Galaxies

44 pages, 8269 KiB

Open AccessArticle

Contributions of AGN to the Morphological Parameters of Their Host Galaxies up to Intermediate Redshifts of z ∼ 2

by Tilahun Getachew-Woreta, Mirjana Pović, Jaime Perea, Isabel Marquez, Josefa Masegosa, Antoine Mahoro and Shimeles Terefe Mengistue

Galaxies 2025, 13(4), 84; https://doi.org/10.3390/galaxies13040084 (registering DOI) - 1 Aug 2025

Abstract

The presence of Active Galaxy Nuclei (AGN) can affect the morphological classification of galaxies. This work aims to determine how the contribution of AGN affects the most-used morphological parameters down to the redshift of z ∼ 2 in COSMOS-like conditions. We use a [...] Read more.

The presence of Active Galaxy Nuclei (AGN) can affect the morphological classification of galaxies. This work aims to determine how the contribution of AGN affects the most-used morphological parameters down to the redshift of z ∼ 2 in COSMOS-like conditions. We use a sample of >2000 local non-active galaxies, with a well-known visual morphological classification, and add an AGN as an unresolved component that contributes to the total galaxy flux with 5–75%. We moved all the galaxies to lower magnitudes (higher redshifts) to map the conditions in the COSMOS field, and we measured six morphological parameters. The greatest impact on morphology occurs when considering the combined effect of magnitude, redshift, and AGN, with spiral galaxies being the most affected. In general, all the concentration parameters change significantly if the AGN contribution is >25% and the magnitude > 23. We find that the GINI coefficient is the most stable in terms of AGN and magnitude/redshift, followed by the moment of light (M20), Conselice–Bershady (CCON), and finally the Abraham (CABR) concentration indexes. We find that, when using morphological parameters, the combination of CABR, CCON, and asymmetry is the most effective in classifying active galaxies at high-redshift, followed by a combination of CABR and GINI. Full article

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20 pages, 1666 KiB

Open AccessArticle

Looking for Signs of Unresolved Binarity in the Continuum of LAMOST Stellar Spectra

by Mikhail Prokhorov, Kefeng Tan, Nikolay Samus, Ali Luo, Dana Kovaleva, Jingkun Zhao, Yujuan Liu, Pavel Kaygorodov, Oleg Malkov, Yihan Song, Sergey Sichevskij, Lev Yungelson and Gang Zhao

Galaxies 2025, 13(4), 83; https://doi.org/10.3390/galaxies13040083 - 30 Jul 2025

Abstract

We describe an attempt to derive the binarity rate of samples of 166 A-, F-, G-, and K-type stars from LAMOST DR5 and 1000 randomly selected presumably single stars from Gaia DR3 catalogs. To this end, we compared continua of the observed spectra [...] Read more.

We describe an attempt to derive the binarity rate of samples of 166 A-, F-, G-, and K-type stars from LAMOST DR5 and 1000 randomly selected presumably single stars from Gaia DR3 catalogs. To this end, we compared continua of the observed spectra with the continua of synthetic spectra from within 3700

< λ < 9097

Å. The latter spectra were reduced to the LAMOST set of wavelengths, while the former ones were smoothed. Next, we searched for every observed star of the nearest synthetic spectrum using a four-parameter representation—

T_{eff}

,

log g

,

[Fe / H]

, and a range of interstellar absorption values. However, rms deviations of observed spectra from synthetic ones appeared to be not sufficient to claim that any of the stars is a binary. We conclude that comparison of the intensity of pairs of spectral lines remains the best way to detect binarity. Full article

(This article belongs to the Special Issue Stellar Spectroscopy, Molecular Astronomy and Atomic Astronomy)

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27 pages, 18125 KiB

Open AccessReview

Molecules and Chemistry in Red Supergiants

by Lucy M. Ziurys and Anita M. S. Richards

Galaxies 2025, 13(4), 82; https://doi.org/10.3390/galaxies13040082 - 21 Jul 2025

Abstract

The envelopes of Red Supergiants (RSGs) have a unique chemical environment not seen in other types of stars. They foster an oxygen-rich synthesis but are tempered by sporadic and chaotic mass loss, which distorts the envelope and creates complex outflow sub-structures consisting of [...] Read more.

The envelopes of Red Supergiants (RSGs) have a unique chemical environment not seen in other types of stars. They foster an oxygen-rich synthesis but are tempered by sporadic and chaotic mass loss, which distorts the envelope and creates complex outflow sub-structures consisting of knots, clumps, and arcs. Near the stellar photosphere, molecules and grains form under approximate LTE conditions, as predicted by chemical models. However, the complicated outflows appear to have distinct chemistries generated by shocks and dust destruction. Various RSG envelopes have been probed for their molecular content, mostly by radio and millimeter observations; however, VY Canis Majoris (VY CMa) and NML Cygni (NML Cyg) display the highest chemical complexity, and also the most complicated envelope structure. Thus far, over 29 different molecules have been identified in the envelopes of RSGs. Some molecules are common for circumstellar gas, including CO, SiO, HCN and H₂O, which have abundances of ∼10⁻⁶–10⁻⁴, relative to H₂. More exotic oxides have additionally been discovered, such as AlO, AlOH, PO, TiO₂, and VO, with abundances of ∼10⁻⁹–10⁻⁷. RSG shells support intricate maser emission in OH, H₂O and SiO, as well. Studies of isotope ratios in molecules suggest dredge-up at least into the H-burning shell, but further exploration is needed. 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, 1982 KiB

Open AccessReview

Stellar Evolution Through the Red Supergiant Phase

by Sylvia Ekström and Cyril Georgy

Galaxies 2025, 13(4), 81; https://doi.org/10.3390/galaxies13040081 - 17 Jul 2025

Abstract

Massive stars less massive than ∼30

M_{⊙}

evolve into a red supergiant after the main sequence. Given a standard IMF, this means about 80% of all single massive stars will experience this phase. RSGs are dominated by convection, with a radius that [...] Read more.

Massive stars less massive than ∼30

M_{⊙}

evolve into a red supergiant after the main sequence. Given a standard IMF, this means about 80% of all single massive stars will experience this phase. RSGs are dominated by convection, with a radius that may extend up to thousands of solar radii. Their low temperature and gravity make them prone to losing large amounts of mass, either through pulsationally driven wind or through mass-loss outburst. RSGs are the progenitors of the most common core-collapse supernovae, type II. In the present review, we give an overview of our theoretical understanding about this spectacular phase of massive star evolution. 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, 5078 KiB

Open AccessArticle

Doppler Tomography of the Be Star HD 698

by Ilfa A. Gabitova, Sergey V. Zharikov, Anatoly S. Miroshnichenko, Alex Carciofi, Azamat A. Khokhlov, Aldiyar Agishev and Peter Prendergast

Galaxies 2025, 13(4), 80; https://doi.org/10.3390/galaxies13040080 - 16 Jul 2025

Abstract

We present a Doppler tomography study of the Be star HD 698, recently resolved via interferometry as a post-mass-transfer binary system consisting of a Be star and a stripped, pre-subdwarf companion. Based on 76 high-resolution optical spectra obtained between 2014 and 2023, we [...] Read more.

We present a Doppler tomography study of the Be star HD 698, recently resolved via interferometry as a post-mass-transfer binary system consisting of a Be star and a stripped, pre-subdwarf companion. Based on 76 high-resolution optical spectra obtained between 2014 and 2023, we analyze the H

α

and H

β

emission lines and apply Doppler tomography to map the structure of the circumstellar disk. The H

α

line reveals an asymmetric, multi-component velocity distribution, with an emission feature closely following the orbital motion of the companion. V/R variations in both H

α

and H

β

lines are phase-locked with the companion’s orbital motion, indicating a tidally induced disk asymmetry. We discuss possible origins of the companion-centered H

α

emission, including a circumsecondary disk, a transient mass-transfer stream, and stellar wind. Full article

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15 pages, 395 KiB

Open AccessArticle

1PN Effective Binary Lagrangian for the Gravity–Kalb–Ramond Sector in the Conservative Regime

by Vegard Undheim, Eirik Eik Svanes and Alex B. Nielsen

Galaxies 2025, 13(4), 79; https://doi.org/10.3390/galaxies13040079 - 8 Jul 2025

Abstract

Within the framework of string theory, a number of new fields can arise that correct the Einstein–Hilbert action, including the Kalb–Ramond two-form field. In this work, we derive explicitly first-order relativistic corrections to conservative dynamics in the presence of a Kalb–Ramond field using [...] Read more.

Within the framework of string theory, a number of new fields can arise that correct the Einstein–Hilbert action, including the Kalb–Ramond two-form field. In this work, we derive explicitly first-order relativistic corrections to conservative dynamics in the presence of a Kalb–Ramond field using the effective field theory approach. The resulting additional terms in the Lagrangian governing conservative binary dynamics are presented explicitly. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum—2nd Edition)

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11 pages, 1002 KiB

Open AccessArticle

Unveiling the Evolution of MWC 728: Non-Conservative Mass Transfer in an FS CMa Binary

by Nadezhda L. Vaidman, Serik A. Khokhlov and Aldiyar T. Agishev

Galaxies 2025, 13(4), 78; https://doi.org/10.3390/galaxies13040078 - 7 Jul 2025

Abstract

We combine corrected Gaia DR3 astrometry with non-conservative MESA modelling to retrace the evolution of the FS-CMa binary MWC 728. The revised parallax sets the distance at

d = 1.2 \pm 0.1

kpc, leading—after Monte-Carlo error propagation—to luminosities of [...] Read more.

We combine corrected Gaia DR3 astrometry with non-conservative MESA modelling to retrace the evolution of the FS-CMa binary MWC 728. The revised parallax sets the distance at

d = 1.2 \pm 0.1

kpc, leading—after Monte-Carlo error propagation—to luminosities of

log {(L / L_{⊙})}_{acc} = 2.6 \pm 0.1

and

log {(L / L_{⊙})}_{don} = 1.5 \pm 0.1

, corresponding to the accretor and donor, respectively. A fiducial binary track that starts with

M_{don} = 3.6 \pm 0.1 M_{⊙}

,

M_{acc} = 1.8 \pm 0.1 M_{⊙}

, and

P_{0} = 21.0 \pm 0.2

d reproduces the observations provided the Roche-lobe overflow, which is moderately non-conservative: only

39 %

of the transferred mass is retained by the accretor, while the remainder leaves the system via (i) a fast isotropic wind from the donor (

α = 0.01

), (ii) isotropic re-emission near the accretor (

β = 0.45

), and (iii) outflow into a circumbinary torus (

δ = 0.15

, lever arm

γ = 1.3

). These channels remove sufficient angular momentum to expand the orbit to the observed

P_{obs} = 27.5 \pm 0.1

d while sustaining the dusty circumbinary outflow. At

t ≃ 223

Myr, the model matches every current observable:

M_{don} = 1.30 \pm 0.05 M_{⊙}

,

M_{acc} = 2.67 \pm 0.05 M_{⊙}

, mass ratio

q = 2.0 \pm 0.1

, and an ongoing transfer rate of

\dot{M} ≃ (1 \pm 0.3) \times 10^{- 6} M_{⊙} {yr}^{- 1}

. MWC 728 thus serves as a benchmark intermediate-mass binary for testing how non-conservative outflows regulate angular-momentum loss and orbital growth. Full article

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16 pages, 1360 KiB

Open AccessReview

Mass Loss in Be Stars: News from Two Fronts

by Alex C. Carciofi, Guilherme P. P. Bolzan, Pâmela R. Querido, Amanda C. Rubio, Jonathan Labadie-Bartz, Tajan H. de Amorim, Ariane C. Fonseca Silva and Vittória L. Schiavolim

Galaxies 2025, 13(4), 77; https://doi.org/10.3390/galaxies13040077 - 7 Jul 2025

Abstract

Be stars are characterized by the presence of a circumstellar Keplerian disk formed from material ejected from the rapidly rotating stellar surface. This article presents recent observational and theoretical progress on two central aspects of this phenomenon: the mechanisms driving mass loss, and [...] Read more.

Be stars are characterized by the presence of a circumstellar Keplerian disk formed from material ejected from the rapidly rotating stellar surface. This article presents recent observational and theoretical progress on two central aspects of this phenomenon: the mechanisms driving mass loss, and the fate of the ejected material. Using simultaneous TESS photometry and ground-based spectroscopy, we examine the short-term variability associated with discrete mass ejection events, or “flickers”, and review strong evidence linking them to pulsational activity near the stellar surface. Complementary 3D hydrodynamic simulations reproduce key observational signatures and establish that disk formation requires compact and asymmetric ejection sites with sufficient angular momentum to overcome re-accretion. In systems with binary companions, new high-resolution simulations resolve the outer disk for the first time and identify five dynamically distinct regions, including a circumsecondary disk and a circumbinary spiral outflow. Together, these results provide a coherent framework that traces the full life cycle of disk material from pulsation-driven ejection near the stellar surface to its final destination, whether re-accreted by the companion or lost from the system entirely. Full article

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

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18 pages, 2282 KiB

Open AccessArticle

Quantifying the Unwinding Due to Ram Pressure Stripping in Simulated Galaxies

by Rubens E. G. Machado, Caroline F. O. Grinberg and Elvis A. Mello-Terencio

Galaxies 2025, 13(4), 76; https://doi.org/10.3390/galaxies13040076 - 7 Jul 2025

Abstract

Galaxies moving through the gas of the intracluster medium (ICM) experience ram pressure stripping, which can leave behind a gas tail. When a disk galaxy receives the wind edge-on, however, the characteristic signature is not a typical jellyfish tail, but rather an unwinding [...] Read more.

Galaxies moving through the gas of the intracluster medium (ICM) experience ram pressure stripping, which can leave behind a gas tail. When a disk galaxy receives the wind edge-on, however, the characteristic signature is not a typical jellyfish tail, but rather an unwinding of the spiral arms. We aim to quantify such asymmetries both in the gas and in the stellar component of a simulated galaxy. To this end, we simulate a gas-rich star-forming spiral galaxy moving through a self-consistent ICM gas. The amplitude and location of the asymmetries were measured via Fourier decomposition. We found that the asymmetry is much more evident in the gas component, but it is also measurable in the stars. The amplitude tends to increase with time and the asymmetry radius migrates inwards. We found that, when considering the gas, the spiral arms extend much further and are more unwound than the corresponding stellar arms. Characterizing the unwinding via simulations should help inform the observational criteria used to classify ram pressure stripped galaxies, as opposed to asymmetries induced by other mechanisms. Full article

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19 pages, 601 KiB

Open AccessArticle

The I-Love Universal Relation for Polytropic Stars Under Newtonian Gravity

by Rui Xu, Alejandro Torres-Orjuela and Pau Amaro Seoane

Galaxies 2025, 13(4), 75; https://doi.org/10.3390/galaxies13040075 - 2 Jul 2025

Abstract

The moment of inertia and tidal deformability of idealized stars with polytropic equations of state (EOSs) are numerically calculated under both Newtonian gravity and general relativity (GR). The results explicitly confirm that the relation between the moment of inertia and tidal deformability, parameterized [...] Read more.

The moment of inertia and tidal deformability of idealized stars with polytropic equations of state (EOSs) are numerically calculated under both Newtonian gravity and general relativity (GR). The results explicitly confirm that the relation between the moment of inertia and tidal deformability, parameterized by the star’s mass, exhibits variations up to

1 %

and

10 %

for different polytropic indices in Newtonian gravity and GR, respectively. This indicates a more robust I-Love universal relation in the Newtonian framework. The theoretically derived I-Love universal relation for polytropic stars is subsequently tested against observational data for the moment of inertia and tidal deformability of the eight planets and some moons in our solar system. The analysis reveals that the theoretical I-Love universal relation aligns well with the observational data, suggesting that it can serve as an empirical relation. Consequently, it enables the estimation of either the moment of inertia or the tidal deformability of an exoplanet if one of these quantities, along with the mass of the exoplanet, is known. Full article

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14 pages, 3543 KiB

Open AccessArticle

The BSN Application-I: Photometric Light Curve Solutions of Contact Binary Systems

by Ehsan Paki, Atila Poro and Minoo Dokht Moosavi Rowzati

Galaxies 2025, 13(4), 74; https://doi.org/10.3390/galaxies13040074 - 30 Jun 2025

Abstract

Light curve analysis of W UMa-type contact binary systems using MCMC or MC methods can be time-consuming, primarily because the repeated generation of synthetic light curves tends to be relatively slow during the fitting process. Although various approaches have been proposed to address [...] Read more.

Light curve analysis of W UMa-type contact binary systems using MCMC or MC methods can be time-consuming, primarily because the repeated generation of synthetic light curves tends to be relatively slow during the fitting process. Although various approaches have been proposed to address this issue, their implementation is often challenging due to complexity or uncertain performance. In this study, we introduce the BSN application, whose name is taken from the BSN project. The application is designed for analyzing contact binary system light curves, supporting photometric data, and employing an MCMC algorithm for efficient parameter estimation. The BSN application generates synthetic light curves more than 40 times faster than PHOEBE during the MCMC fitting process. The BSN application enhances light curve analysis with an expanded feature set and a more intuitive interface while maintaining compliance with established scientific standards. In addition, we present the first light curve analyses of four contact binary systems based on the TESS data, utilizing the BSN application version 1.0. We also conducted a light curve analysis using the PHOEBE Python code and compared the resulting outputs. Two of the target systems exhibited asymmetries in the maxima of their light curves, which were appropriately modeled by introducing a cold starspot on one of the components. The estimated mass ratios of these total-eclipse systems place them within the category of low mass ratio contact binary stars. The estimation of the absolute parameters for the selected systems was carried out using the

P - a

empirical relationship. Based on the effective temperatures and masses of the components, three of the target systems were classified as A-subtype, while TIC 434222993 was identified as a W-subtype system. Full article

(This article belongs to the Special Issue Study on Contact Binary Stars)

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15 pages, 1991 KiB

Open AccessArticle

ALMA Observations of G333.6-0.2: Molecular and Ionized Gas Environment

by Aruzhan Omar, Aidana Abdirakhman, Nazgul Alimgazinova, Meiramgul Kyzgarina, Aisha Naurzbayeva, Zhomartkali Islyam, Kunduz Turekhanova, Aizat Demessinova and Arailym Manapbayeva

Galaxies 2025, 13(4), 73; https://doi.org/10.3390/galaxies13040073 - 27 Jun 2025

Abstract

We present high-angular resolution observations, conducted with the Atacama Large Millimeter/Submillimeter Array (ALMA) in Band 6, of high-excitation molecular lines of

{CH}_{3} CN

, CH₃OH, and the H29

α

radio recombination line, towards the G333.6-0.2 ultracompact (UC) H ii region. [...] Read more.

We present high-angular resolution observations, conducted with the Atacama Large Millimeter/Submillimeter Array (ALMA) in Band 6, of high-excitation molecular lines of

{CH}_{3} CN

, CH₃OH, and the H29

α

radio recombination line, towards the G333.6-0.2 ultracompact (UC) H ii region. Our observations reveal three hot molecular cores: A, B, and C, where emission is detected in ten components of the

J = 14 \to 13

rotational ladder of

{CH}_{3} CN

and in the CH₃OH

J = 5_{1, 4} \to 4_{1, 3}

transition. Rotational diagram analysis of

{CH}_{3} CN

reveals excitation temperatures ranging from 380 to 430 K. First-order moment maps of

{CH}_{3} CN

and CH₃OH reveal distinct velocity gradients in all cores, suggesting rotating structures, with core A also showing evidence of expansion motions. The H29

α

recombination line shows a linewidth of

30.2 \pm 0.12

km s⁻¹, dominated by dynamical and thermal broadening, indicative of large-scale motions in ionized gas. Analysis of the ionized gas properties yields an electron density of

(4.8 \pm 0.4) \times 10^{5}

cm⁻³, an emission measure of

(1.23 \pm 0.06) \times 10^{9}

pc cm⁻⁶, and a Lyman continuum photon flux consistent with an O5–O6 V (Zero-Age Main Sequence; ZAMS) star. Our results suggest that G333.6-0.2 is in an intermediate evolutionary stage between hypercompact (HC) and ultracompact (UC) H ii regions, hosting active high-mass star formation with rotating hot cores and ionized gas dynamics. Full article

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

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26 pages, 4473 KiB

Open AccessReview

Red Supergiant Mass Loss and Mass-Loss Rates

by Jacco Th. van Loon

Galaxies 2025, 13(4), 72; https://doi.org/10.3390/galaxies13040072 - 20 Jun 2025

Abstract

This review discusses the causes, nature, importance and observational evidence of mass loss by red supergiants. It arrives at the perception that mass loss finds its origin in the gravity which makes the star a star in the first place, and is a [...] Read more.

This review discusses the causes, nature, importance and observational evidence of mass loss by red supergiants. It arrives at the perception that mass loss finds its origin in the gravity which makes the star a star in the first place, and is a mechanism for the star to equilibrate. This is corroborated by a careful examination of various popular historical and recent empirical mass-loss rate prescriptions and theoretical works, and which provides no evidence for an explicit dependence of red supergiant mass loss on metallicity though dust-associated mass loss becomes less prevalent at lower metallicity. It also identifies a common problem in methods that use tracers of mass loss, which do not correct for varying scaling factors (often because there is no information available on which to base such correction) and as a result tend to underestimate mass-loss rates at the lower end. Conversely, dense, extended chromospheres in themselves do not translate into high mass-loss rates, and the significance of stochastic mass loss can be overstated. On a population scale, on the other hand, binary interaction acts as a stochastic agent of mass loss of great import. In all, evidence is overwhelming that points at red supergiants at the lower mass end losing mass at insufficient rates to shed their mantles before core collapse, but massive (at birth) red supergiants to be prone to intense, dusty mass loss which sees them become hotter stars before meeting their fate. This is consistent with the identified progenitors of hydrogen-rich supernovae. Supernova evolution holds great promise to probe the mass loss but we caution against confusing atmospheres with winds. Finally, promising avenues are looked into, which could forge step-change progress in what has been a long and arduous search for the holy grail of red supergiant mass loss. We may yet find it! Full article

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

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17 pages, 320 KiB

Open AccessArticle

CMB Multipole Expansion in a Frame Dragging-Sustained Milky Way

by Federico Re, Marco Galoppo and Massimo Dotti

Galaxies 2025, 13(3), 71; https://doi.org/10.3390/galaxies13030071 - 13 Jun 2025

Abstract

We study the impact on the cosmic microwave background (CMB) landscape of peculiar rotational general relativistic effects. These effects, on galactic scales, do not possess a Newtonian analogue, and therefore could a priori impact CMB analysis. We find that the velocity inferred from [...] Read more.

We study the impact on the cosmic microwave background (CMB) landscape of peculiar rotational general relativistic effects. These effects, on galactic scales, do not possess a Newtonian analogue, and therefore could a priori impact CMB analysis. We find that the velocity inferred from the CMB dipole, under the kinematic interpretation, coincides with that measured by a stationary observer within the Milky Way and not with the one measured by the zero angular momentum observer. We show that the galaxy peculiar frame-dragging effects do not impact the standard CMB analysis, as these modify the multipole coefficients only at higher orders with respect to the dominant terms. Moreover, we prove that no general relativistic framework at the galactic scale patched within the standard cosmological model can account for the current tension on the CMB quadrupole amplitude. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum—2nd Edition)

7 pages, 1181 KiB

Open AccessCommunication

The Enigmatic, Highly Variable, High-Mass Young Stellar Object Mol 12: A New Extreme Herbig Be (Proto)star

by Mauricio Tapia, Paolo Persi, Jesús Hernández and Nuria Calvet

Galaxies 2025, 13(3), 70; https://doi.org/10.3390/galaxies13030070 - 13 Jun 2025

Abstract

We report new medium-resolution spectroscopy covering the wavelength range from 0.6 to 2.4

μ

m, as well as multi-epoch, multi-wavelength photometry, of the Class I high-mass embedded young stellar object Mol 12 (IRAS 05373+2349). It is embedded (

A_{V} ≃ 12

) [...] Read more.

We report new medium-resolution spectroscopy covering the wavelength range from 0.6 to 2.4

μ

m, as well as multi-epoch, multi-wavelength photometry, of the Class I high-mass embedded young stellar object Mol 12 (IRAS 05373+2349). It is embedded (

A_{V} ≃ 12

) in the centre of a dense core at a distance of 1.59 kpc from the Sun and has a total luminosity of

1.74 \times 10^{3} L_{⊙}

. The spectra show a large number of permitted atomic emission lines, mostly for Fe, H, C, N, and Ca, that originate in the inner zones of a very active protoplanetary disc and no photospheric absorption lines. Conspicuously, the He I line at 1.0830

μ

m displays a complex P-Cygni profile. Also, the first overtone CO emission band-heads at 2.3

μ

m are seen in emission. From the strengths of the principal emission lines, we determined the accretion rate and luminosity to be

\dot{M} \sim 10^{- 5} M_{⊙}

y⁻¹ and

L_{acc} \sim 10^{3} L_{⊙}

, respectively. Decade-long light curves show a series of irregular brightness dips of more than four magnitudes in r, becoming shallower as the wavelength increases and disappearing at

λ > 3 μ

m. The colour–magnitude diagrams suggest the occurrence of a series of eclipses caused by the passage of small dust cloudlets in front of the star, producing more than 10 magnitudes of extra extinction. Full article

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

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28 pages, 1004 KiB

Open AccessArticle

Numerical Solutions and Stability Analysis of White Dwarfs with a Generalized Anisotropic Factor

by Ayazhan Orazymbet, Aray Muratkhan, Daniya Utepova, Nurzada Beissen, Gulzada Baimbetova and Saken Toktarbay

Galaxies 2025, 13(3), 69; https://doi.org/10.3390/galaxies13030069 - 12 Jun 2025

Abstract

This study examines the equilibrium structure and stability of white dwarfs, incorporating both isotropic and anisotropic pressure distributions. The Tolman–Oppenheimer–Volkoff (TOV) equation is numerically solved using the Chandrasekhar equation of state (EoS) to analyze the effects of pressure anisotropy. A general anisotropy function [...] Read more.

This study examines the equilibrium structure and stability of white dwarfs, incorporating both isotropic and anisotropic pressure distributions. The Tolman–Oppenheimer–Volkoff (TOV) equation is numerically solved using the Chandrasekhar equation of state (EoS) to analyze the effects of pressure anisotropy. A general anisotropy function is introduced to close and solve the system of differential equations. The results indicate that anisotropy remains negligible at the center and increases toward the stellar surface. Stability is assessed using the speed of sound criterion,

v_{s}^{2} = d p / d ρ

, and the Buchdahl bound,

2 M / R < 8 / 9

, confirming that white dwarfs remain within stability limits. We performed a sensitivity analysis to examine how variations in the anisotropy parameter

α_{0}

and central density affect the mass, radius, and compactness of white dwarfs. Additionally, we calculated the gravitational redshift at the stellar surface and found that it varies with anisotropy, ranging from

z_{s} \sim 3.15 \times 10^{- 3}

in isotropic cases to

z_{s} \sim 0.2 \times 10^{- 3}

in highly anisotropic models. These results link anisotropy to potentially observable features. The findings suggest that while anisotropy does not significantly affect the overall equilibrium structure, it may play a role in astrophysical scenarios involving strong magnetic fields, rotational deformations, or accretion processes in binary systems. Full article

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10 pages, 1248 KiB

Open AccessArticle

Post-AGB Binaries as Interacting Systems

by Hans Van Winckel

Galaxies 2025, 13(3), 68; https://doi.org/10.3390/galaxies13030068 - 12 Jun 2025

Abstract

We present recent progress in our understanding of the physical interaction mechanisms at work in evolved binaries of low-to-intermediate initial mass, which are surrounded by a stable disc of gas and dust. These systems are known as post-asymptotic giant-branch (post-AGB) binaries, but recently, [...] Read more.

We present recent progress in our understanding of the physical interaction mechanisms at work in evolved binaries of low-to-intermediate initial mass, which are surrounded by a stable disc of gas and dust. These systems are known as post-asymptotic giant-branch (post-AGB) binaries, but recently, it has been shown that some systems are too low in luminosity and should be considered as post-red-giant branch (post-RGB) instead. While the systems are currently well within their Roche lobe, they still show signs of active ongoing interaction between the different building blocks. We end this contribution with some future research plans. Full article

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

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15 pages, 922 KiB

Open AccessArticle

Searching for New Objects with the B[e] Phenomenon

by Aisha Zh. Naurzbayeva, Nazgul Sh. Alimgazinova, Anatoly S. Miroshnichenko, Corinne Rossi, Inna V. Reva, Raushan I. Kokumbayeva, Chingis T. Omarov, Sergei V. Zharikov, Nadine Manset, Ashish Raj, Arti Joshi, Richard J. Rudy, Richard C. Puetter, Raleigh B. Perry and Kunduz M. Turekhanova

Galaxies 2025, 13(3), 67; https://doi.org/10.3390/galaxies13030067 - 11 Jun 2025

Abstract

Objects with the B[e] phenomenon, whose defining features are the presence of forbidden emission lines and infrared excess coming from circumstellar dust, represent a broad range of evolutionary stages from pre-main-sequence to planetary nebulae. They are important for understanding mechanisms of the circumstellar [...] Read more.

Objects with the B[e] phenomenon, whose defining features are the presence of forbidden emission lines and infrared excess coming from circumstellar dust, represent a broad range of evolutionary stages from pre-main-sequence to planetary nebulae. They are important for understanding mechanisms of the circumstellar matter formation and evolution. However, it is not easy to discover them, especially among faint stars, as forbidden emission lines are usually weak and hardly noticeable in low-resolution spectra. We developed photometric criteria to search for candidate objects with this phenomenon based on a combination of optical and near-infrared color indices and found nearly 40 objects that satisfy these criteria. Spectroscopy of the candidates allows us to make more confident conclusions on their classification. We present the results of our photometric and spectroscopic observations of six objects, which are part of a large list of ∼40 objects that satisfy our photometric selection criteria for candidate objects with the B[e] phenomenon. Forbidden lines of neutral oxygen were clearly detected in the optical spectrum of one object (VES 683) and suspected in three others. One object, AS 415, is most likely a binary system with components that exhibit partial eclipses but without the B[e] phenomenon, while IRAS 20402 + 4638 may be a luminous member of the FS CMa objects group. Full article

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

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19 pages, 2881 KiB

Open AccessReview

Red Supergiants in the Milky Way and Nearby Galaxies

by Alceste Z. Bonanos

Galaxies 2025, 13(3), 66; https://doi.org/10.3390/galaxies13030066 - 5 Jun 2025

Abstract

Identifications of red supergiants (RSGs) in the Milky Way and nearby galaxies have experienced an exponential increase in recent years, driven by advancements in selection techniques, the continued expansion of archival datasets, and a steady increase in spectroscopic data. This review describes the [...] Read more.

Identifications of red supergiants (RSGs) in the Milky Way and nearby galaxies have experienced an exponential increase in recent years, driven by advancements in selection techniques, the continued expansion of archival datasets, and a steady increase in spectroscopic data. This review describes the advances in methodologies and selection criteria for identifying RSGs and presents the current census of these stars in our own Galaxy and nearby galaxies. It also describes the insights gained from resolving nearby RSGs and their complex circumstellar material in the Milky Way and from the growing samples of RSGs being discovered in the Local Group and beyond. These advances impact the Humphreys–Davidson limit in the cool part of the Hertzsprung–Russell diagram. Furthermore, they provide insight into extreme RSGs and the role of photometric variability and, in particular, of the newly discovered phenomenon of dimming events. Recent observations have enabled the determination of the binarity fraction among RSGs, offering new constraints to stellar evolution. Looking ahead, the synergy between large-scale surveys, high-resolution observations, and emerging machine-learning tools promises to further transform our understanding of the final evolutionary stages of massive stars in the coming decade. Full article

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

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