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11 pages, 682 KiB

Open AccessArticle

A Spectroscopic and Photometric Study of MWC 342 and Its B[e] Phenomenon over the Last 40 Years

by Aigerim Zh. Akniyazova, Anatoly S. Miroshnichenko, Sergey V. Zharikov, Hans Van Winckel, Nadine Manset, Ashish Raj, Stephen Drew Chojnowski, Serik A. Khokhlov, Inna V. Reva, Raushan I. Kokumbaeva, Chingis T. Omarov, Konstantin N. Grankin, Aldiyar T. Agishev and Nadezhda L. Vaidman

Galaxies 2025, 13(3), 63; https://doi.org/10.3390/galaxies13030063 - 20 May 2025

Viewed by 82

Abstract

MWC 342 (V1972 Cyg) was discovered nearly 90 years ago as an early-type emission-line star. It was among the first hot stars whose strong infrared excess was detected in the early 1970s. Several mostly short-term photometric and spectroscopic studies resulted in contradictory conclusions [...] Read more.

MWC 342 (V1972 Cyg) was discovered nearly 90 years ago as an early-type emission-line star. It was among the first hot stars whose strong infrared excess was detected in the early 1970s. Several mostly short-term photometric and spectroscopic studies resulted in contradictory conclusions about the nature and evolutionary status of MWC 342. It has been classified as a pre-main-sequence Herbig Be star, an evolved suspected binary system, and a long-period variable star. Suggestions on the nature of the secondary component to this B0/B1 primary included a cool M-type giant and an X-ray source. We collected medium- and high-resolution optical spectra of MWC 342 taken in 1994–2024 as well as optical photometric data taken in 1986–2024. Analysis of these data shows strong variations in the object’s brightness and spectral line properties at various time scales, but no strictly periodic phenomena have been found. Inparticular, such a long-term dataset allowed us to reveal the optical brightness variations over a nearly 20-year-long quasi-period, as well as their anti-correlation with the H

α

emission-line strength. Also, we did not confirm the presence of He ii emission lines and absorption lines of the star’s atmosphere that were suspected in previously published studies. Full article

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

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11 pages, 887 KiB

Open AccessArticle

Investigation of the Nature of the B[e] Star CI Cam in the Optical Range

by Elena A. Barsukova, Vitaly P. Goranskij, Aleksandr N. Burenkov and Ilya A. Yakunin

Galaxies 2025, 13(3), 61; https://doi.org/10.3390/galaxies13030061 - 19 May 2025

Viewed by 126

Abstract

The B[e] phenomenon is observed in a wide range of stars at various evolutionary stages. Its nature remains uncertain. The B[e] phenomenon is defined as the simultaneous presence of low-excitation forbidden line emission and strong infrared excess in the spectra of early-type stars. [...] Read more.

The B[e] phenomenon is observed in a wide range of stars at various evolutionary stages. Its nature remains uncertain. The B[e] phenomenon is defined as the simultaneous presence of low-excitation forbidden line emission and strong infrared excess in the spectra of early-type stars. Here, we present new spectral observations of a representative of this class: the star CI Cam. A monitoring campaign was carried out for the He II 4686 Å emission line, which serves as an indicator of binarity in this system. The aim was to detect variations in this line not only due to orbital motion but also those associated with the pulsations of the system’s primary component, the B[e] star. Two maxima in the equivalent width were detected over the pulsation period, during which the equivalent width increased by a factor of three. We refine the classification of CI Cam, assigning it to the FS CMa group of B[e] stars by all criteria, and we refer the secondary component of the system to a group of recently discovered “stripped” stars. Full article

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

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13 pages, 5876 KiB

Open AccessArticle

V694 Mon: A Recent Event of Mass Transfer in the Dynamical Mode

by Vitaly P. Goranskij, Elena A. Barsukova, Aleksandr N. Burenkov, Natalia V. Metlova, Alla V. Zharova and Ilya A. Yakunin

Galaxies 2025, 13(3), 59; https://doi.org/10.3390/galaxies13030059 - 14 May 2025

Viewed by 135

Abstract

The phenomenon of runaway mass transfers between components of binary systems on a dynamical timescale has been theoretically predicted. However, this phenomenon has been observed for the first time in the history of astronomy just now in a symbiotic system V694 Mon. We [...] Read more.

The phenomenon of runaway mass transfers between components of binary systems on a dynamical timescale has been theoretically predicted. However, this phenomenon has been observed for the first time in the history of astronomy just now in a symbiotic system V694 Mon. We employed medium- and high-dispersion spectroscopy, along with multicolor photometry, to study this event in detail. Over 6 years, beginning in 2018, we observed the cessation of disk accretion, the filling of the accretor’s Roche lobe, and the subsequent formation of an A-type star within it. The pulsating envelope of the M giant donor was transferred to the accretor down to its base. Thus, the products from the hydrogen-burning layer appeared on the donor’s surface, and a flash of an emission-line spectrum enriched with s-process elements was detected. We discuss discrepancies between theoretical predictions and observations, as well as other phenomena potentially related to dynamical mass transfer. Full article

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

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8 pages, 246 KiB

Open AccessArticle

Studies of Hot Stars and Other Observational Programs Using the 1-Meter Optical Telescope Zeiss-1000 of SAO RAS

by Vladimir V. Komarov, Victoria N. Komarova and Alexander S. Moskvitin

Galaxies 2025, 13(3), 58; https://doi.org/10.3390/galaxies13030058 - 13 May 2025

Viewed by 177

Abstract

Here, we briefly describe the current state of the Zeiss-1000 telescope of the Special Astrophysical Observatory of Russian Academy of Sciences (SAO RAS). Principal attention is given to research programs from recent years. The observations made according to allocated requests both by researchers [...] Read more.

Here, we briefly describe the current state of the Zeiss-1000 telescope of the Special Astrophysical Observatory of Russian Academy of Sciences (SAO RAS). Principal attention is given to research programs from recent years. The observations made according to allocated requests both by researchers from the observatory and scientists from other institutions and organizations are planned within a half-year schedule. The instrumental facilities provide a wide range of methods for studying objects of interest. They include standard photometry, moderate- and high-resolution spectroscopy, and polarimetry, along with unique “guest” methods (e.g., emission line imaging). The research programs cover different fields of astrophysics. The topic of “hot stars” and other important developments have become possible due to the introduction of new research methods at the telescope. Blazars, gamma-ray burst optical transients, massive supernovae, cataclysmic variables, magnetic stars, white dwarfs, luminous blue variables, red dwarfs, and many others are among its targets. Full article

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

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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

Viewed by 144

Abstract

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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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

Viewed by 173

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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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 262

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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6 pages, 228 KiB

Open AccessArticle

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

by Nina Beskrovnaya, Nazar Ikhsanov and Vitaliy Kim

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

Viewed by 245

Abstract

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

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

km s^{- 1}

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

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

11 pages, 2062 KiB

Open AccessArticle

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

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

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

Viewed by 325

Abstract

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

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

T_{eff} = 9800 \pm 300

K, log L/

L_{⊙} = 1.86 \pm 0.06

,

v sin i = 9 \pm 2

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

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

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11 pages, 520 KiB

Open AccessArticle

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

by Natalia Ikonnikova, Marina Burlak and Alexander Dodin

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

Viewed by 277

Abstract

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

U B V R_{C} I_{C}

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

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

U B V R_{C} I_{C}

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

0 .^{m} 3

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

N_{e}

∼ 10⁴ cm⁻³,

T_{e}

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

V_{r} = - 141 \pm 7

km s⁻¹. Full article

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

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18 pages, 1207 KiB

Open AccessArticle

Multiplicity of Luminous Blue Variable Stars

by A. Lobel and N. Gorlova

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

Viewed by 336

Abstract

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

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

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

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11 pages, 479 KiB

Open AccessArticle

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

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

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

Cited by 1 | Viewed by 284

Abstract

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

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

P = 428.474 \pm 0.002

days, eccentricity

e = 0.29 \pm 0.01

, argument of periastron

ω = {242.3}^{\circ} \pm {0.3}^{\circ}

, epoch of periastron

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

, heliocentric systemic radial velocity

γ = - 30.12 \pm 0.09

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

K_{1} = 15.52 \pm 0.13

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

0.71 \pm 0.10

mas), corresponding to a distance of

1397 \pm 170

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

m_{V} = 5.35

mag). The derived luminosity,

log (L / L_{⊙}) = 4.22 \pm 0.12

, suggests an initial mass of 3.0–4.0

M_{⊙}

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

\sim 0.75 M_{⊙}

for the primary and

0.70

–

0.82 M_{⊙}

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

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

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10 pages, 551 KiB

Open AccessArticle

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

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

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

Viewed by 494

Abstract

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

μ

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

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

μ

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

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

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Review

Jump to: Research

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 230

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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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 274

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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15 pages, 507 KiB

Open AccessReview

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

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

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

Viewed by 347

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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8 pages, 1341 KiB

Open AccessReview

Chemical Synthesis in the Circumstellar Environment

by Sun Kwok

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

Viewed by 271

Abstract

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

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

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

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13 pages, 4528 KiB

Open AccessReview

Hot Stars in Stellar Populations of Galaxies

by Claus Leitherer

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

Viewed by 576

Abstract

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

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

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

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10 pages, 426 KiB

Open AccessReview

The Blue Supergiant Problem and the Main-Sequence Width

by Jorick S. Vink and Rene D. Oudmaijer

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

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Abstract

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

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

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

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7 pages, 215 KiB

Open AccessReview

Going Forward to Unveil the Nature of γ Cas Analogs

by Yaël Nazé

Galaxies 2025, 13(1), 8; https://doi.org/10.3390/galaxies13010008 - 24 Jan 2025

Viewed by 502

Abstract

The star

γ

Cas and its analogs are a subset of Be stars that display particularly hard and bright thermal X-ray emission, which has no equivalent among other massive stars. Here, I will review their characteristics and present the latest results of our [...] Read more.

The star

γ

Cas and its analogs are a subset of Be stars that display particularly hard and bright thermal X-ray emission, which has no equivalent among other massive stars. Here, I will review their characteristics and present the latest results of our optical and X-ray monitoring campaigns, including an assessment of the links between the circumstellar environment and the high-energy properties. Possible scenarios to explain this phenomenon will be presented in light of these observational results. Full article

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

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