Galaxies

12 pages, 3593 KiB

Open AccessArticle

Spectral Variability of Herbig Ae Star V1295, Aquila

by Hemayil Adigozalzade, Silva Jarvinen, Swetlana Hubrig, Ulviyya Bashirova and Nariman Ismailov

Galaxies 2025, 13(3), 56; https://doi.org/10.3390/galaxies13030056 - 6 May 2025

This article presents the results of long-term spectral studies of the unusual Herbig Ae star V1295 Aql. The variability in the spectrophotometric parameters of the emission and absorption components of the hydrogen lines Hα, Hβ, and D Na I—and of various metallic lines—is [...] Read more.

This article presents the results of long-term spectral studies of the unusual Herbig Ae star V1295 Aql. The variability in the spectrophotometric parameters of the emission and absorption components of the hydrogen lines Hα, Hβ, and D Na I—and of various metallic lines—is studied. The periodic variability in these parameters over a period of 51.7 days, recently established using mean longitudinal magnetic field measurements, is confirmed. The obtained average value of the radial velocities of metal absorption lines is Vr = 0.75 ± 1.85 km/s. According to our long-term observations, the radial velocities of the star do not change over time. This indicates the absence of a close stellar-mass companion. Full article

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

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17 pages, 1201 KiB

Open AccessArticle

Time Dilation Observed in Type Ia Supernova Light Curves and Its Cosmological Consequences

by Václav Vavryčuk

Galaxies 2025, 13(3), 55; https://doi.org/10.3390/galaxies13030055 - 3 May 2025

Abstract

The cosmic time dilation observed in Type Ia supernova light curves suggests that the passage of cosmic time varies throughout the evolution of the Universe. This observation implies that the rate of proper time is not constant, as assumed in the standard FLRW [...] Read more.

The cosmic time dilation observed in Type Ia supernova light curves suggests that the passage of cosmic time varies throughout the evolution of the Universe. This observation implies that the rate of proper time is not constant, as assumed in the standard FLRW metric, but instead is time-dependent. Consequently, the commonly used FLRW metric should be replaced by a more general framework, known as the Conformal Cosmology (CC) metric, to properly account for cosmic time dilation. The CC metric incorporates both spatial expansion and time dilation during cosmic evolution. As a result, it is necessary to distinguish between comoving and proper (physical) time, similar to the distinction made between comoving and proper distances. In addition to successfully explaining cosmic time dilation, the CC metric offers several further advantages: (1) it preserves Lorentz invariance, (2) it maintains the form of Maxwell’s equations as in Minkowski spacetime, (3) it eliminates the need for dark matter and dark energy in the Friedmann equations, and (4) it successfully predicts the expansion and morphology of spiral galaxies in agreement with observations. Full article

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

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8 pages, 350 KiB

Open AccessArticle

The Effect of Viscosity on the Temperature of Ae Star Disks

by R. Anusha and T. A. A. Sigut

Galaxies 2025, 13(3), 54; https://doi.org/10.3390/galaxies13030054 - 3 May 2025

Abstract

This study explores the impact of viscous heating on decretion disks around Classical Ae (CAe) stars, with a focus on modeling the disk’s thermal structure. While photoionization is the dominant heating mechanism, viscous dissipation can play an important role in shaping the disk [...] Read more.

This study explores the impact of viscous heating on decretion disks around Classical Ae (CAe) stars, with a focus on modeling the disk’s thermal structure. While photoionization is the dominant heating mechanism, viscous dissipation can play an important role in shaping the disk temperature, particularly for cooler CAe subtypes. Our models incorporate viscosity-driven heating and predict that shear heating has a negligible effect for early A-type stars (A0–A1), but it becomes increasingly significant for later spectral types, especially as the viscosity parameter (

α

) increases. This heating also influences the strength of H

α

emission. Furthermore, our models predict a sharp decline in the number of emission-line stars beyond spectral type A2, a trend observed in CAe populations. However, for sufficiently high

α

values (≥0.3), a higher fraction of emission-line objects is expected even among later subtypes, such as A5, despite the lack of well-characterized CAe stars observed beyond the spectral type A4. Full article

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

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14 pages, 644 KiB

Open AccessReview

Very-High-Energy Gamma-Ray Observations as a Probe to the Nature of Dark Matter and Prospects for MACE

by Mani Khurana, Krishna Kumar Singh, Atul Pathania, Pawan Kumar Netrakanti and Kuldeep Kumar Yadav

Galaxies 2025, 13(3), 53; https://doi.org/10.3390/galaxies13030053 - 2 May 2025

Abstract

Searching for very-high-energy photons arising from dark matter interactions in selected astrophysical environments is a promising strategy to probe the existence and particle nature of dark matter. Among the many particle candidates, motivated by the extensions of the Standard Model, Weakly Interacting Massive [...] Read more.

Searching for very-high-energy photons arising from dark matter interactions in selected astrophysical environments is a promising strategy to probe the existence and particle nature of dark matter. Among the many particle candidates, motivated by the extensions of the Standard Model, Weakly Interacting Massive Particles (WIMPs) are considered the most compelling candidate for the elusive dark matter in the universe. In this contribution, we report an overview of the important developments in the field of indirect searching for dark matter through cosmic gamma-ray observations. We mainly focus on the role of atmospheric Cherenkov telescopes in probing the dark matter. Finally, we emphasize the opportunities for the Major Atmospheric Cherenkov Experiment (MACE) situated in Hanle, India, to explore WIMPs in the mass range of 200 GeV to 10 TeV for Segue1 and Draco dwarf–spheroidal galaxies. Full article

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20 pages, 3417 KiB

Open AccessFeature PaperArticle

Statistical Classification and an Optimized Red-Sequence Technique for the Determination of Galaxy Clusters

by Dagoberto R. Mares-Rincón, Josué J. Trejo-Alonso, José A. Guerrero-Díaz-de-León and Jorge E. Macías-Díaz

Galaxies 2025, 13(3), 52; https://doi.org/10.3390/galaxies13030052 - 1 May 2025

Abstract

This study presents a novel method for characterizing galaxy clusters by integrating statistical classification techniques with an optimized adaptation of the red sequence approach. The proposed algorithm employs Gaussian mixture models to analyze the distribution of three key variables: r magnitude, [...] Read more.

This study presents a novel method for characterizing galaxy clusters by integrating statistical classification techniques with an optimized adaptation of the red sequence approach. The proposed algorithm employs Gaussian mixture models to analyze the distribution of three key variables: r magnitude,

g - r

color index, and redshift z. To enhance cluster discrimination, we incorporate Mahalanobis distance metrics and modify the conventional red sequence technique by adopting the principal eigenvector as the slope of the cluster. A sample of 114 galaxy groups and clusters within the redshift range

0.002 < z < 0.45

was used to validate the method. Comparative analyses demonstrate that the proposed approach achieves comparable or, in certain cases, superior performance in cluster characterization relative to the standard red sequence technique. These results highlight the algorithm’s potential as a robust tool for the exploratory identification and initial parameter determination of galaxy clusters, particularly in large-scale surveys. The methodology bridges statistical rigor with established astrophysical techniques, offering a promising avenue for advancing cluster detection in observational cosmology. Full article

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20 pages, 1318 KiB

Open AccessArticle

The Galactic Pizza: Flat Rotation Curves in the Context of Cosmological Time-Energy Coupling

by Artur Novais and André L. B. Ribeiro

Galaxies 2025, 13(3), 51; https://doi.org/10.3390/galaxies13030051 - 27 Apr 2025

Abstract

The phenomenon of augmented gravity on the scale of galaxies, conventionally attributed to dark matter halos, is shown to possibly result from the incremental growth of galactic masses and radii over time. This approach elucidates the cosmological origins of the acceleration scale [...] Read more.

The phenomenon of augmented gravity on the scale of galaxies, conventionally attributed to dark matter halos, is shown to possibly result from the incremental growth of galactic masses and radii over time. This approach elucidates the cosmological origins of the acceleration scale

a_{0} \approx c H_{0} / 2 π \approx 10^{- 10}

ms⁻² at which galaxy rotation curves deviate from Keplerian behavior, with no need for new particles or modifications to the laws of gravity, i.e., it constitutes a new explanatory path beyond Cold Dark Matter (CDM) and Modified Newtonian Dynamics (MOND). Once one formally equates the energy density of the universe to the critical value (

ρ = ρ_{c}

) and the cosmic age to the reciprocal of the Hubble parameter (

t = H^{- 1}

), independently of the epoch of observation, the result is the Zero-Energy condition for the cosmic fluid’s equation of state, with key repercussions for the study of dark energy since the observables can be explained in the absence of a cosmological constant. Furthermore, this mass-energy evolution framework is able to reconcile the success of CDM models in describing structure assembly at

z ≲ 6

with the unexpected discovery of massive objects at

z ≳ 10

. Models that feature a strong coupling between cosmic time and energy are favored by this analysis. Full article

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

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17 pages, 7779 KiB

Open AccessReview

Betelgeuse, the Prototypical Red Supergiant

by Andrea K. Dupree and Miguel Montargès

Galaxies 2025, 13(3), 50; https://doi.org/10.3390/galaxies13030050 - 24 Apr 2025

Abstract

The behavior of the bright red supergiant, Betelgeuse, is described with results principally from the past 6 years. The review includes imaging, photometry, and spectroscopy to record the Great Dimming of 2019–2020. This event was followed by a slow ongoing recovery from the [...] Read more.

The behavior of the bright red supergiant, Betelgeuse, is described with results principally from the past 6 years. The review includes imaging, photometry, and spectroscopy to record the Great Dimming of 2019–2020. This event was followed by a slow ongoing recovery from the massive surface mass ejection after which the stellar characteristics changed. Theoretical simulations address the cause of this episodic mass ejection and the optical Dimming. Recent publications evaluating the perplexing 2100 day periodicity in the star’s brightness and radial velocity provide evidence that Betelgeuse may harbor a companion object. Current attempts at direct detection of this companion are discussed. Betelgeuse provides a well-studied and meaningful example for supergiant stars in our Galaxy and others. Full article

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

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24 pages, 629 KiB

Open AccessReview

Symbiotic Stars in the Era of Modern Ground- and Space-Based Surveys

by Jaroslav Merc

Galaxies 2025, 13(3), 49; https://doi.org/10.3390/galaxies13030049 - 23 Apr 2025

Abstract

Symbiotic stars, interacting binaries composed of a cool giant and a hot compact companion, exhibit complex variability across the electromagnetic spectrum. Over the past decades, large-scale photometric and spectroscopic surveys from ground- and space-based observatories have significantly advanced their discovery and characterization. These [...] Read more.

Symbiotic stars, interacting binaries composed of a cool giant and a hot compact companion, exhibit complex variability across the electromagnetic spectrum. Over the past decades, large-scale photometric and spectroscopic surveys from ground- and space-based observatories have significantly advanced their discovery and characterization. These datasets have transformed the search for new symbiotic candidates, providing extensive time-domain information crucial for their classification and analysis. This review highlights recent observational results that have expanded the known population of symbiotic stars, refined classification criteria, and enhanced our understanding of their variability. Despite these advances, fundamental questions remain regarding their long-term evolution, mass transfer and accretion processes, or their potential role as progenitors of Type Ia supernovae. With ongoing and upcoming surveys, the coming years promise new discoveries and a more comprehensive picture of these intriguing interacting systems. Full article

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

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12 pages, 590 KiB

Open AccessArticle

Necessary Conditions for Earthly Life Floating in the Venusian Atmosphere

by Jennifer J. Abreu, Alyxander R. Anchordoqui, Nyamekye J. Fosu, Michael G. Kwakye, Danijela Kyriakakis, Krystal Reynoso and Luis A. Anchordoqui

Galaxies 2025, 13(3), 48; https://doi.org/10.3390/galaxies13030048 - 22 Apr 2025

Abstract

Millimeter-waveband spectra of Venus from both the James Clerk Maxwell Telescope (JCMT) and the Atacama Large Millimeter/submillimeter Array (ALMA) seem to indicate there may be evidence (signal-to-noise ratio of about 15σ) of a phosphine absorption-line profile against the thermal background from deeper, hotter [...] Read more.

Millimeter-waveband spectra of Venus from both the James Clerk Maxwell Telescope (JCMT) and the Atacama Large Millimeter/submillimeter Array (ALMA) seem to indicate there may be evidence (signal-to-noise ratio of about 15σ) of a phosphine absorption-line profile against the thermal background from deeper, hotter layers of the atmosphere. Phosphine is an important biomarker; e.g., the trace of phosphine in the Earth’s atmosphere is unequivocally associated with anthropogenic activity and microbial life (which produces this highly reducing gas even in an overall oxidizing environment). Motivated by the JCMT and ALMA tantalizing observations, we reexamine whether Venus could accommodate Earthly life. More concretely, we hypothesize that the microorganisms populating the Venusian atmosphere are not free floating but confined to the liquid environment inside cloud aerosols or droplets. Armed with this hypothesis, we generalize a study of airborne germ transmission to constrain the maximum size of droplets that could be floating in the Venusian atmosphere by demanding that their Stokes fallout times to reach moderately high temperatures are pronouncedly larger than the microbe’s replication time. We also comment on the effect of cosmic ray showers on the evolution of aerial microbial life. Full article

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10 pages, 539 KiB

Open AccessArticle

Fundamental Parameters and Evolutionary Scenario of HD 327083

by Nadezhda L. Vaidman, Anatoly S. Miroshnichenko, Sergey V. Zharikov, Serik A. Khokhlov, Aldiyar T. Agishev and Berik S. Yermekbayev

Galaxies 2025, 13(3), 47; https://doi.org/10.3390/galaxies13030047 - 22 Apr 2025

Abstract

In this study, we present refined orbital and fundamental parameters of the Galactic B[e] supergiant binary system HD 327083 using the Bayesian Markov Chain Monte Carlo (MCMC) method applied to the radial velocities data of HD 327083. We found that the system is [...] Read more.

In this study, we present refined orbital and fundamental parameters of the Galactic B[e] supergiant binary system HD 327083 using the Bayesian Markov Chain Monte Carlo (MCMC) method applied to the radial velocities data of HD 327083. We found that the system is well described by a circular orbital model with the mass ratio of the components of

q = 1.15 \pm 0.07

. We modeled the evolutionary history of the system using MESA code. Initially, the system was formed by a binary with the orbital period of

P o r b = 108

day, which contained stars with 13.00

\pm 0.05

M_{⊙}

and

11.50 \pm 0.05

M_{⊙}

masses. They had a relatively slow rotation

υ_{r o t} = 0.40 \pm 0.13 υ_{c r i t}

and provided a strong stellar wind. The current system age is

13.6 \pm 0.1

Myr, and the state of the system corresponds to a close filling of the high massive component’s Roche lobe and a beginning of the mass transfer. The mass-transfer event will occur in a short interval of ≲0.1 Myr only. After that, the mass of the post-primary drops to ≈5

M_{⊙}

, the post-secondary mass grows until ≈20

M_{⊙}

, and the binary will convert to a detached system with a long orbital period of ≈700 days. Full article

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

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14 pages, 3588 KiB

Open AccessReview

Intermediate-Mass Mergers: A New Scenario for Several FS CMa Stars

by Daniela Korčáková, Nela Dvořáková, Iris Bermejo Lozano, Gregg A. Wade, Alicia Moranchel Basurto, Pavel Kroupa, Raul Ortega Chametla, Colin Peter Folsom and Ondrej Juhás

Galaxies 2025, 13(3), 46; https://doi.org/10.3390/galaxies13030046 - 22 Apr 2025

Abstract

We summarise the properties and nature of a peculiar group of B-type stars called FS CMa stars. These stars show the B[e] phenomenon, i.e., their spectra exhibit both forbidden emission lines and infrared excess. Such properties point to an extended circumstellar gas and [...] Read more.

We summarise the properties and nature of a peculiar group of B-type stars called FS CMa stars. These stars show the B[e] phenomenon, i.e., their spectra exhibit both forbidden emission lines and infrared excess. Such properties point to an extended circumstellar gas and dust component. Although the phenomenon has been explained in most B[e] stars, the origin and nature of FS CMa stars is disputed. Here, we focus on the merger hypothesis, for which evidence has recently been discovered. Full article

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

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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

Abstract

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

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

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

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

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

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

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

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)

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