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Galaxies, Volume 13, Issue 3 (June 2025) – 6 articles

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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

Viewed by 64

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.

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

Viewed by 135

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 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

Viewed by 77

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 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

Viewed by 93

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 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

Viewed by 155

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.

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

Viewed by 185

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 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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