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Galaxies, Volume 13, Issue 4 (August 2025) – 16 articles

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10 pages, 335 KiB  
Article
On the Probability of the Astrometric Resolution of Spectroscopic Binaries into Components
by Dugasa Belay Zeleke, Alexey Yu. Sytov, Seblu Humne Negu, Arseniy M. Sachkov, Solomon Belay Tessema and Oleg Yu. Malkov
Galaxies 2025, 13(4), 87; https://doi.org/10.3390/galaxies13040087 (registering DOI) - 5 Aug 2025
Abstract
Resolved spectroscopic binaries (RSBs) are an extremely valuable class of objects, being the only (apart from trigonometric) supplier of dynamical stellar parallaxes. This circumstance, as well as the comparative paucity of studied RSBs, makes the problem of identifying binary systems potentially capable of [...] Read more.
Resolved spectroscopic binaries (RSBs) are an extremely valuable class of objects, being the only (apart from trigonometric) supplier of dynamical stellar parallaxes. This circumstance, as well as the comparative paucity of studied RSBs, makes the problem of identifying binary systems potentially capable of being added to the list of known RSBs extremely urgent. In this paper, we propose a methodology for estimating the probability of a spectroscopic binary system to be resolved into components, perform the first step of its application to the SB9 catalogue, and present preliminary results, in particular, a list of the most promising RSB candidates. Full article
(This article belongs to the Special Issue Stellar Spectroscopy, Molecular Astronomy and Atomic Astronomy)
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13 pages, 1787 KiB  
Article
A New Activity Cycle of V1413 Aql
by Anna Tatarnikova, Andrey Tatarnikov, Nataliia Maslennikova, Alexander Dodin, Marina Burlak, Natalia Ikonnikova, Alexander Belinski and Georgy Nikishev
Galaxies 2025, 13(4), 86; https://doi.org/10.3390/galaxies13040086 (registering DOI) - 4 Aug 2025
Viewed by 2
Abstract
V1413 Aql is an anomalous classical symbiotic star that has remained in an active state almost continuously. After failing to return to quiescence following the 2020 outburst, it underwent a new flare in 2025. We present an analysis of new photometric and spectroscopic [...] Read more.
V1413 Aql is an anomalous classical symbiotic star that has remained in an active state almost continuously. After failing to return to quiescence following the 2020 outburst, it underwent a new flare in 2025. We present an analysis of new photometric and spectroscopic observations, which provide estimates for the binary components at different stages of activity: Lcool=1700L, Rcool=130R; Lhot=800L, Rhot=28R during minimum; Lhot=7000L, Rhot=64R at the 2025 maximum. The significant increase in the luminosity of the hot component during the 2025 outburst deviates from the typical behavior observed in classical symbiotic stars, where outbursts are generally attributed to a redistribution of spectral energy rather than intrinsic changes in luminosity. Full article
(This article belongs to the Special Issue Stellar Spectroscopy, Molecular Astronomy and Atomic Astronomy)
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14 pages, 3378 KiB  
Article
The pcGR Within the Hořava-Lifshitz Gravity and the Wheeler-deWitt Quantization
by Peter O. Hess, César A. Zen Vasconcellos and Dimiter Hadjimichef
Galaxies 2025, 13(4), 85; https://doi.org/10.3390/galaxies13040085 (registering DOI) - 1 Aug 2025
Viewed by 156
Abstract
We investigate pseudo-complex General Relativity (pcGR)—a coordinate-extended formulation of General Relativity (GR)—within the framework of Hořava-Lifshitz gravity, a regularized theory featuring anisotropic scaling. The pcGR framework bridges GR with modified gravitational theories through the introduction of a minimal length scale. Focusing on Schwarzschild [...] Read more.
We investigate pseudo-complex General Relativity (pcGR)—a coordinate-extended formulation of General Relativity (GR)—within the framework of Hořava-Lifshitz gravity, a regularized theory featuring anisotropic scaling. The pcGR framework bridges GR with modified gravitational theories through the introduction of a minimal length scale. Focusing on Schwarzschild black holes, we derive the Wheeler-deWitt equation, obtaining a quantized description of pcGR. Using perturbative methods and semi-classical approximations, we analyze the solutions of the equations and their physical implications. A key finding is the avoidance of the central singularity due to nonlinear interaction terms in the Hořava-Lifshitz action. Notably, extrinsic curvature (kinetic energy) contributions prove essential for singularity resolution, even in standard GR. Furthermore, the theory offers new perspectives on dark energy, proposing an alternative mechanism for its accumulation. Full article
(This article belongs to the Special Issue Cosmology and the Quantum Vacuum—2nd Edition)
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44 pages, 8269 KiB  
Article
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
Viewed by 250
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  
Article
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
Viewed by 244
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—Teff, logg, [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  
Review
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
Viewed by 424
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  
Review
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
Viewed by 402
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  
Article
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
Viewed by 492
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 Hα and Hβ emission lines and apply Doppler tomography to map the structure of the circumstellar disk. The Hα 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 Hα and Hβ lines are phase-locked with the companion’s orbital motion, indicating a tidally induced disk asymmetry. We discuss possible origins of the companion-centered Hα emission, including a circumsecondary disk, a transient mass-transfer stream, and stellar wind. Full article
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15 pages, 395 KiB  
Article
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
Viewed by 315
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  
Article
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
Viewed by 432
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±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±0.1 kpc, leading—after Monte-Carlo error propagation—to luminosities of log(L/L)acc=2.6±0.1 and log(L/L)don=1.5±0.1, corresponding to the accretor and donor, respectively. A fiducial binary track that starts with Mdon=3.6±0.1M, Macc=1.8±0.1M, and P0=21.0±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 Pobs=27.5±0.1 d while sustaining the dusty circumbinary outflow. At t223 Myr, the model matches every current observable: Mdon=1.30±0.05M, Macc=2.67±0.05M, mass ratio q=2.0±0.1, and an ongoing transfer rate of M˙(1±0.3)×106Myr1. 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  
Review
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
Viewed by 521
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  
Article
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
Viewed by 456
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  
Article
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
Viewed by 466
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  
Article
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
Viewed by 506
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 Pa 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  
Article
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
Viewed by 510
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 CH3CN, CH3OH, and the H29α 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 CH3CN, CH3OH, and the H29α 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=1413 rotational ladder of CH3CN and in the CH3OH J=51,441,3 transition. Rotational diagram analysis of CH3CN reveals excitation temperatures ranging from 380 to 430 K. First-order moment maps of CH3CN and CH3OH reveal distinct velocity gradients in all cores, suggesting rotating structures, with core A also showing evidence of expansion motions. The H29α recombination line shows a linewidth of 30.2±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±0.4)×105 cm−3, an emission measure of (1.23±0.06)×109 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  
Review
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
Viewed by 850
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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