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Galaxies
Volume 11
Issue 5
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Galaxies, Volume 11, Issue 5 (October 2023) – 16 articles

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In this introductory chapter of the Special Issue entitled ‘The Structure and Evolution of Stars’, we highlight the recent major progress made in our understanding of the physics that governs the inner workings of stars. To accomplish this, we combine insights from observations, one-dimensional models of stellar evolution, as well as two-dimensional and three-dimensional simulations that account for rotation and magnetohydrodynamics. This amalgamation provides a complete and compelling picture of the essential components within contemporary theories of stellar structure while also highlighting areas that demand further refinement. Additionally, we offer an overarching perspective that ties together the central themes explored in the subsequent Chapters. View this paper

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35 pages, 19230 KiB  
Article
Reference Array and Design Consideration for the Next-Generation Event Horizon Telescope
by Sheperd S. Doeleman, John Barrett, Lindy Blackburn, Katherine L. Bouman, Avery E. Broderick, Ryan Chaves, Vincent L. Fish, Garret Fitzpatrick, Mark Freeman, Antonio Fuentes, José L. Gómez, Kari Haworth, Janice Houston, Sara Issaoun, Michael D. Johnson, Mark Kettenis, Laurent Loinard, Neil Nagar, Gopal Narayanan, Aaron Oppenheimer, Daniel C. M. Palumbo, Nimesh Patel, Dominic W. Pesce, Alexander W. Raymond, Freek Roelofs, Ranjani Srinivasan, Paul Tiede, Jonathan Weintroub and Maciek Wielgusadd Show full author list remove Hide full author list
Galaxies 2023, 11(5), 107; https://doi.org/10.3390/galaxies11050107 - 18 Oct 2023
Cited by 29 | Viewed by 3256
Abstract
We describe the process to design, architect, and implement a transformative enhancement of the Event Horizon Telescope (EHT). This program—the next-generation Event Horizon Telescope (ngEHT)—will form a networked global array of radio dishes capable of making high-fidelity real-time movies of supermassive black holes [...] Read more.
We describe the process to design, architect, and implement a transformative enhancement of the Event Horizon Telescope (EHT). This program—the next-generation Event Horizon Telescope (ngEHT)—will form a networked global array of radio dishes capable of making high-fidelity real-time movies of supermassive black holes (SMBH) and their emanating jets. This builds upon the EHT principally by deploying additional modest-diameter dishes to optimized geographic locations to enhance the current global mm/submm wavelength Very Long Baseline Interferometric (VLBI) array, which has, to date, utilized mostly pre-existing radio telescopes. The ngEHT program further focuses on observing at three frequencies simultaneously for increased sensitivity and Fourier spatial frequency coverage. Here, the concept, science goals, design considerations, station siting, and instrument prototyping are discussed, and a preliminary reference array to be implemented in phases is described. 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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17 pages, 3815 KiB  
Article
A Comprehensive Study of Soft X-ray Absorption Features in GX 13+1 Using XMM-Newton Observations
by Rabindra Mahato and Monmoyuri Baruah
Galaxies 2023, 11(5), 106; https://doi.org/10.3390/galaxies11050106 - 15 Oct 2023
Cited by 1 | Viewed by 1632
Abstract
In this study, a comprehensive analysis of the reflection grating spectrometer (RGS) data (energy range 0.33 keV to 2.5 keV) of GX 13+1 from the XMM-Newton space observatory was conducted. Numerous absorption lines have been described in earlier publications, although the majority of [...] Read more.
In this study, a comprehensive analysis of the reflection grating spectrometer (RGS) data (energy range 0.33 keV to 2.5 keV) of GX 13+1 from the XMM-Newton space observatory was conducted. Numerous absorption lines have been described in earlier publications, although the majority of these investigations focused on the energy range between 3 keV and 10 keV. We considered all ten on-axis observations for spectral analysis, but during timing analysis, off-axis observations were also analysed. A fresh dip in off-axis observation 0122340701(B) was observed together with the previously reported dip in on-axis observation 0505480501(F). No burst or eclipsing was observed in any of the observations. The spectral analysis revealed the presence of the highly ionized elements, Si XIII 1s2–1s2p and 1s2–1s3p transitions with energies of 2.02 keV and 2.24 keV, respectively, as well as Mg XII 1s–6p transitions with energies of 1.91 keV and Al XIII 1s–4p transitions with energies of 2.15 keV. Before this study, no analyses of XMM-Newton data reported Mg XII and Al XIII elements. Moreover, the 1s–6p transitions of Mg XII and 1s–4p transitions of Al XIII were not even reported in any Chandra data analysis. The equivalent hydrogen column densities obtained (2.35 × 1022 cm−2 to 4.18 × 1022 cm−2) are consistent with previously reported values. The equivalent widths of neighbouring observations are found to be nearly the same. This supports us in suggesting that the absorptions may be due to the presence of an interstellar medium (ISM) in the line of sight (LOS) of the source. Full article
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24 pages, 1727 KiB  
Review
Three Dimensional Natures of Massive Star Envelopes
by Yan-Fei Jiang
Galaxies 2023, 11(5), 105; https://doi.org/10.3390/galaxies11050105 - 11 Oct 2023
Cited by 4 | Viewed by 1705
Abstract
In this paper, we review our current understanding of the outer envelope structures of massive stars based on three-dimensional (3D) radiation hydrodynamic simulations. We briefly summarize the fundamental issues in constructing hydrostatic one-dimensional (1D) stellar evolution models when stellar luminosity approaches the Eddington [...] Read more.
In this paper, we review our current understanding of the outer envelope structures of massive stars based on three-dimensional (3D) radiation hydrodynamic simulations. We briefly summarize the fundamental issues in constructing hydrostatic one-dimensional (1D) stellar evolution models when stellar luminosity approaches the Eddington value. Radiation hydrodynamic simulations in 3D covering the mass range from 13M⊙ to 80M⊙ always find a dynamic envelope structure with the time-averaged radial profiles matching 1D models with an adjusted mixing-length parameter when convection is subsonic. Supersonic turbulence and episodic mass loss are generally found in 3D models when stellar luminosity is super-Eddington locally due to the opacity peaks and convection being inefficient. Turbulent pressure plays an important role in supporting the outer envelope, which makes the photosphere more extended than predictions from 1D models. Massive star lightcurves are always found to vary with a characteristic timescale consistent with the thermal time scale at the location of the iron opacity peak. The amplitude of the variability as well as the power spectrum can explain the commonly observed stochastic low-frequency variability of mass stars observed by TESS over a wide range of parameters in an HR diagram. The 3D simulations can also explain the ubiquitous macro-turbulence that is needed for spectroscopic fitting in massive stars. Implications of 3D simulations for improving 1D stellar evolution models are also discussed. Full article
(This article belongs to the Special Issue The Structure and Evolution of Stars)
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17 pages, 2434 KiB  
Review
Complex Organics in Space: A Changing View of the Cosmos
by Sun Kwok
Galaxies 2023, 11(5), 104; https://doi.org/10.3390/galaxies11050104 - 8 Oct 2023
Cited by 2 | Viewed by 2950
Abstract
Planetary explorations have revealed that complex organics are widely present in the solar system. Astronomical infrared spectroscopic observations have discovered that complex organics are synthesized in large quantities in planetary nebulae and distributed throughout the galaxy. Signatures of organics have been found in [...] Read more.
Planetary explorations have revealed that complex organics are widely present in the solar system. Astronomical infrared spectroscopic observations have discovered that complex organics are synthesized in large quantities in planetary nebulae and distributed throughout the galaxy. Signatures of organics have been found in distant galaxies, as early as 1.5 billion years after the Big Bang. A number of unsolved spectral phenomena such as diffuse interstellar bands, extended red emissions, 220 nm feature, and unidentified infrared emission bands are likely to originate from organics. In this paper, we discuss the possible chemical structures of the carriers of these unexplained phenomena, and how these organics are synthesized abiotically in the universe. We raise the possibility that the primordial solar system was enriched by complex organics synthesized and ejected by evolved stars. The implications of possible stellar organics in primordial Earth are also discussed. Full article
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11 pages, 326 KiB  
Review
Spin Equilibrium of Rapidly Spinning Neutron Stars via Transient Accretion
by Sudip Bhattacharyya
Galaxies 2023, 11(5), 103; https://doi.org/10.3390/galaxies11050103 - 1 Oct 2023
Viewed by 1467
Abstract
The concept of spin equilibrium due to an interaction between the stellar magnetosphere and a thin, Keplerian accretion disk, and a well-known formula of the corresponding equilibrium spin frequency, provide a key understanding of spin evolution and the distribution of rapidly spinning neutron [...] Read more.
The concept of spin equilibrium due to an interaction between the stellar magnetosphere and a thin, Keplerian accretion disk, and a well-known formula of the corresponding equilibrium spin frequency, provide a key understanding of spin evolution and the distribution of rapidly spinning neutron stars, viz., millisecond pulsars. However, this concept and formula are for stable accretion, but the mass transfer to most accreting millisecond pulsars is transient and the accretion rate evolves by orders of magnitude during an outburst. In this short and focussed review, we briefly discuss a relatively new concept of the spin equilibrium condition and a new formula for the equilibrium spin frequency for transiently accreting millisecond pulsars. We also review a new method to estimate this equilibrium spin frequency for observed transiently accreting millisecond pulsars, even when a pulsar has not yet attained the spin equilibrium. These will be crucial to probe the spin evolution and distribution of millisecond pulsars, and should also be applicable to all magnetic stars transiently accreting via a thin, Keplerian accretion disk. Full article
(This article belongs to the Special Issue The 10th Anniversary of Galaxies: The Astrophysics of Neutron Stars)
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29 pages, 1159 KiB  
Article
A Walk through AGN Country—For the Somewhat Initiated!
by Robert R. J. Antonucci
Galaxies 2023, 11(5), 102; https://doi.org/10.3390/galaxies11050102 - 30 Sep 2023
Cited by 6 | Viewed by 1666
Abstract
Key issues in AGN and galaxy formation are discussed. Very successful Unified Models explain much of the variety of AGN with orientation effects; the ingredients are shadowing by a dusty “torus” and relativistic beaming. A spinoff result is described which is important for [...] Read more.
Key issues in AGN and galaxy formation are discussed. Very successful Unified Models explain much of the variety of AGN with orientation effects; the ingredients are shadowing by a dusty “torus” and relativistic beaming. A spinoff result is described which is important for the formation of massive elliptical galaxies, the most spectacular and unequivocal AGN feedback phenomenon known. This is the so-called “alignment effect” in powerful radio galaxies at z∼>1. One of them is a BAL radio galaxy! Next, I explain a very robust derivation of the reddening law for nuclear dust, which reveals a dearth of small grains on parsec scales. Then, the quasistatic thin accretion disk model, thought by many to explain the energetically dominant optical/UV continuum, is thoroughly debunked. Much of this was known when the model was proposed 35 years ago. A new argument is provided that trivially falsifies a huge superset of such models. I then show that it is possible to see the central engine spectrum with the atomic and dust emission surgically removed! Few have noticed this breakthrough work. Finally, the far IR dust emission in Cygnus A is 10% polarized; to date, high nuclear dust polarization has been seen in all radio loud objects and no radio quiet ones. Full article
(This article belongs to the Special Issue The Symbiosis between Radio Source and Galaxy Evolution)
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15 pages, 759 KiB  
Communication
On Exotic Objects Made of Dark Energy and Dark Matter: Mass-to-Radius Profiles and Tidal Love Numbers
by Camila Sepúlveda and Grigoris Panotopoulos
Galaxies 2023, 11(5), 101; https://doi.org/10.3390/galaxies11050101 - 22 Sep 2023
Cited by 2 | Viewed by 1528
Abstract
We investigate some properties of exotic spherical configurations made of dark matter and dark energy. For the former, we adopt a polytropic equation-of-state, while for the latter, we adopt the extended Chaplygin gas equation-of-state. Solving the Tolman–Oppenheimer–Volkoff equations, within the two-fluid formalism, we [...] Read more.
We investigate some properties of exotic spherical configurations made of dark matter and dark energy. For the former, we adopt a polytropic equation-of-state, while for the latter, we adopt the extended Chaplygin gas equation-of-state. Solving the Tolman–Oppenheimer–Volkoff equations, within the two-fluid formalism, we compute the factor of compactness, the mass-to-radius relationships, as well as the tidal Love numbers and dimensionless deformabilities. A comparison between single-fluid objects and two-fluid configurations is made as well. Full article
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40 pages, 15479 KiB  
Article
Baryonic Mass Inventory for Galaxies and Rarefied Media from Theory and Observations of Rotation and Luminosity
by Anne M. Hofmeister, Robert E. Criss and Hugh Chou
Galaxies 2023, 11(5), 100; https://doi.org/10.3390/galaxies11050100 - 20 Sep 2023
Cited by 1 | Viewed by 1865
Abstract
Available inventories of baryonic mass in the universe are based largely on galactic data and empirical calculations made >20 years ago. Values falling below cosmological estimates underlie proposals that certain rarified gassy regions could have extremely high T, which motivated absorption measurements [...] Read more.
Available inventories of baryonic mass in the universe are based largely on galactic data and empirical calculations made >20 years ago. Values falling below cosmological estimates underlie proposals that certain rarified gassy regions could have extremely high T, which motivated absorption measurements and hydrodynamic models. Yet, the shortfall remains. We inventory the total baryonic mass, focusing on gravitational interactions and updated measurements. A recent analytical inverse method for analyzing galactic rotation curves quantified how baryon mass and associated volumetric density (ρ) depend on distance (r) from galactic centers. The model is based on the dynamical consequences of the observed oblate shape of galaxies and the Virial Theorem. The parameter-free solution provides ρ(r) ∝ 1/r2 which describes star-rich galactic interiors, gas-rich outer discoids, circumgalactic media, and gradation into intergalactic media. Independent observational determinations of baryonic ρ validate that our 1/r2 result describes baryons alone. This solution shows that total baryonic mass associated with any galaxy is 2.4 to 40 times detectable luminosity, depending on galaxy size and spacing. Luminosity data within 50 Mpc show that Andromeda equivalents separated by ~1 Mpc represent the local universe. Combining the above yields (6 ± 2) × 10−25 kg m−3 for the present-day universe. Three other approaches support this high density: (1) evaluating trends and luminosity data near 1000 Mpc; (2) using a recent estimate for the number of galaxies in the universe; (3) calculating an energy balance. We discuss uncertainties in the critical density. Implications of large baryonic ρ are briefly discussed. Full article
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21 pages, 2518 KiB  
Article
A Scattered Star Group in the Orion A Region of the Milky Way
by Sergei Vereshchagin, Natalya Chupina, Kristina Lyzenko, Anatoly Kalinkin, Nikolay Kondratev, Dana Kovaleva and Sergei Sapozhnikov
Galaxies 2023, 11(5), 99; https://doi.org/10.3390/galaxies11050099 - 19 Sep 2023
Cited by 1 | Viewed by 1559
Abstract
Using Gaia DR3 data, we identified an extended a ~60 pc group of stars sharing common motion but scattered in space, including from 150 to 300 probable members, named Group V. It can be associated with a group identified by Getman et al. [...] Read more.
Using Gaia DR3 data, we identified an extended a ~60 pc group of stars sharing common motion but scattered in space, including from 150 to 300 probable members, named Group V. It can be associated with a group identified by Getman et al. (2019) and by Jerabkova et al. (2019) as a relic of a gas filament, traced by the mutual position of stars after the gas is swept out. We estimate its age to be approximately 16 million years. A combination of methods is applied to select probable members of Group V. We discuss the kinematic characteristics of the stars of Group V and the controversial clues they provide for understanding its nature. Due to the vicinity of a number of open clusters in the space, differentiating between members of the group and of the clusters is problematic, and mutual contamination is inevitable. The pair of clusters Gulliver 6 and UBC 17b is wrapped inside Group V but differs from it in kinematics. Full article
(This article belongs to the Special Issue Galactic Structure and Dynamics)
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8 pages, 294 KiB  
Communication
Gaia BH1: A Key for Understanding the Demography of Low-q Binaries in the Milky Way Galaxy
by Oleg Malkov
Galaxies 2023, 11(5), 98; https://doi.org/10.3390/galaxies11050098 - 18 Sep 2023
Viewed by 1482
Abstract
The recently discovered Gaia BH1 binary system, a Sun-like star and a dark object (presumably a black hole), may significantly change our understanding of the population of binaries. The paper presents the components mass ratio (q) distributions of binary systems of [...] Read more.
The recently discovered Gaia BH1 binary system, a Sun-like star and a dark object (presumably a black hole), may significantly change our understanding of the population of binaries. The paper presents the components mass ratio (q) distributions of binary systems of different observational classes. They all show a significant shortage of low-q systems. In this work, I demonstrate (quantitatively) how our ignorance extends, and point out the importance of discovering and studying systems like Gaia BH1. In addition, an approximate mass–temperature relation and mass ratio–magnitude difference relation for main-sequence stars are presented here. Full article
(This article belongs to the Special Issue Galactic Structure and Dynamics)
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19 pages, 15129 KiB  
Article
Resonant Effects of a Bar on the Galactic Disk Kinematics Perpendicular to Its Plane
by Vladimir Korchagin, Artem Lutsenko, Roman Tkachenko, Giovanni Carraro and Katherine Vieira
Galaxies 2023, 11(5), 97; https://doi.org/10.3390/galaxies11050097 - 6 Sep 2023
Cited by 2 | Viewed by 1403
Abstract
Detailed analysis of kinematics of the Milky Way disk in the solar neighborhood based on the GAIA DR3 catalog reveals the existence of peculiarities in the stellar velocity distribution perpendicular to the galactic plane. We study the influence of resonances—the outer Lindblad resonance [...] Read more.
Detailed analysis of kinematics of the Milky Way disk in the solar neighborhood based on the GAIA DR3 catalog reveals the existence of peculiarities in the stellar velocity distribution perpendicular to the galactic plane. We study the influence of resonances—the outer Lindblad resonance and the outer vertical Lindblad resonance—of a rotating bar with stellar oscillations perpendicular to the plane of the disk, and their role in shaping the spatial and the velocity distributions of stars. We find that the Z and VZ distributions of stars with respect to LZ are affected by the outer Lindblad resonance. The existence of bar resonance with stellar oscillations perpendicular to the plane of the disk is demonstrated for a long (large semi-axis 5 kpc) and fast rotating bar with Ωb=60.0kms−1kpc−1. We show also that, in the model with the long and fast rotating bar, some stars in the 2:1 OLR region deviate far from their original places, entering the bar region. A combination of resonance excitation of stellar motions at the 2:1 OLR region together with strong interaction of the stars with the bar potential leads to the formation of the group of ‘escapees’, i.e., stars that deviate in R and Z—directions at large distances from the resonance region. Simulations, however, do not demonstrate any noticeable effect on VZ-distribution of stars in the solar neighborhood. Full article
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13 pages, 4224 KiB  
Article
A Close Binary Supermassive Black Hole Model for the Galaxy 3C 273
by Alexandr Volvach, Larisa Volvach and Mikhail Larionov
Galaxies 2023, 11(5), 96; https://doi.org/10.3390/galaxies11050096 - 5 Sep 2023
Cited by 7 | Viewed by 1428
Abstract
The data from the last 60 years on the programs of long-term multi-frequency monitoring of active galactic nucleus (AGN) 3C 273 were analyzed. A model is proposed for finding the parameters of close binary systems (CBSs) from supermassive black holes (SMBHs), including a [...] Read more.
The data from the last 60 years on the programs of long-term multi-frequency monitoring of active galactic nucleus (AGN) 3C 273 were analyzed. A model is proposed for finding the parameters of close binary systems (CBSs) from supermassive black holes (SMBHs), including a harmonic analysis of observational data series obtained in the optical and radio ranges. The purpose of this research was to show that in the absence of optical information on AGNs, only radio data can be used and the necessary information on the physical objects can be obtained. Regarding the example of the blazar 3C 273, the following parameters were obtained: the masses of the companions; their orbital characteristics, such as the speeds of movement in orbits; the reserves of the kinetic energy of the system; and others. It was found that AGN 3C 273 can be a very massive binary system at the stage of evolution close to merging. Based on the obtained parameters, the characteristics of the gravitational waves (GWs) of this system, its lifetime before the merger, and the possible observation of 3C 273 using gravitational wave detectors were considered. Full article
(This article belongs to the Special Issue Distant Glowing Objects: Quest for Quasars)
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18 pages, 1196 KiB  
Article
Test Particles and Quasiperiodic Oscillations around Gravitational Aether Black Holes
by Javlon Rayimbaev, Farrux Abdulxamidov, Sardor Tojiev, Ahmadjon Abdujabbarov and Farhod Holmurodov
Galaxies 2023, 11(5), 95; https://doi.org/10.3390/galaxies11050095 - 1 Sep 2023
Cited by 13 | Viewed by 1483
Abstract
This paper is devoted to the analysis of the dynamics of test particles in the vicinity of a black hole within the framework of a gravitational aether model. First, we explored the structure of spacetime by analyzing the curvature scalars. Then, we studied [...] Read more.
This paper is devoted to the analysis of the dynamics of test particles in the vicinity of a black hole within the framework of a gravitational aether model. First, we explored the structure of spacetime by analyzing the curvature scalars. Then, we studied particle dynamics around a black hole using the Hamilton–Jacobi equation.The influence of the aether on the effective potential of the radial motion of test particles around the black hole has been investigated. The dependence of the innermost stable circular orbits (ISCO) on the aether parameter has also been investigated. We also considered particle collision near the black hole in the presence of aether, and studied the fundamental frequencies of the orbital motion of the test particles around the black hole in the presence of aether. Further, we applied the obtained results to the analysis of the upper and lower frequencies of twin-peaked quasiperiodic oscillations (QPOs) occurring near black holes. Finally, we use theoretical and numerical results to obtain constraints on model parameters using observation data in QPO. 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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12 pages, 261 KiB  
Review
The Structure and Evolution of Stars: Introductory Remarks
by Dominic M. Bowman, Jennifer van Saders and Jorick S. Vink
Galaxies 2023, 11(5), 94; https://doi.org/10.3390/galaxies11050094 - 31 Aug 2023
Cited by 1 | Viewed by 2343
Abstract
In this introductory chapter of the Special Issue entitled ‘The Structure and Evolution of Stars’, we highlight the recent major progress made in our understanding of the physics that governs stellar interiors. In so doing, we combine insight from observations, 1D evolutionary modelling [...] Read more.
In this introductory chapter of the Special Issue entitled ‘The Structure and Evolution of Stars’, we highlight the recent major progress made in our understanding of the physics that governs stellar interiors. In so doing, we combine insight from observations, 1D evolutionary modelling and 2D + 3D rotating (magneto)hydrodynamical simulations. Therefore, a complete and compelling picture of the necessary ingredients in state-of-the-art stellar structure theory and areas in which improvements still need to be made are contextualised. Additionally, the over-arching perspective linking all the themes of subsequent chapters is presented. Full article
(This article belongs to the Special Issue The Structure and Evolution of Stars)
28 pages, 1902 KiB  
Article
Unveiling the Evolutionary State of Three B Supergiant Stars: PU Gem, ϵ CMa, and η CMa
by Julieta Paz Sánchez Arias, Péter Németh, Elisson Saldanha da Gama de Almeida, Matias Agustin Ruiz Diaz, Michaela Kraus and Maximiliano Haucke
Galaxies 2023, 11(5), 93; https://doi.org/10.3390/galaxies11050093 - 29 Aug 2023
Cited by 3 | Viewed by 2035
Abstract
We aim to combine asteroseismology, spectroscopy, and evolutionary models to establish a comprehensive picture of the evolution of Galactic blue supergiant stars (BSG). To start such an investigation, we selected three BSG candidates for our analysis: HD 42087 (PU Gem), HD 52089 ( [...] Read more.
We aim to combine asteroseismology, spectroscopy, and evolutionary models to establish a comprehensive picture of the evolution of Galactic blue supergiant stars (BSG). To start such an investigation, we selected three BSG candidates for our analysis: HD 42087 (PU Gem), HD 52089 (ϵ CMa), and HD 58350 (η CMa). These stars show pulsations and were suspected to be in an evolutionary stage either preceding or succeding the red supergiant (RSG) stage. For our analysis, we utilized the 2-min cadence TESS data to study the photometric variability, and we obtained new spectroscopic observations at the CASLEO observatory. We used non-LTE radiative transfer models calculated with CMFGEN to derive their stellar and wind parameters. For the fitting procedure, we included CMFGEN models in the iterative spectral analysis pipeline XTgrid to determine their CNO abundances. The spectral modeling was limited to changing only the effective temperature, surface gravity, CNO abundances, and mass-loss rates. Finally, we compared the derived metal abundances with prediction from Geneva stellar evolution models. The frequency spectra of all three stars show stochastic oscillations and indications of one nonradial strange mode, fr= 0.09321 d−1 in HD 42087 and a rotational splitting centred in f2= 0.36366 d−1 in HD 52089. We conclude that the rather short sectoral observing windows of TESS prevent establishing a reliable mode identification of low frequencies connected to mass-loss variabilities. The spectral analysis confirmed gradual changes in the mass-loss rates, and the derived CNO abundances comply with the values reported in the literature. We were able to achieve a quantitative match with stellar evolution models for the stellar masses and luminosities. However, the spectroscopic surface abundances turned out to be inconsistent with the theoretical predictions. The stars show N enrichment, typical for CNO cycle processed material, but the abundance ratios did not reflect the associated levels of C and O depletion. We found HD 42087 to be the most consistent with a pre-RSG evolutionary stage, HD 58350 is most likely in a post-RSG evolution and HD 52089 shows stellar parameters compatible with a star at the TAMS. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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3 pages, 2759 KiB  
Editorial
From Vision to Instrument: Creating a Next-Generation Event Horizon Telescope for a New Era of Black Hole Science
by Michael D. Johnson, Sheperd S. Doeleman, José L. Gómez and Avery E. Broderick
Galaxies 2023, 11(5), 92; https://doi.org/10.3390/galaxies11050092 - 22 Aug 2023
Cited by 1 | Viewed by 1310
Abstract
In April 2019, the Event Horizon Telescope (EHT) Collaboration successfully imaged a supermassive black hole (SMBH) for the first time, revealing the apparent “shadow” cast by the dark compact object M87* in the center of the elliptical galaxy Virgo A [...] [...] Read more.
In April 2019, the Event Horizon Telescope (EHT) Collaboration successfully imaged a supermassive black hole (SMBH) for the first time, revealing the apparent “shadow” cast by the dark compact object M87* in the center of the elliptical galaxy Virgo A [...] 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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