microphysical processes in numerical simulations, highlighting both the current state of the field, as well as the open questions and potential ways in which these questions can be addressed in the future. Full article

(This article belongs to the Special Issue New Perspectives on Radio Galaxy Dynamics)

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20 pages, 936 KiB

Open AccessArticle

New Insight into the FS CMa System MWC 645 from Near-Infrared and Optical Spectroscopy

by Andrea Fabiana Torres, María Laura Arias, Michaela Kraus, Lorena Verónica Mercanti and Tõnis Eenmäe

Galaxies 2023, 11(3), 72; https://doi.org/10.3390/galaxies11030072 - 10 Jun 2023

Viewed by 1427

Abstract

The B[e] phenomenon is manifested by a heterogeneous group of stars surrounded by gaseous and dusty circumstellar envelopes with similar physical conditions. Among these stars, the FS CMa-type objects are suspected to be binary systems, which could be experiencing or have undergone a [...] Read more.

The B[e] phenomenon is manifested by a heterogeneous group of stars surrounded by gaseous and dusty circumstellar envelopes with similar physical conditions. Among these stars, the FS CMa-type objects are suspected to be binary systems, which could be experiencing or have undergone a mass-transfer process that could explain the large amount of material surrounding them. We aim to contribute to the knowledge of a recently confirmed binary, MWC 645, which could be undergoing an active mass-transfer process. We present near-infrared and optical spectra, identify atomic and molecular spectral features, and derive different quantitative properties of line profiles. Based on publicly available photometric data, we search for periodicity in the light curve and model the spectral energy distribution. We have detected molecular bands of CO in absorption at 1.62

μ

m and 2.3

μ

m for the first time. We derive an upper limit for the effective temperature of the cool binary component. We found a correlation between the enhancement of the H

α

emission and the decrease in optical brightness that could be associated with mass-ejection events or an increase in mass loss. We outline the global properties of the envelope, possibly responsible for brightness variations due to a variable extinction, and briefly speculate on different possible scenarios. Full article

(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)

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12 pages, 338 KiB

Open AccessArticle

Investigating Gravitationally Lensed Quasars Observable by Nancy Grace Roman Space Telescope

by Lindita Hamolli, Mimoza Hafizi, Francesco De Paolis and Esmeralda Guliqani

Galaxies 2023, 11(3), 71; https://doi.org/10.3390/galaxies11030071 - 1 Jun 2023

Viewed by 1399

Abstract

In this work, we investigate the possibility of observing quasars, particularly lensed quasars, by the Nancy Grace Roman Space Telescope (Roman). To this aim, based on the capabilities of the Roman Space Telescope and the results from the quasar luminosity function (QLF) in [...] Read more.

In this work, we investigate the possibility of observing quasars, particularly lensed quasars, by the Nancy Grace Roman Space Telescope (Roman). To this aim, based on the capabilities of the Roman Space Telescope and the results from the quasar luminosity function (QLF) in the infrared band of the Spitzer Space Telescope imaging survey, we calculated the number of quasars expected to be in its field of view. In order to estimate the number of lensed quasars, we develop a Monte Carlo simulation to estimate the probability that a quasar is lensed once or more times by foreground galaxies. Using the mass–luminosity distribution function of galaxies and the redshift distributions of galaxies and quasars, we find that 1 per 180 observed quasars will be lensed by foreground galaxies. Further on, adopting a singular isothermal sphere (SIS) model for lens galaxies, we calculate the time delay between lensed images for single and multiple lensing systems and present their distributions. We emphasize that detailed studies of these lensing systems will provide a powerful probe of the physical properties of quasars and may allow testing the mass distribution models of galaxies in addition to being extremely helpful for constraining the cosmological parameters. Full article

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13 pages, 627 KiB

Open AccessArticle

Hawking Radiation and Lifetime of Primordial Black Holes in Braneworld

by Bobur Turimov, Akhror Mamadjanov and Ozodbek Rahimov

Galaxies 2023, 11(3), 70; https://doi.org/10.3390/galaxies11030070 - 31 May 2023

Cited by 3 | Viewed by 1639

Abstract

The paper explores the thermodynamic properties of primordial black holes (PBHs) in the braneworld. Specifically, the researchers examined Hawking radiation and the lifetime of PBHs. Through their analysis, an exact analytical expression for the Bekenstein–Hawking entropy, temperature, and heat capacity was derived. Their [...] Read more.

The paper explores the thermodynamic properties of primordial black holes (PBHs) in the braneworld. Specifically, the researchers examined Hawking radiation and the lifetime of PBHs. Through their analysis, an exact analytical expression for the Bekenstein–Hawking entropy, temperature, and heat capacity was derived. Their findings suggest that the lifetime of PBHs in the early universe is reduced by at least one order of magnitude, ultimately leading to their evaporation. This could explain why we have not observed the final rapid evaporation of PBHs in the recent epoch of the universe. Full article

(This article belongs to the Special Issue The 10th Anniversary of Galaxies: The Physics of Black Holes and Gravitational Waves)

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40 pages, 90706 KiB

Open AccessArticle

Synthetic Light Curve Design for Pulsating Binary Stars to Compare the Efficiency in the Detection of Periodicities

by Aldana Alberici Adam, Gunther F. Avila Marín, Alejandra Christen and Lydia Sonia Cidale

Galaxies 2023, 11(3), 69; https://doi.org/10.3390/galaxies11030069 - 31 May 2023

Cited by 1 | Viewed by 1275

Abstract

B supergiant stars pulsate in regular and quasi-regular oscillations resulting in intricate light variations that might conceal their binary nature. To discuss possible observational bias in a light curve, we performed a simulation design of a binary star affected by sinusoidal functions emulating [...] Read more.

B supergiant stars pulsate in regular and quasi-regular oscillations resulting in intricate light variations that might conceal their binary nature. To discuss possible observational bias in a light curve, we performed a simulation design of a binary star affected by sinusoidal functions emulating pulsation phenomena. The Period04 tool and the WaveletComp package of R were used for this purpose. Thirty-two models were analysed based on a combination of two values on each of the k = 6 variables, such as multiple pulsations, the amplitude of the pulsation, the pulsation frequency, the beating phenomenon, the light-time effect, and regular or quasi-regular periods. These synthetic models, unlike others, consider an ARMA (1, 1) statistical noise, irregular sampling, and a gap of about 4 days. Comparing Morlet wavelet with Fourier methods, we observed that the orbital period and its harmonics were well detected in most cases. Although the Fourier method provided more accurate period detection, the wavelet analysis found it more times. Periods seen with the wavelet method have a shift due to the slightly irregular time scale used. The pulsation period hitting rate depends on the wave amplitude and frequency with respect to eclipse depth and orbital period. None of the methods was able to distinguish accurate periods leading to a beating phenomenon when they were longer than the orbital period, resulting, in both cases, in an intermediate value. When the beating period was shorter, the Fourier analysis found it in all cases except for unsolved quasi-regular periods. Overall, the Morlet wavelet analysis performance was lower than the Fourier analysis. Considering the strengths and disadvantages found in these methods, we recommend using at least two diagnosis tools for a detailed time series data analysis to obtain confident results. Moreover, a fine-tuning of trial periods by applying phase diagrams would be helpful for recovering accurate values. The combined analysis could reduce observational bias in searching binaries using photometric techniques. Full article

(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)

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33 pages, 2909 KiB

Open AccessReview

Radiation-Driven Wind Hydrodynamics of Massive Stars: A Review

by Michel Curé and Ignacio Araya

Galaxies 2023, 11(3), 68; https://doi.org/10.3390/galaxies11030068 - 12 May 2023

Cited by 6 | Viewed by 2190

Abstract

Mass loss from massive stars plays a determining role in their evolution through the upper Hertzsprung–Russell diagram. The hydrodynamic theory that describes their steady-state winds is the line-driven wind theory (m-CAK). From this theory, the mass loss rate and the velocity profile of [...] Read more.

Mass loss from massive stars plays a determining role in their evolution through the upper Hertzsprung–Russell diagram. The hydrodynamic theory that describes their steady-state winds is the line-driven wind theory (m-CAK). From this theory, the mass loss rate and the velocity profile of the wind can be derived, and estimating these properly will have a profound impact on quantitative spectroscopy analyses from the spectra of these objects. Currently, the so-called

β

law, which is an approximation for the fast solution, is widely used instead of m-CAK hydrodynamics, and when the derived value is

β ≳ 1.2

, there is no hydrodynamic justification for these values. This review focuses on (1) a detailed topological analysis of the equation of motion (EoM), (2) solving the EoM numerically for all three different (fast and two slow) wind solutions, (3) deriving analytical approximations for the velocity profile via the LambertW function and (4) presenting a discussion of the applicability of the slow solutions. Full article

(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)

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20 pages, 930 KiB

Open AccessReview

Wide-Angle-Tail (WAT) Radio Sources

by Christopher P. O’Dea and Stefi A. Baum

Galaxies 2023, 11(3), 67; https://doi.org/10.3390/galaxies11030067 - 12 May 2023

Cited by 14 | Viewed by 2750

Abstract

We review the properties of Wide-Angle-Tail (WAT) radio sources. The WAT radio sources are powerful, bent radio sources typically associated with the dominant galaxy in a cluster or group. For the purpose of this review, we define the radio morphology properties of WATs [...] Read more.

We review the properties of Wide-Angle-Tail (WAT) radio sources. The WAT radio sources are powerful, bent radio sources typically associated with the dominant galaxy in a cluster or group. For the purpose of this review, we define the radio morphology properties of WATs as (1) a sudden jet-tail transition, (2) overall bending of the tails to one side, and (3) non-parallel tails. The mechanism for the rapid jet-tail transition is uncertain but it seems to occur near the transition from the host ISM to ICM. The jet-tail transition may make the jets easier to bend. The narrow range in radio luminosity can be understood if there is a minimum luminosity required to allow the jets to propagate undisturbed for tens of kpc and a maximum luminosity required to allow the jet disruption mechanism to act. WATs are typically hosted by the brightest cluster galaxies in clusters which are currently merging. Thus, WATs can be used as tracers of merging clusters. The merging produces large-scale bulk motions in the ICM which can provide sufficient ram pressure to bend the jets. We suggest that although the Lorentz force may not bend the jets in WATs, it may be relevant in other sources, e.g., protostellar jets. Full article

(This article belongs to the Special Issue New Perspectives on Radio Galaxy Dynamics)

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8 pages, 904 KiB

Open AccessCommunication

Advanced Life Peaked Billions of Years Ago According to Black Holes

by David Garofalo

Galaxies 2023, 11(3), 66; https://doi.org/10.3390/galaxies11030066 - 11 May 2023

Cited by 1 | Viewed by 2183

Abstract

The link between black holes and star formation allows for us to draw a connection between black holes and the places and times when extraterrestrial intelligences (ETIs) had a greater chance of emerging. Within the context of the gap paradigm for black holes, [...] Read more.

The link between black holes and star formation allows for us to draw a connection between black holes and the places and times when extraterrestrial intelligences (ETIs) had a greater chance of emerging. Within the context of the gap paradigm for black holes, we show that denser cluster environments that led to gas-rich mergers and copious star formation were places less compatible on average with the emergence of ETIs compared to isolated elliptical galaxies by almost two orders of magnitude. The probability for ETIs peaked in these isolated environments around 6 billion years ago and cosmic downsizing shifted the likelihood of ETIs emerging to galaxies with weak black hole feedback, such as in spiral galaxies, at late times. Full article

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17 pages, 1987 KiB

Open AccessArticle

Faint Galaxy Number Counts in the Durham and SDSS Catalogues

by John H. Marr

Galaxies 2023, 11(3), 65; https://doi.org/10.3390/galaxies11030065 - 7 May 2023

Cited by 1 | Viewed by 1840

Abstract

Galaxy number counts in the K-, H-, I-, R-, B- and U-bands from the Durham Extragalactic Astronomy and Cosmology catalogue could be well-fitted over their whole range using luminosity function (LF) parameters derived from the SDSS at [...] Read more.

Galaxy number counts in the K-, H-, I-, R-, B- and U-bands from the Durham Extragalactic Astronomy and Cosmology catalogue could be well-fitted over their whole range using luminosity function (LF) parameters derived from the SDSS at the bright region and required only modest luminosity evolution with the steepening of the LF slope (

α

), except for a sudden steep increase in the B-band and a less steep increase in the U-band at faint magnitudes that required a starburst evolutionary model to account for the excess faint number counts. A cosmological model treating Hubble expansion as an Einstein curvature required less correction at faint magnitudes than a standard

Λ

CDM model, without requiring dark matter or dark energy. Data from DR17 of the SDSS in the g, i, r, u and z bands over two areas of the sky centred on the North Galactic Cap (NGC) and above the South Galactic Cap (SGC), with areas of 5954 and 859 sq. deg., respectively, and a combined count of 622,121 galaxies, were used to construct bright galaxy number counts and galaxy redshift/density plots within the limits of redshift

\leq 0.4

and mag

\leq 20

. Their comparative densities confirmed an extensive void in the Southern sky with a deficit of 26% out to a redshift z ≤ 0.15. Although not included in the number count data set because of its incompleteness at fainter magnitudes, extending the SDSS redshift-number count survey to fainter and more distant galaxies with redshift ≤ 1.20 showed a secondary peak in the number counts with many QSOs, bright X-ray and radio sources, and evolving irregular galaxies with rapid star formation rates. This sub-population at redshifts of 0.45–0.65 may account for the excess counts observed in the B-band. Recent observations from the HST and James Webb Space Telescope (JWST) have also begun to reveal a high density of massive galaxies at high redshifts (

z > 7

) with high UV and X-ray emissions, and future observations by the JWST may reveal the assembly of galaxies in the early universe going back to the first light in the universe. Full article

(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)

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14 pages, 37354 KiB

Open AccessArticle

Large-Scale Ejecta of Z CMa—Proper Motion Study and New Features Discovered

by Tiina Liimets, Michaela Kraus, Lydia Cidale, Sergey Karpov and Anthony Marston

Galaxies 2023, 11(3), 64; https://doi.org/10.3390/galaxies11030064 - 4 May 2023

Cited by 1 | Viewed by 1425

Abstract

Z Canis Majoris is a fascinating early-type binary with a Herbig Be primary and a FU Orionis-type secondary. Both of the stars exhibit sub-arcsecond jet-like ejecta. In addition, the primary is associated with the extended jet as well as with the large-scale outflow. [...] Read more.

Z Canis Majoris is a fascinating early-type binary with a Herbig Be primary and a FU Orionis-type secondary. Both of the stars exhibit sub-arcsecond jet-like ejecta. In addition, the primary is associated with the extended jet as well as with the large-scale outflow. In this study, we investigate further the nature of the large-scale outflow, which has not been studied since its discovery almost three and a half decades ago. We present proper motion measurements of individual features of the large-scale outflow and determine their kinematical ages. Furthermore, with our newly acquired deep images, we have discovered additional faint arc-shaped features that can be associated with the central binary. Full article

(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)

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Figure 1
(Left): Schematic view of the various extended nebular features surrounding Z CMa. The stars in the field of view are drawn as black filled circles, large-scale outflow features are marked with grayish areas or circles with black contures. The dashed line is drawn at the 60° position angle. Individual numbers refer to radial velocities (Poetzel et al. [<a href="#B5-galaxies-11-00064" class="html-bibr">5</a>]). Field of view is <math display="inline"><semantics> <mrow> <msup> <mn>9</mn> <mo>′</mo> </msup> <mo>.</mo> <mn>5</mn> <mo>×</mo> <msup> <mn>5</mn> <mo>′</mo> </msup> <mo>.</mo> <mn>6</mn> </mrow> </semantics></math>. The base of the figure is depicted from Poetzel et al. [<a href="#B5-galaxies-11-00064" class="html-bibr">5</a>]. Reproduced with permission © ESO. (Right): Schematic view of the sub-arcsecond features around Z CMa presented in true proportions. The lengths of the micro-jets are taken from Whelan et al. [<a href="#B6-galaxies-11-00064" class="html-bibr">6</a>] and those for the streamer are taken from Dong et al. [<a href="#B2-galaxies-11-00064" class="html-bibr">2</a>]. The dashed line represents the position angle of the large-scale outflow. Field of view is <math display="inline"><semantics> <mrow> <msup> <mn>1</mn> <mrow> <mo>″</mo> </mrow> </msup> <mo>.</mo> <mn>6</mn> <mo>×</mo> <msup> <mn>2</mn> <mrow> <mo>″</mo> </mrow> </msup> <mo>.</mo> <mn>7</mn> </mrow> </semantics></math>. A white square representing the same size is drawn on the left panel at the position of the central binary inside the comma nebula. On both panels, north is up and east is to the left. Full article ">Figure 2
(Left): [S ii] image of Z CMa acquired with GMOS attached to Gemini-South. The white square shape feature, indicated with a white arrow, is an artifact from vignetting of the guiding probe. (Right): Insets of the resolved features in the [S ii] image. The FOVs of the smallest insets are <math display="inline"><semantics> <mrow> <msup> <mn>20</mn> <mrow> <mo>″</mo> </mrow> </msup> <mo>×</mo> <msup> <mn>15</mn> <mrow> <mo>″</mo> </mrow> </msup> </mrow> </semantics></math> each. On all images, north is up, east is to the left, and the intensity is in log scale to improve the contrast. See text for more details. Full article ">Figure 3
(Left): H<math display="inline"><semantics> <mi>α</mi> </semantics></math> image of Z CMa taken in 2002. GMOS FOV is shown for comparison. (Right): Insets of the features resolvable in the H<math display="inline"><semantics> <mi>α</mi> </semantics></math> image and which are outside GMOS FOV. On all images, north is up, east is to the left, and the intensity is in log scale to improve the contrast. See text for more details. Full article ">Figure 4
Pan-STARRS RGB image presenting the faint extended features around Z CMa. Red channel corresponds to z filter, green channel corresponds to i, and blue channel corresponds to g filter. The intensity is in log scale to improve the contrast. North is up and east is to the left. FOV <math display="inline"><semantics> <mrow> <msup> <mn>9</mn> <mo>′</mo> </msup> <mo>×</mo> <msup> <mn>9</mn> <mo>′</mo> </msup> </mrow> </semantics></math>. See text for more details. Full article ">

16 pages, 433 KiB

Open AccessArticle

Language Models for Multimessenger Astronomy

by Vladimir Sotnikov and Anastasiia Chaikova

Galaxies 2023, 11(3), 63; https://doi.org/10.3390/galaxies11030063 - 1 May 2023

Cited by 4 | Viewed by 3493

Abstract

With the increasing reliance of astronomy on multi-instrument and multi-messenger observations for detecting transient phenomena, communication among astronomers has become more critical. Apart from automatic prompt follow-up observations, short reports, e.g., GCN circulars and ATels, provide essential human-written interpretations and discussions of observations. [...] Read more.

With the increasing reliance of astronomy on multi-instrument and multi-messenger observations for detecting transient phenomena, communication among astronomers has become more critical. Apart from automatic prompt follow-up observations, short reports, e.g., GCN circulars and ATels, provide essential human-written interpretations and discussions of observations. These reports lack a defined format, unlike machine-readable messages, making it challenging to associate phenomena with specific objects or coordinates in the sky. This paper examines the use of large language models (LLMs)—machine learning models with billions of trainable parameters or more that are trained on text—such as InstructGPT-3 and open-source Flan-T5-XXL for extracting information from astronomical reports. The study investigates the zero-shot and few-shot learning capabilities of LLMs and demonstrates various techniques to improve the accuracy of predictions. The study shows the importance of careful prompt engineering while working with LLMs, as demonstrated through edge case examples. The study’s findings have significant implications for the development of data-driven applications for astrophysical text analysis. Full article

(This article belongs to the Special Issue The New Era of Real-Time Multi-Messenger Astronomy)

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20 pages, 2221 KiB

Open AccessReview

Opacities and Atomic Diffusion

by Georges Alecian and Morgan Deal

Galaxies 2023, 11(3), 62; https://doi.org/10.3390/galaxies11030062 - 25 Apr 2023

Cited by 1 | Viewed by 1819

Abstract

Opacity is a fundamental quantity for stellar modeling, and it plays an essential role throughout the life of stars. After gravity drives the collapse of interstellar matter into a protostar, the opacity determines how this matter is structured around the stellar core. The [...] Read more.

Opacity is a fundamental quantity for stellar modeling, and it plays an essential role throughout the life of stars. After gravity drives the collapse of interstellar matter into a protostar, the opacity determines how this matter is structured around the stellar core. The opacity explains how the radiation field interacts with the matter and how a major part of the energy flows through the star. It results from all the microscopic interactions of photons with atoms. Part of the momentum exchange between photons and atoms gives rise to radiative accelerations (specific to each type of atom), which are strongly involved in a second-order process: atomic diffusion. Although this process is a slow one, it can have a significant impact on stellar structure and chemical composition measurements. In this review, we discuss the way opacities are presently computed and used in numerical codes. Atomic diffusion is described, and the current status of the consideration of this process is presented. Full article

(This article belongs to the Special Issue The Structure and Evolution of Stars)

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38 pages, 9925 KiB

Open AccessArticle

Key Science Goals for the Next-Generation Event Horizon Telescope

by Michael D. Johnson, Kazunori Akiyama, Lindy Blackburn, Katherine L. Bouman, Avery E. Broderick, Vitor Cardoso, Rob P. Fender, Christian M. Fromm, Peter Galison, José L. Gómez, Daryl Haggard, Matthew L. Lister, Andrei P. Lobanov, Sera Markoff, Ramesh Narayan, Priyamvada Natarajan, Tiffany Nichols, Dominic W. Pesce, Ziri Younsi, Andrew Chael, Koushik Chatterjee, Ryan Chaves, Juliusz Doboszewski, Richard Dodson, Sheperd S. Doeleman, Jamee Elder, Garret Fitzpatrick, Kari Haworth, Janice Houston, Sara Issaoun, Yuri Y. Kovalev, Aviad Levis, Rocco Lico, Alexandru Marcoci, Niels C. M. Martens, Neil M. Nagar, Aaron Oppenheimer, Daniel C. M. Palumbo, Angelo Ricarte, María J. Rioja, Freek Roelofs, Ann C. Thresher, Paul Tiede, Jonathan Weintroub and Maciek Wielgus Show full author list Hide full author list

Galaxies 2023, 11(3), 61; https://doi.org/10.3390/galaxies11030061 - 24 Apr 2023

Cited by 55 | Viewed by 6653

Abstract

The Event Horizon Telescope (EHT) has led to the first images of a supermassive black hole, revealing the central compact objects in the elliptical galaxy M87 and the Milky Way. Proposed upgrades to this array through the next-generation EHT (ngEHT) program would sharply [...] Read more.

The Event Horizon Telescope (EHT) has led to the first images of a supermassive black hole, revealing the central compact objects in the elliptical galaxy M87 and the Milky Way. Proposed upgrades to this array through the next-generation EHT (ngEHT) program would sharply improve the angular resolution, dynamic range, and temporal coverage of the existing EHT observations. These improvements will uniquely enable a wealth of transformative new discoveries related to black hole science, extending from event-horizon-scale studies of strong gravity to studies of explosive transients to the cosmological growth and influence of supermassive black holes. Here, we present the key science goals for the ngEHT and their associated instrument requirements, both of which have been formulated through a multi-year international effort involving hundreds of scientists worldwide. Full article

(This article belongs to the Special Issue From Vision to Instrument: Creating a Next-Generation Event Horizon Telescope for a New Era of Black Hole Science)

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