Galaxies

34 pages, 2012 KiB

Open AccessArticle

Numerical Study of Bar Suppression in Galaxy Models Due to Disc Heating

by Alejandro López Gómez, Ruslan Gabbasov and Isaura Luisa Fuentes-Carrera

Galaxies 2025, 13(2), 45; https://doi.org/10.3390/galaxies13020045 - 21 Apr 2025

Viewed by 94

The process of bar formation, evolution and destruction is still a controversial topic regarding galaxy dynamics. Numerical simulations show that these phenomena strongly depend on physical and numerical parameters. In this work, we study the combined influence of the softening parameter,

ϵ

and [...] Read more.

The process of bar formation, evolution and destruction is still a controversial topic regarding galaxy dynamics. Numerical simulations show that these phenomena strongly depend on physical and numerical parameters. In this work, we study the combined influence of the softening parameter,

ϵ

and disc mass fraction,

m_{d}

, on the formation and evolution of bars in isolated disc-halo models via N-body simulations with different particle resolutions. Previous studies indicate that the bar strength depends on

m_{d}

as

\propto m_{d}^{- 1}

, which is seen as a delay in bar formation. However, the distorsion parameter,

η

, which measures the bar’s momentum through time, shows that an increase in

m_{d}

does not always induce a delay in bar formation. This suggests that

ϵ

interact to either enhance or weaken the bar. Moreover, numerical heating dominates in models with small softening values, creating highly accelerated particles at the centre of discs, regardless of

m_{d}

or resolution. These enhanced particle accelerations produce chaotic orbits for

ϵ \leq 5

pc, resulting in bar suppression due to collisional dynamics in the centre. In our high-resolution models (

N \approx 10^{7}

), small softening values are incapable of reproducing the bar instability. The role of disc mass is as follows: increasing

m_{d}

for moderate

ϵ

(≥10 pc) reduces the amount of drift in the acceleration profile, without affecting the bar’s behaviour. Models with lower

m_{d}

values, coupled with small softening values, have an excess of highly accelerated particles, introducing unwanted effects into otherwise reliable simulations. Finally, we show that the evolution of the disc’s vertical acceleration profile is a reliable indicator of numerical heating introduced by

ϵ

and the bar. Full article

(This article belongs to the Special Issue From Tides to Waves: Understanding the Formation Mechanisms of Galactic Spirals)

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35 pages, 6349 KiB

Open AccessArticle

Examination of the Functional Form of the Light and Mass Distribution in Spiral Arms

by Ilia V. Chugunov, Alexander A. Marchuk and Sergey S. Savchenko

Galaxies 2025, 13(2), 44; https://doi.org/10.3390/galaxies13020044 - 19 Apr 2025

Viewed by 86

Abstract

Spiral arms are a common feature of local galaxies, but the exact form of the distribution of mass and light in them is not well known. In this work, we aim to measure this distribution as accurately as possible, focusing on individual spiral [...] Read more.

Spiral arms are a common feature of local galaxies, but the exact form of the distribution of mass and light in them is not well known. In this work, we aim to measure this distribution as accurately as possible, focusing on individual spiral arms and using the so-called slicing method. The sample consists of 19 well-resolved, viewed face-on spiral galaxies from the S⁴G survey. We work primarily with infrared images at 3.6 μm from the same survey and, secondarily, with ultraviolet data from the GALEX telescope. We derive the properties of the spiral arms step by step, starting from their overall shape, then measuring their brightness profile and width variation along the arm and then examining the fine structure of the profile across the arm, namely, its skewness and Sérsic index. We construct a 2D photometric function of the spiral arm that can be used in further decomposition studies, validate it and identify the most and least important parameters. Finally, we show how our results can be used to unravel the nature of the spiral arms, supporting the evidence that NGC 4535 has a density wave in its disc. Full article

(This article belongs to the Special Issue From Tides to Waves: Understanding the Formation Mechanisms of Galactic Spirals)

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16 pages, 1546 KiB

Open AccessReview

Red and Yellow Hypergiants

by Terry Jones

Galaxies 2025, 13(2), 43; https://doi.org/10.3390/galaxies13020043 - 18 Apr 2025

Viewed by 99

Abstract

The red and yellow hypergiants are a rare and important phase in the evolution of the most massive stars that can reach the cool part of the HR Diagram. The hypergiant phase is commonly characterized by high, often episodic mass-loss rates and significant [...] Read more.

The red and yellow hypergiants are a rare and important phase in the evolution of the most massive stars that can reach the cool part of the HR Diagram. The hypergiant phase is commonly characterized by high, often episodic mass-loss rates and significant changes in spectral type, probably due to the formation of a pseudo photopsphere during a high mass-loss episode. Many of the yellow hypergiants are the immediate successors to the most luminous red supergiants, and often show evidence in their dusty, circumstellar envelopes from past red supergiant activity. In this paper we review the yellow and red hypergiants with an emphasis on how they differ from more normal red supergiants. Full article

(This article belongs to the Special Issue The Red Supergiants: Crucial Signposts for the Fate of Massive Stars)

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15 pages, 477 KiB

Open AccessArticle

Global Mean-Motion Resonances: Part I—An Exceptional Multiplanetary Resonant Chain in TOI-270 and an Exact Laplace-like Resonance in HD 110067

by Dimitris M. Christodoulou, Nicholas M. Sorabella, Sayantan Bhattacharya, Silas G. T. Laycock and Demosthenes Kazanas

Galaxies 2025, 13(2), 42; https://doi.org/10.3390/galaxies13020042 - 15 Apr 2025

Viewed by 222

Abstract

Super-Earth b and sub-Neptunes c and d are orbiting about the M3.0V dwarf TOI-270 in that order from the star. Their global resonant chain (3:5, 1:1, 2:1) is extremely surprising because planet d appears to be the only known planet occupying the 2:1 [...] Read more.

Super-Earth b and sub-Neptunes c and d are orbiting about the M3.0V dwarf TOI-270 in that order from the star. Their global resonant chain (3:5, 1:1, 2:1) is extremely surprising because planet d appears to be the only known planet occupying the 2:1 resonant orbit without participating in a Laplace resonance (LR) or another planet intervening between the 1:1 and 2:1 orbits as in HD 110067. We do not believe that TOI-270 d is an exception to the empirical rule calling for 2:1 vacancy except in 1:2:4 LRs and Laplace-like 2:3:4 chains. Instead, a LR might exist in this system, and we searched (to no avail) the TESS light curves of TOI-270 for hints of an outer planet that would complete the LR chain. Alternative explanations would be an unknown planet more massive than planet c (

M_{c} = 6.20 M_{\oplus}

) establishing the actual 1:1 orbit, or planet b residing in the 1:2 Laplace orbit with a period shorter by 0.53 days. However, these possibilities are ruled out by current data. This leaves only one other option to explore: the observed orbits could be in a stable

\frac{3}{5}

:1:2 resonant chain. Preliminary calculations do not preclude this possibility that should be investigated further by numerical orbit integrations. To this end, we determine two potentially resonant angles,

φ

and

\hat{φ}

, related via the Laplace phase

φ_{L}

by

\hat{φ} = φ_{L} + 2 φ

. In contrast, HD 110067 is shown to have planets d-e-f in a Laplace-like 1:

\frac{3}{2}

:2 resonance with phase

φ = 2 φ_{L}

precisely. Full article

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

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18 pages, 531 KiB

Open AccessArticle

Global Mean-Motion Resonances: Part II—Laplace-like Phase Angles to Facilitate Libration Searches in Multiplanetary N-body Simulations

by Dimitris M. Christodoulou, Nicholas M. Sorabella, Sayantan Bhattacharya, Silas G. T. Laycock and Demosthenes Kazanas

Galaxies 2025, 13(2), 41; https://doi.org/10.3390/galaxies13020041 - 14 Apr 2025

Viewed by 142

Abstract

We describe a method of determining three-body and four-body Laplace-like phase angles with the potential to librate about a mean value in multiplanet extrasolar systems. Unlike in past searches of N-body results, this method relies on global mean-motion resonances (MMRs) and takes into [...] Read more.

We describe a method of determining three-body and four-body Laplace-like phase angles with the potential to librate about a mean value in multiplanet extrasolar systems. Unlike in past searches of N-body results, this method relies on global mean-motion resonances (MMRs) and takes into consideration the location of the most massive planet that defines the 1:1 global MMR in each (sub)system. We compiled lists of potentially librating phase angles and prevalent MMRs in 35 real multibody systems, and we discuss their properties in conjunction with recent investigations of librations discovered in sophisticated N-body simulations. We hope that our results will facilitate systematic libration searches in dynamical models of compact systems with three or more orbiting bodies. Full article

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

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15 pages, 507 KiB

Open AccessReview

Spectropolarimetry for Discerning Geometry and Structure in Circumstellar Media of Hot Massive Stars

by Richard Ignace, Kenneth G. Gayley, Roberto Casini, Paul Scowen, Christiana Erba and Jeremy Drake

Galaxies 2025, 13(2), 40; https://doi.org/10.3390/galaxies13020040 - 11 Apr 2025

Viewed by 225

Abstract

Spectropolarimetric techniques are a mainstay of astrophysical inquiry, ranging from Solar System objects to the Cosmic Background Radiation. This review highlights applications of stellar polarimetry for massive hot stars, particularly in the context of ultraviolet (UV) spaceborne missions. The prevalence of binarity in [...] Read more.

Spectropolarimetric techniques are a mainstay of astrophysical inquiry, ranging from Solar System objects to the Cosmic Background Radiation. This review highlights applications of stellar polarimetry for massive hot stars, particularly in the context of ultraviolet (UV) spaceborne missions. The prevalence of binarity in the massive star population and uncertainties regarding the degree of rotational criticality among hot stars raises important questions about stellar interactions, interior structure, and even the lifetimes of evolutionary phases. These uncertainties have consequences for stellar population synthesis calculations. Spectropolarimetry is a key tool for extracting information about stellar and binary geometries. We review methodologies involving electron scattering in circumstellar envelopes; gravity darkening from rapid rotation; spectral line effects, including the (a) “line effect”, (b) Öhman effect, and (c) Hanle effect; and the imprint of interstellar polarization on measurements. Finally, we describe the Polstar UV spectropolarimetric SMEX mission concept as one means for employing these diagnostics to clarify the state of high rotation and its impacts for massive stars. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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31 pages, 4553 KiB

Open AccessArticle

Accurate Decomposition of Galaxies with Spiral Arms: Dust Properties and Distribution

by Alexander A. Marchuk, Ilia V. Chugunov, Frédéric Galliano, Aleksandr V. Mosenkov, Polina V. Strekalova, Sergey S. Savchenko, Valeria S. Kostiuk, George A. Gontcharov, Vladimir B. Il’in, Anton A. Smirnov and Denis M. Poliakov

Galaxies 2025, 13(2), 39; https://doi.org/10.3390/galaxies13020039 - 9 Apr 2025

Viewed by 375

Abstract

We analyze three nearby spiral galaxies—NGC 1097, NGC 1566, and NGC 3627—using images from the DustPedia database in seven infrared bands (3.6, 8, 24, 70, 100, 160, and 250 μm). For each image, we perform photometric decomposition and construct a multi-component model, including [...] Read more.

We analyze three nearby spiral galaxies—NGC 1097, NGC 1566, and NGC 3627—using images from the DustPedia database in seven infrared bands (3.6, 8, 24, 70, 100, 160, and 250 μm). For each image, we perform photometric decomposition and construct a multi-component model, including a detailed representation of the spiral arms. Our results show that the light distribution is well described by an exponential disk and a Sérsic bulge when non-axisymmetric components are properly taken into account. We test the predictions of the stationary density wave theory using the derived models in bands, tracing both old stars and recent star formation. Our findings suggest that the spiral arms in all three galaxies are unlikely to originate from stationary density waves. Additionally, we perform spectral energy distribution (SED) modeling using the hierarchical Bayesian code HerBIE, fitting individual components to derive dust properties. We find that spiral arms contain a significant (>10%) fraction of cold dust, with an average temperature of approximately 18–20 K. The estimated fraction of polycyclic aromatic hydrocarbons (PAHs) declines significantly toward the galactic center but remains similar between the arm and interarm regions. Full article

(This article belongs to the Special Issue From Tides to Waves: Understanding the Formation Mechanisms of Galactic Spirals)

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16 pages, 310 KiB

Open AccessArticle

Non-Zero Coriolis Field in Ehlers’ Frame Theory

by Federico Re and Oliver F. Piattella

Galaxies 2025, 13(2), 38; https://doi.org/10.3390/galaxies13020038 - 5 Apr 2025

Viewed by 230

Abstract

Ehlers’ Frame Theory is a class of geometric theories parameterized by

λ : = 1 / c^{2}

and identical to the General Theory of Relativity for

λ \neq 0

. The limit

λ \to 0

does not recover Newtonian gravity, as one [...] Read more.

Ehlers’ Frame Theory is a class of geometric theories parameterized by

λ : = 1 / c^{2}

and identical to the General Theory of Relativity for

λ \neq 0

. The limit

λ \to 0

does not recover Newtonian gravity, as one might expect, but yields the so-called Newton–Cartan theory of gravity, which is characterized by a second gravitational field

ω

, called the Coriolis field. Such a field encodes at a non-relativistic level the dragging feature of general spacetimes, as we show explicitly for the case of the

(η, H)

geometries. Taking advantage of the Coriolis field, we apply Ehlers’ theory to an axially symmetric distribution of matter, mimicking, for example, a disc galaxy, and show how its dynamics might reproduce a flattish rotation curve. In the same setting, we further exploit the formal simplicity of Ehlers’ formalism in addressing non-stationary cases, which are remarkably difficult to treat with the General Theory of Relativity. We show that the time derivative of the Coriolis field gives rise to a tangential acceleration which allows for studying a possible formation in time of the rotation curve’s flattish feature. Full article

(This article belongs to the Special Issue From Tides to Waves: Understanding the Formation Mechanisms of Galactic Spirals)

6 pages, 228 KiB

Open AccessArticle

Stellar Wind Parameters of Massive Stars in Accretion-Powered High-Mass X-Ray Binary Pulsars

by Nina Beskrovnaya, Nazar Ikhsanov and Vitaliy Kim

Galaxies 2025, 13(2), 37; https://doi.org/10.3390/galaxies13020037 - 5 Apr 2025

Viewed by 152

Abstract

The process of mass exchange between the components of High-Mass X-ray Binary (HMXB) systems with neutron stars undergoing wind-fed accretion is discussed. The X-ray luminosity of these systems allows us to evaluate the mass capture rate by the neutron star from the stellar [...] Read more.

The process of mass exchange between the components of High-Mass X-ray Binary (HMXB) systems with neutron stars undergoing wind-fed accretion is discussed. The X-ray luminosity of these systems allows us to evaluate the mass capture rate by the neutron star from the stellar wind of its massive companion and set limits on the relative velocity between the neutron star and the wind. We found that the upper limit to the wind velocity in the orbital plane during the high state of the X-ray source is in the range of 120–1000

km s^{- 1}

, which is by a factor of 2–4 lower than both the terminal wind velocity and the speed of the wind flowing out from the polar regions of massive stars for all the objects under investigation. This finding is valid not only for the systems with Be stars, but also for the systems in which the optical components do not exhibit the Be phenomenon. We also show that the lower limit to the radial wind velocity in these systems can unlikely be smaller than a few percent of the orbital velocity of the neutron star. This provides us with a new constraint on the mass transfer process in the outflowing disks of Be-type stars. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

8 pages, 1341 KiB

Open AccessReview

Chemical Synthesis in the Circumstellar Environment

by Sun Kwok

Galaxies 2025, 13(2), 36; https://doi.org/10.3390/galaxies13020036 - 3 Apr 2025

Viewed by 174

Abstract

We discuss the spectral distinctions between B[e] stars and compact planetary nebulae. The differentiation between proto-planetary nebulae, transition objects between the asymptotic giant branch and planetary nebulae, and reflection nebulae in binary systems is also discussed. Infrared and millimeter-wave observations have identified many [...] Read more.

We discuss the spectral distinctions between B[e] stars and compact planetary nebulae. The differentiation between proto-planetary nebulae, transition objects between the asymptotic giant branch and planetary nebulae, and reflection nebulae in binary systems is also discussed. Infrared and millimeter-wave observations have identified many inorganic and organic molecules, as well as solid-state minerals, in the circumstellar environment. There is evidence that complex organics in the form of mixed aromatic/aliphatic nanoparticles (MAONs) are synthesized during the proto-planetary nebulae phase of evolution. Their ejection into the interstellar medium may have enriched the primordial Solar System, and the complex organics found in comets, asteroids, and planetary satellites could be stellar in origin. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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10 pages, 1540 KiB

Open AccessArticle

Why Jet Power and Star Formation Are Uncorrelated in Active Galaxies

by David Garofalo, Brent McDaniel and Max North

Galaxies 2025, 13(2), 35; https://doi.org/10.3390/galaxies13020035 - 3 Apr 2025

Viewed by 227

Abstract

Jet luminosity from active galaxies and the rate of star formation have recently been found to be uncorrelated observationally. We show how to understand this in the context of a model in which powerful AGN jets enhance star formation for up to hundreds [...] Read more.

Jet luminosity from active galaxies and the rate of star formation have recently been found to be uncorrelated observationally. We show how to understand this in the context of a model in which powerful AGN jets enhance star formation for up to hundreds of millions of years while jet power decreases in time, followed by a longer phase in which star formation is suppressed but coupled to jet power increasing with time. We also highlight characteristic differences, depending on environment richness in a way that is compatible with the observed SEDs of high redshift radio galaxies. While the absence of a direct correlation between jet power and star formation rate emerges naturally, our framework allows us to also predict the environment richness, range of excitation, and redshift values of radio AGN in the jet power-star formation rate plane. Full article

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15 pages, 685 KiB

Open AccessReview

An Updated Repository of Sub-mJy Extragalactic Source-Count Measurements in the Radio Domain

by Vincenzo Galluzzi, Meriem Behiri, Marika Giulietti and Andrea Lapi

Galaxies 2025, 13(2), 34; https://doi.org/10.3390/galaxies13020034 - 2 Apr 2025

Viewed by 219

Abstract

We present an updated repository of sub-mJy extragalactic radio source counts between 150 MHz and 10 GHz, incorporating recent advances in radio surveys and observational techniques. By compiling and refining previous datasets, we provide a comprehensive catalog that enhances the understanding of faint [...] Read more.

We present an updated repository of sub-mJy extragalactic radio source counts between 150 MHz and 10 GHz, incorporating recent advances in radio surveys and observational techniques. By compiling and refining previous datasets, we provide a comprehensive catalog that enhances the understanding of faint radio-source populations, including Dusty Star-Forming Galaxies (DSFGs) and Radio-Quiet Active Galactic Nuclei (RQAGNs), from intermediate to high redshifts. Our analysis accounts for observational biases, such as resolution effects and Eddington bias, ensuring improved accuracy in flux-density estimations. We also discuss the implications of new-generation radio telescopes, such as the Square-Kilometer Array Observatory (SKAO) and its precursors and pathfinders, to further resolve these populations. Our collection contributes to constraining evolutionary models of radio sources, highlighting the increasing role of polarization studies in distinguishing different classes. This work serves as a key reference for future deep radio surveys targeting the faintest end of the extragalactic radio sky. Full article

(This article belongs to the Special Issue The Observation and Detection of Dusty Star-Forming Galaxies)

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24 pages, 1031 KiB

Open AccessReview

Red Supergiants as Supernova Progenitors

by Schuyler D. Van Dyk

Galaxies 2025, 13(2), 33; https://doi.org/10.3390/galaxies13020033 - 2 Apr 2025

Viewed by 306

Abstract

The inevitable fate of massive stars in the initial mass range of ≈8–

30 M_{⊙}

in the red supergiant (RSG) phase is a core-collapse supernova (SN) explosion, although some stars may collapse directly to a black hole. We know that this is [...] Read more.

The inevitable fate of massive stars in the initial mass range of ≈8–

30 M_{⊙}

in the red supergiant (RSG) phase is a core-collapse supernova (SN) explosion, although some stars may collapse directly to a black hole. We know that this is the case, since RSGs have been directly identified and characterized for a number of supernovae (SNe) in pre-explosion archival optical and infrared images. RSGs likely all have some amount of circumstellar matter (CSM), through nominal mass loss, although evidence exists that some RSGs must experience enhanced mass loss during their lifetimes. The SNe from RSGs are hydrogen-rich Type II-Plateau (II-P), and SNe II-P at the low end of the luminosity range tend to arise from low-luminosity RSGs. The typical spectral energy distribution (SED) for such RSGs can generally be fit with a cool photospheric model, whereas the more luminous RSG progenitors of more luminous SNe II-P tend to require a greater quantity of dust in their CSM to account for their SEDs. The SN II-P progenitor luminosity range is

log (L_{bol} / L_{⊙}) \sim 4.0

–5.2. The fact RSGs are known up to

log (L_{bol} / L_{⊙}) \sim 5.7

leads to the so-called “RSG problem”, which may, in the end, be a result of small number of available statistics to date. Full article

(This article belongs to the Special Issue The Red Supergiants: Crucial Signposts for the Fate of Massive Stars)

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11 pages, 2062 KiB

Open AccessArticle

IRAS 17449+2320: A Possible Binary System with the B[e] Phenomenon and a Strong Magnetic Field

by Sergey Zharikov, Anatoly Miroshnichenko, Inna Reva, Raushan Kokumbaeva, Chingis Omarov, Steve Danford, Alicia Aarnio, Nadine Manset, Ashish Raj, S. Drew Chojnowski and Joseph Daglen

Galaxies 2025, 13(2), 32; https://doi.org/10.3390/galaxies13020032 - 31 Mar 2025

Viewed by 232

Abstract

We report the recent results of a long-term spectroscopic and photometric monitoring of IRAS 17449+2320, a member of the least studied group of objects with the B[e] phenomenon called FS CMa-type objects. The main hypothesis for explaining the strong emission-line spectra and infrared [...] Read more.

We report the recent results of a long-term spectroscopic and photometric monitoring of IRAS 17449+2320, a member of the least studied group of objects with the B[e] phenomenon called FS CMa-type objects. The main hypothesis for explaining the strong emission-line spectra and infrared excesses of these objects assumes an ongoing or past mass transfer between the components in binary systems. The object is the only star with a gaseous and dusty envelope, where a strong and variable magnetic field (5.5–7.2 kG) was found through the splitting of some spectral lines. Additionally, we discovered the regular appearance of a red-shifted absorption component in spectral lines of neutral hydrogen, helium, and oxygen as well as one of ionized silicon with a period of 36.13 ± 0.20 days. We show that the magnetic field strength also followed this period. The process was accompanied by increasing emission component strengths for the hydrogen lines as well as the helium and metallic absorption lines. We refined the fundamental parameters of the optical counterpart of IRAS 17449+2320 (

T_{eff} = 9800 \pm 300

K, log L/

L_{⊙} = 1.86 \pm 0.06

,

v sin i = 9 \pm 2

km s⁻¹) and concluded that the star was slightly metal-deficient and viewed nearly pole-on. No signs of a secondary component were found. Possible interpretations of the observed phenomena are suggested, and some earlier findings about the object’s nature are revised. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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11 pages, 520 KiB

Open AccessArticle

The Photometric Variability and Spectrum of the Hot Post-AGB Star IRAS 21546+4721

by Natalia Ikonnikova, Marina Burlak and Alexander Dodin

Galaxies 2025, 13(2), 31; https://doi.org/10.3390/galaxies13020031 - 31 Mar 2025

Viewed by 213

Abstract

We present the results of photometric and spectroscopic observations of a poorly studied B-type supergiant with infrared excess, the hot post-AGB star IRAS 21546+4721. Based on our photometric observations in the

U B V R_{C} I_{C}

bands, we detected rapid, night-to-night, [...] Read more.

We present the results of photometric and spectroscopic observations of a poorly studied B-type supergiant with infrared excess, the hot post-AGB star IRAS 21546+4721. Based on our photometric observations in the

U B V R_{C} I_{C}

bands, we detected rapid, night-to-night, non-periodic brightness variations in the star with peak-to-peak amplitudes up to

0 .^{m} 3

in the V band, as well as color–color and color–brightness correlations. Based on its variability characteristics, IRAS 21546+4721 appears similar to other hot post-AGB stars. Possible causes of the photometric variability are discussed. Additionally, we acquired low-resolution spectra in a wavelength range from 3500 to 7500 Å. The spectrum contains absorption lines typical of an early B-type star, along with a set of emission lines of H I, He I, [O I], [O II], [N II], [S II], and C II originating from an ionized circumstellar envelope. An analysis of the emission spectrum allowed us to estimate the parameters of the gas envelope (

N_{e}

∼ 10⁴ cm⁻³,

T_{e}

∼ 10,000 K) and the star’s temperature (∼26,500 K). The radial velocity measured from the emission lines was

V_{r} = - 141 \pm 7

km s⁻¹. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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13 pages, 2295 KiB

Open AccessArticle

Nonlinear Stability of the Bardeen–De Sitter Wormhole in f(R) Gravity

by A. Eid

Galaxies 2025, 13(2), 30; https://doi.org/10.3390/galaxies13020030 - 28 Mar 2025

Viewed by 121

Abstract

This paper discusses the nonlinear stability of a thin-shell wormhole from a regular black hole in Bardeen–de Sitter spacetime in the

f (R)

gravity framework. The stability is examined under the linear perturbation about static solution and a nonlinear variable equation [...] Read more.

This paper discusses the nonlinear stability of a thin-shell wormhole from a regular black hole in Bardeen–de Sitter spacetime in the

f (R)

gravity framework. The stability is examined under the linear perturbation about static solution and a nonlinear variable equation of state, such as the modified generalized Chaplygin gas. The stability solutions for a suitable choice of different parameters included in the variable equation of state and

f (R)

gravity models, as well as the metric space–time, are illustrated. Full article

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18 pages, 1207 KiB

Open AccessArticle

Multiplicity of Luminous Blue Variable Stars

by A. Lobel and N. Gorlova

Galaxies 2025, 13(2), 29; https://doi.org/10.3390/galaxies13020029 - 28 Mar 2025

Viewed by 262

Abstract

The study of Luminous Blue Variables (LBVs) is critical to understanding the mechanisms behind their recurring S Dor-like outbursts, which can span decades and feature dramatic spectral changes. These outbursts may result from atmospheric instability or interactions with a companion star, but their [...] Read more.

The study of Luminous Blue Variables (LBVs) is critical to understanding the mechanisms behind their recurring S Dor-like outbursts, which can span decades and feature dramatic spectral changes. These outbursts may result from atmospheric instability or interactions with a companion star, but their causes remain poorly understood. Detecting binarity in LBVs through long-term radial velocity (RV) measurements, which can identify orbital motion via shifts in absorption lines, is a promising method. Periodic line shifts and variability can reveal the presence of a companion star. We report on the monitoring of four LBVs and two candidate LBVs (cLBVs), observing high-resolution spectra from 2009 to 2024. Although we do not find a clear periodic RV signal for LBVs MWC 930, P Cyg, or HD 168607, our long-term monitoring campaign previously detected the binarity of cLBV MWC 314 in 2013. For the first time, we detected a periodic signal in the RV data of the cLBV Schulte 12. In addition, in LBV HD 168607, we observed Discrete Absorption Components, indicative of large-scale structures in a rotating wind. These findings advance our understanding of the binary nature of LBVs and their complex outburst behaviors. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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36 pages, 538 KiB

Open AccessReview

Advanced Suspension Techniques in Interferometric Gravitational Wave Detectors: An Overview

by Vishnu G. Nair

Galaxies 2025, 13(2), 28; https://doi.org/10.3390/galaxies13020028 - 26 Mar 2025

Viewed by 1556

Abstract

Interferometric gravitational wave (GW) detectors are sophisticated instruments that require suspended mirrors to be effectively isolated from all forms of vibrations and noise. This isolation is crucial for enabling the detectors to function efficiently at low frequencies, which directly impacts their capacity to [...] Read more.

Interferometric gravitational wave (GW) detectors are sophisticated instruments that require suspended mirrors to be effectively isolated from all forms of vibrations and noise. This isolation is crucial for enabling the detectors to function efficiently at low frequencies, which directly impacts their capacity to detect distant events from the universe’s past. To address this challenge, various suspension systems have been developed, utilizing passive, active, or hybrid control mechanisms. The effectiveness of these systems in suppressing noise determines the lowest detectable frequencies. Designing and managing mirror suspensions present significant challenges across all interferometric GW detectors. Detectors such as LIGO, VIRGO, TAMA300, KAGRA, and GEO600 implement unique suspension designs and techniques to enhance their performance. A comprehensive comparison of these systems would offer valuable insights. This paper provides an overview of the different suspension systems employed in major global interferometric GW detectors, alongside a brief examination of proposed future detectors. It discusses the rationale behind each design, the materials utilized, and other relevant details, serving as a useful resource for the gravitational wave detector community. Full article

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30 pages, 10621 KiB

Open AccessArticle

A Comprehensive Analysis on the Nature of the Spiral Arms in NGC 3686, NGC 4321, and NGC 2403

by Valeria Kostiuk, Alexander Marchuk, Alexander Gusev and Ilia V. Chugunov

Galaxies 2025, 13(2), 27; https://doi.org/10.3390/galaxies13020027 - 24 Mar 2025

Cited by 1 | Viewed by 246

Abstract

In theoretical investigations, various mechanisms have been put forward to explain the emergence of spiral patterns in galaxies. One of the few ways to find out the nature of spirals in a particular galaxy is to consider the so-called corotation radius, or corotation [...] Read more.

In theoretical investigations, various mechanisms have been put forward to explain the emergence of spiral patterns in galaxies. One of the few ways to find out the nature of spirals in a particular galaxy is to consider the so-called corotation radius, or corotation resonance. A distinctly defined corotation resonance is likely to indicate the existence of a spiral density wave, while the chaotic distribution of their positions may suggest a dynamic nature to the spiral structure. In this study, we analyzed measurements of the corotation radius obtained using several methods for three galaxies (NGC 3686, NGC 4321, and NGC 2403) that exhibit different morphologies of spiral structures. We also performed independent measurements to estimate the location of the resonance, which allowed us to determine whether each galaxy has a clear corotation radius position. This examination, along with other tests such as stellar age gradient, interlocking resonances, and the radial distribution of metallicity, enables us to understand the mechanism that may be responsible for the formation of spiral arms in the studied galaxies. Full article

(This article belongs to the Special Issue From Tides to Waves: Understanding the Formation Mechanisms of Galactic Spirals)

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11 pages, 479 KiB

Open AccessArticle

HR 4049: A Spectroscopic Analysis of a Post-AGB Object

by Shakhida T. Nurmakhametova, Nadezhda L. Vaidman, Anatoly S. Miroshnichenko, Azamat A. Khokhlov, Aldiyar T. Agishev, Berik S. Yermekbayev, Stephen Danford and Alicia N. Aarnio

Galaxies 2025, 13(2), 26; https://doi.org/10.3390/galaxies13020026 - 22 Mar 2025

Viewed by 226

Abstract

A new spectroscopic study of HR 4049, a post-AGB star in a binary system, based on échelle spectra taken between 2019 and 2025 with the 0.81 m telescope of the Three College Observatory (North Carolina, USA) at a resolution of R ≈ 12,000 [...] Read more.

A new spectroscopic study of HR 4049, a post-AGB star in a binary system, based on échelle spectra taken between 2019 and 2025 with the 0.81 m telescope of the Three College Observatory (North Carolina, USA) at a resolution of R ≈ 12,000 is reported. A cross-correlation analysis of 73 spectra of a single C i multiplet in the 4760–4780 Å range yielded the following orbital parameters: the orbital period

P = 428.474 \pm 0.002

days, eccentricity

e = 0.29 \pm 0.01

, argument of periastron

ω = {242.3}^{\circ} \pm {0.3}^{\circ}

, epoch of periastron

T_{0} = 2,458,383.2 \pm 0.6

, heliocentric systemic radial velocity

γ = - 30.12 \pm 0.09

km s⁻¹, and semi-amplitude of the radial velocity curve

K_{1} = 15.52 \pm 0.13

km s⁻¹. Phase-dependent variations of the Hα line profile indicate dynamic processes in the circumstellar environment. The luminosity of HR 4049 was refined using the Gaia EDR3 parallax (

0.71 \pm 0.10

mas), corresponding to a distance of

1397 \pm 170

pc, and the average visual magnitude in the brightest state (

m_{V} = 5.35

mag). The derived luminosity,

log (L / L_{⊙}) = 4.22 \pm 0.12

, suggests an initial mass of 3.0–4.0

M_{⊙}

. Analysis of the mass function and most probable orbital inclinations (60°–75°) leads to current masses of

\sim 0.75 M_{⊙}

for the primary and

0.70

–

0.82 M_{⊙}

for the secondary component. The results confirm the system’s long-term orbital stability and provide further insights for future research into the nature of post-AGB binaries. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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12 pages, 1218 KiB

Open AccessReview

Red Supergiants—The Other Side of the H-R Diagram

by Roberta M. Humphreys

Galaxies 2025, 13(2), 25; https://doi.org/10.3390/galaxies13020025 - 20 Mar 2025

Viewed by 288

Abstract

Red supergiants are the largest stars known with some of the highest mass loss rates observed. They are the final stage in the evolution of the majority of massive stars. The unexpected discovery of high mass loss episodes in many red supergiants have [...] Read more.

Red supergiants are the largest stars known with some of the highest mass loss rates observed. They are the final stage in the evolution of the majority of massive stars. The unexpected discovery of high mass loss episodes in many red supergiants have posed questions about the role of mass loss on their final stages. The papers in this volume are timely reviews of our current understanding of this often surprising population of massive stars. This introductory paper is a brief summary of their observed properties and a historical perspective on some of the current problems on mass loss, their circumstellar environments, and their evolutionary state. Full article

(This article belongs to the Special Issue The Red Supergiants: Crucial Signposts for the Fate of Massive Stars)

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10 pages, 1232 KiB

Open AccessArticle

The Relative Position of the Solar Magnetic Dipole Axis and Rotation Axis of the Sun

by Alexandr Riehokainen, Victoria Smirnova, Alexander Solov’ev and Polina Strekalova

Galaxies 2025, 13(2), 24; https://doi.org/10.3390/galaxies13020024 - 19 Mar 2025

Viewed by 234

Abstract

We estimated the relative location of the solar rotation axis and the magnetic axis of the solar dipole, which were defined as centers of polar coronal holes. We used observations of polar coronal hole data, which were originally obtained with Solar Dynamic Observatory [...] Read more.

We estimated the relative location of the solar rotation axis and the magnetic axis of the solar dipole, which were defined as centers of polar coronal holes. We used observations of polar coronal hole data, which were originally obtained with Solar Dynamic Observatory (SDO) spacecraft. To calculate the tilt of the magnetic axis relative to the rotation axis of the Sun, an empirical method for the estimation of the coronal hole centers is proposed. As a result, it was found that these axes do not coincide. The average deviation of the magnetic dipole axis from the rotation axis is ∼5 degrees of latitude. Using the wavelet transform method, it was found that the magnetic axis rotates around the rotation axis with a main period of 15–16 days. This period is related to the sector structure of the global magnetic field in the polar zones of the Sun. Full article

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22 pages, 1122 KiB

Open AccessArticle

Propagation Times and Energy Losses of Cosmic Protons and Antiprotons in Interplanetary Space

by Nicola Tomassetti, Bruna Bertucci, Emanuele Fiandrini and Behrouz Khiali

Galaxies 2025, 13(2), 23; https://doi.org/10.3390/galaxies13020023 - 14 Mar 2025

Viewed by 310

Abstract

In this paper, we investigate the heliospheric modulation of cosmic rays in interplanetary space, focusing on their propagation times and energy losses over the solar cycle. To perform the calculations, we employed a data-driven model based on the stochastic method. Our model was [...] Read more.

In this paper, we investigate the heliospheric modulation of cosmic rays in interplanetary space, focusing on their propagation times and energy losses over the solar cycle. To perform the calculations, we employed a data-driven model based on the stochastic method. Our model was calibrated using time-resolved and energy-resolved data from several missions including AMS-02, PAMELA, EPHIN/SOHO, BESS, and data from Voyager-1. This approach allows us to calculate probability density functions for the propagation time and energy losses of cosmic protons and antiprotons in the heliosphere. Furthermore, we explore the temporal evolution of these probabilities spanning from 1993 to 2018, covering a full 22-year cycle of magnetic polarity, which includes two solar minima and two magnetic reversals. Our calculations were carried out for cosmic protons and antiprotons, enabling us to investigate the role of charge-sign dependent effects in cosmic ray transport. These findings provide valuable insights into the physical processes of cosmic-ray propagation in the heliosphere and contribute to a deeper understanding of the solar modulation phenomenon. Full article

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19 pages, 5901 KiB

Open AccessArticle

Scaling Relations of Early-Type Galaxies in MOND

by Robin Eappen and Pavel Kroupa

Galaxies 2025, 13(2), 22; https://doi.org/10.3390/galaxies13020022 - 14 Mar 2025

Viewed by 374

Abstract

We investigate the shape and morphology of early-type galaxies (ETGs) within the framework of Modified Newtonian Dynamics (MOND). Building on our previous studies, which demonstrated that the monolithic collapse of primordial gas clouds in MOND produces galaxies (noted throughout as ‘model relics’ in [...] Read more.

We investigate the shape and morphology of early-type galaxies (ETGs) within the framework of Modified Newtonian Dynamics (MOND). Building on our previous studies, which demonstrated that the monolithic collapse of primordial gas clouds in MOND produces galaxies (noted throughout as ‘model relics’ in the context of this work) with short star formation timescales and a downsizing effect as observationally found, we present new analyses on the resulting structural and morphological properties of these systems. Initially, the monolithically formed galaxies display disk-like structures. In this study, we further analyze the transformations that occur when these galaxies merge, observing that the resulting systems (noted throughout as ‘merged galaxies’ in the context of this work) take on elliptical-like shapes, with the

(V_{rot} / V_{σ})

–ellipticity relations closely matching observational data across various projections. We extend this analysis by examining the isophotal shapes and rotational parameter

(λ_{R})

of both individual relics and merged galaxies. The results indicate that ETGs may originate in pairs in dense environments, with mergers subsequently producing elliptical structures that align well with the observed kinematic and morphological characteristics. Finally, we compare both the model relics and merged galaxies with the fundamental plane and Kormendy relation of observed ETGs, finding close agreement. Together, these findings suggest that MOND provides a viable physical framework for the rapid formation and morphological evolution of ETGs. Full article

(This article belongs to the Special Issue Alternative Interpretations of Observed Galactic Behaviors)

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14 pages, 1007 KiB

Open AccessArticle

A Nested Inverted Pendulum as a Possible Pre-Isolator for the ET-LF Seismic Isolation System

by Lucia Trozzo, Alcide Bertocco, Matteo Bruno, Rosario De Rosa, Luciano Di Fiore, Domenico D’Urso, Franco Frasconi, Alberto Gennai, Leonardo Lucchesi, Moreno Nacca, Federico Pilo, Paolo Prosperi, Davide Rozza, Paolo Ruggi, Valeria Sipala and Francesca Spada

Galaxies 2025, 13(2), 21; https://doi.org/10.3390/galaxies13020021 - 7 Mar 2025

Viewed by 491

Abstract

The third-generation instrument era is approaching, and the Einstein Telescope (ET) giant interferometer is becoming a reality, with the potential to be installed at an underground site where seismic noise is about 100 times lower than at the surface. Moreover, new available technologies [...] Read more.

The third-generation instrument era is approaching, and the Einstein Telescope (ET) giant interferometer is becoming a reality, with the potential to be installed at an underground site where seismic noise is about 100 times lower than at the surface. Moreover, new available technologies and the experience acquired from operating advanced detectors are key to further extending the detection bandwidth down to 2–3 Hz, with the possibility of suspending a cryogenic payload. The New Generation of Super-Attenuator (NGSA) is an R&D project aimed at the improvement of vibration isolation performance for thirrd-generation detectors of gravitational waves, assuming that the present mechanical system adopted for the advanced VIRGO interferometer (second generation) is compliant with a third-generation detector. In this paper, we report the preliminary results obtained from a simulation activity devoted to the characterization of a mechanical system based on a multi-stage pendulum and a double-inverted pendulum in a nested configuration (NIP). The final outcomes provide guidelines for the construction of a reduced-scale prototype to be assembled and tested in the “PLANET” laboratory at INFN Naples, where the multi-stage pendulum—equipped with a new magnetic anti-spring (nMAS)—will be hung from the NIP structure. Full article

(This article belongs to the Special Issue Challenges for Third-Generation Gravitational Wave Detectors and Beyond)

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13 pages, 4528 KiB

Open AccessReview

Hot Stars in Stellar Populations of Galaxies

by Claus Leitherer

Galaxies 2025, 13(2), 20; https://doi.org/10.3390/galaxies13020020 - 7 Mar 2025

Viewed by 481

Abstract

Star-forming galaxies are hosts of dominant populations of recently formed, hot, massive stars, which give rise to conspicuous stellar spectral features and provide the ionizing fluxes. Strong outflows of these stars shape their properties. These winds affect the evolution and the output of [...] Read more.

Star-forming galaxies are hosts of dominant populations of recently formed, hot, massive stars, which give rise to conspicuous stellar spectral features and provide the ionizing fluxes. Strong outflows of these stars shape their properties. These winds affect the evolution and the output of ionizing radiation, as well as the energy and momentum input in the interstellar medium and the chemical enrichment. Many properties of massive stars become even more extreme at a low metallicity. Owing to the pioneering observations of young, metal-poor stellar populations, both locally with HST and large ground-based facilities and at high redshift with JWST, we are at a key moment to assess our understanding of hot massive stars in these galaxies. Stellar population synthesis is a key tool. I will demonstrate how population models of hot, massive stars help to address some issues at the forefront of current research. The recent advent of new evolutionary and atmosphere models of massive stars probing new parameter space allows us to characterize the properties of nearby and distant populations. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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10 pages, 426 KiB

Open AccessReview

The Blue Supergiant Problem and the Main-Sequence Width

by Jorick S. Vink and Rene D. Oudmaijer

Galaxies 2025, 13(2), 19; https://doi.org/10.3390/galaxies13020019 - 6 Mar 2025

Viewed by 386

Abstract

Using Gaia DR3 we derive new distances and luminosities for a sample of Galactic B supergiants which were thought to be post main-sequence (MS) objects from their HR diagram location beyond the terminal-age MS (TAMS). When applying the newer Gaia distances in addition [...] Read more.

Using Gaia DR3 we derive new distances and luminosities for a sample of Galactic B supergiants which were thought to be post main-sequence (MS) objects from their HR diagram location beyond the terminal-age MS (TAMS). When applying the newer Gaia distances in addition to enhanced amounts of core-boundary mixing, aka convective overshooting, we show that these Galactic B supergiants are likely enclosed within the MS band, indicating an evolutionary stage of steady core hydrogen burning. We discuss the importance of considering enhanced overshooting and how vectors in the mass-luminosity plane (ML-plane) can be used to disentangle the effects of wind mass loss from interior mixing. We finish with the key message that any proposed solution to the BSG problem should consider not only an explanation for the sheer number of B supergiants inside the Hertzsprung gap, but should at the same time also account for the steep drop in rotation rates identified at spectral type B1—corresponding to an effective temperature of ∼21 kK, and for which two distinct families of solutions have been proposed. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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13 pages, 2612 KiB

Open AccessFeature PaperArticle

Adaptive Optics for Aberration Control in Einstein Telescope

by Maria Cifaldi, Claudia Taranto, Lorenzo Aiello and Diana Lumaca

Galaxies 2025, 13(2), 18; https://doi.org/10.3390/galaxies13020018 - 5 Mar 2025

Viewed by 540

Abstract

Optical aberrations represent a critical issue for gravitational wave interferometers, as they impact the stability and controllability of the experiment. In the next generation of detectors, the circulating power in the cavity arms is expected to increase by up to a factor of [...] Read more.

Optical aberrations represent a critical issue for gravitational wave interferometers, as they impact the stability and controllability of the experiment. In the next generation of detectors, the circulating power in the cavity arms is expected to increase by up to a factor of 20 compared to current ones. This significant increase makes the mitigation of power-dependent optical aberrations extremely challenging. In this paper, we describe the problem of absorption in the optics and its role in generating some of the most important wavefront distortions, along with the present compensation strategy. To meet the new stringent requirements, new technologies must be designed, and existing ones upgraded. We present a review of the strategies and concepts in the field of aberration control in gravitational wave detectors and discuss the challenges for future detectors like the high-power operation of the Einstein Telescope. Full article

(This article belongs to the Special Issue Challenges for Third-Generation Gravitational Wave Detectors and Beyond)

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10 pages, 551 KiB

Open AccessArticle

AS 314: A Massive Dusty Hypergiant or a Low-Mass Post-Asymptotic Giant Branch Object?

by Aigerim Bakhytkyzy, Anatoly S. Miroshnichenko, Valentina G. Klochkova, Vladimir E. Panchuk, Sergey V. Zharikov, Laurent Mahy, Hans Van Winckel, Aldiyar T. Agishev and Serik A. Khokhlov

Galaxies 2025, 13(2), 17; https://doi.org/10.3390/galaxies13020017 - 28 Feb 2025

Viewed by 438

Abstract

AS 314 (V452 Sct) is a poorly studied early-type emission-line star, which exhibits an infrared excess at wavelengths longer than 10

μ

m. Its earlier studies have been limited to small amounts of observational data and led to controversial conclusions about its fundamental [...] Read more.

AS 314 (V452 Sct) is a poorly studied early-type emission-line star, which exhibits an infrared excess at wavelengths longer than 10

μ

m. Its earlier studies have been limited to small amounts of observational data and led to controversial conclusions about its fundamental parameters and evolutionary status. Comparison of high-resolution spectra of AS 314 taken over 20 years ago with those of Luminous Blue Variables and other high-luminosity objects suggested its observed properties can be explained by a strong stellar wind from a distant (D∼10 kpc) massive star, possibly in a binary system. However, a recent assessment of its low-resolution spectrum along with a new distance from a Gaia parallax (∼1.6 kpc) resulted in an alternative hypothesis that AS 314 is a low-mass post-asymptotic giant branch (post-AGB) star. The latter hypothesis ignored the high-resolution data, which gave rise to the former explanation. We collected over 30 mostly high-resolution spectra taken in 1997–2023, supplemented them with results of long-term photometric surveys, compared the spectra and the spectral energy distribution with those of post-AGB objects and B/A supergiants, and concluded that the observed properties AS 314 are more consistent with those of the latter. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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49 pages, 5523 KiB

Open AccessReview

Gamma-Ray Bursts: The Energy Monsters of the Universe

by Franco Giovannelli

Galaxies 2025, 13(2), 16; https://doi.org/10.3390/galaxies13020016 - 25 Feb 2025

Viewed by 683

Abstract

Gamma-Ray Bursts(GRBs) are the most violent and energetic astrophysical phenomena, which I dare call “the Energy Monsters of the Universe”. Indeed, they show an enormous emitted isotropic energy ranging from ∼3 ×

10^{46}

erg (GRB 170817A) to ∼

10^{55}

[...] Read more.

Gamma-Ray Bursts(GRBs) are the most violent and energetic astrophysical phenomena, which I dare call “the Energy Monsters of the Universe”. Indeed, they show an enormous emitted isotropic energy ranging from ∼3 ×

10^{46}

erg (GRB 170817A) to ∼

10^{55}

erg (GRB 221009A) and a duration ranging from ≈milliseconds to ∼

10^{4}

s. In this review—which I agreed to write as a scientist not directly involved in the field of GRBs—I will present the history of GRBs from the time of their discovery by chance until the new era whose beginning was marked by the detection of gravitational waves coming from the merger of two neutron stars. I will discuss the experimental results and their physical interpretation, which is still a source of heated debate within the scientific community. Due to the reasonable length of this review and especially given my limited knowledge, I do not claim to have exhausted the complicated topic of GRBs, but to have contributed in making this subject easy to read for non-experts, providing a critical contribution that is hopefully useful to the whole community. Full article

(This article belongs to the Special Issue Gamma-Ray Bursts in Multiwavelength: Theory, Observational Correlations and GRB Cosmology)

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