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Theory and Observation of Active B-type Stars
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Theory and Observation of Active B-type Stars

A special issue of Galaxies (ISSN 2075-4434).

Deadline for manuscript submissions: closed (15 March 2023) | Viewed by 27300

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editors

Dr. Lydia Sonia Cidale

E-Mail Website
Guest Editor
Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, La Plata (1900), Argentina
Interests: B-type emission line stars
Dr. Michaela Kraus

E-Mail Website
Guest Editor
Astronomický Ústav, Akademie ved Ceské Republiky, Fricova 298, CZ-251 65 Ondrejov, Czech Republic
Interests: B-type emission line stars
Dr. María Laura Arias

E-Mail Website
Guest Editor
Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, La Plata (1900), Argentina
Interests: B-type emission line stars

Special Issue Information

Dear Colleagues,

Active B-type stars are defined by the presence of emission lines formed in a dense environment, often in the form of a circumstellar disk. Among them are the classical Be stars, which are stars surrounded by an ionized gaseous disk, and B[e] stars, whose disks are so dense that even molecules and dust can form within them. In either case, the disks are the source of characteristic infrared excess continuum emission and linear polarization. Active B-type stars often exhibit signatures of variability on different temporal and spatial scales. Of particular importance are variations associated with the formation and dissipation of the gaseous, molecular and dusty disks around the various types of objects that occur on time scales spanning from years to decades. Although mechanisms such as radiation, rotation, pulsation, binarity and magnetic fields and the role they might play in pulling material off the stars were explored in detail over the past decades, the relevant physical processes involved in the formation of sustainable disks and their dynamical evolution are still unclear and a matter of debate.

This Special Issue intends to provide a comprehensive update on the status of the art in the field of active B-type stars. It aims to combine reviews and selected contributions on recent groundbreaking advances in the knowledge of these peculiar objects from both a theoretical and an observational perspective. Latest observations from high-resolution, ground-based facilities and from satellite missions, collected over a wide wavelength range, reveal fascinating details about the shape and structure of circumstellar disks and their interaction with interstellar matter. At the same time, the progress in numerical models combined with ever-increasing computer power facilitate the analysis of complex, multi-dimensional models for a more realistic representation and treatment of the mechanisms behind mass ejection phases and the realistic estimation of the amount of mass lost during some eruptive events. These new insights, from both theory and observations, provide a basis for exploring complex phenomena such as pulsation–wind connections, the transport of angular momentum, viscosity, internal rotation law, and mass loss, and for verifying the various scenarios proposed for the origin, evolution and variability of the circumstellar environment of the diverse classes of active B-type stars.

Potential topics include, but are not limited to:

  • Observational properties of active B-type stars. Surveys, catalogues, tools and novel techniques;
  • Stellar and fundamental parameters;
  • Chemistry and physical structure of ejected material;
  • Outflows and mass loss events; 
  • Wind structure and properties;
  • Rotation, pulsation and pulsation–wind connections;
  • Magnetic fields, magnetospheres and magnetic activity;
  • Binarity and multiple systems;
  • Stellar evolution and evolutionary phases;
  • Decretion disks and circumstellar disk formation;
  • Dynamical evolution of disk, disk growth and dissipation;
  • Evolutionary links among related objects.

References:

Baade et al. 2016, A&A, 588, 56.
Cochetti, Y. R. ; Arias, M. L. ; Kraus, M; et al.  2021, A&A, 647, 164C.
Ghoreyshi et al. 2018, MNRAs, 479, 2214
Halonen, R. & Jones, C. 2013, APJS. 208, 3
Hofmann, K. et al. 2022, A&A, 658,81
Kraus, M. 2019, Galax, 7, 83K
Kriz, S. & Harmanec, P. 1975, Bull. Astron. Inst. Czech., 26, p. 65
Porter, J & Rivinius, T 2003, PASP, Vol. 115, pp. 1153-1170.
Rivinius, T. 2013, ASSP, 31, 253R

Previous proceedings books:

  • The B[e] Phenomenon: Forty Years of Studies: Proceedings of a Conference Held at Charles University, Prague, Czech Republic, 27 June- 1 July 2016, Volume 508 of Astronomical Society of the Pacific conference series. Eds.: Anatoly S. Miroshnichenko, Sergey Zharikov, Daniela Korčáková, Marek Wolf.
  • New windows on massive stars: asteroseismology, interferometry, and spectropolarimetry, Proceedings of the International Astronomical Union, IAU Symposium, 2015, Volume 307, pp. 154-164.
  • Bright Emissaries: Be Stars as Messengers of Star-Disk Physics, Proceedings of a Meeting held at The University of Western Ontario, in London, Ontario, Canada, 11-13 August 2014. Edited by T. A. A. Sigut and C. E. Jones. San Francisco: Astronomical Society of the Pacific, 2016, p.175
  • Circumstellar Dynamics at High Resolution. Proceedings of a Joint ESP/Brazilian Workshop held at Foz do Iguacu, Brazil, 27 February-2 March, 2012. ASP Conference Proceedings, Vol. 464. Edited by A. Carciofi and Th. Rivinius. San Francisco: Astronomical Society of the Pacific, 2012.
  • Active OB stars: structure, evolution, mass loss, and critical limits, Proceedings of the International Astronomical Union, IAU Symposium, Volume 272, 2011. 

Dr. Lydia Sonia Cidale
Dr. Michaela Kraus
Dr. María Laura Arias
Guest Editors

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Keywords

  • stars: emission-line, Be
  • stars: fundamental parameters
  • circumstellar matter
  • stars: activity
  • binaries: general
  • stellar winds
  • hydrodynamics
  • magnetic fields
  • mass loss
  • rotation

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Published Papers (14 papers)

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Research

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28 pages, 1902 KiB  
Article
Unveiling the Evolutionary State of Three B Supergiant Stars: PU Gem, ϵ CMa, and η CMa
by Julieta Paz Sánchez Arias, Péter Németh, Elisson Saldanha da Gama de Almeida, Matias Agustin Ruiz Diaz, Michaela Kraus and Maximiliano Haucke
Galaxies 2023, 11(5), 93; https://doi.org/10.3390/galaxies11050093 - 29 Aug 2023
Cited by 3 | Viewed by 2078
Abstract
We aim to combine asteroseismology, spectroscopy, and evolutionary models to establish a comprehensive picture of the evolution of Galactic blue supergiant stars (BSG). To start such an investigation, we selected three BSG candidates for our analysis: HD 42087 (PU Gem), HD 52089 ( [...] Read more.
We aim to combine asteroseismology, spectroscopy, and evolutionary models to establish a comprehensive picture of the evolution of Galactic blue supergiant stars (BSG). To start such an investigation, we selected three BSG candidates for our analysis: HD 42087 (PU Gem), HD 52089 (ϵ CMa), and HD 58350 (η CMa). These stars show pulsations and were suspected to be in an evolutionary stage either preceding or succeding the red supergiant (RSG) stage. For our analysis, we utilized the 2-min cadence TESS data to study the photometric variability, and we obtained new spectroscopic observations at the CASLEO observatory. We used non-LTE radiative transfer models calculated with CMFGEN to derive their stellar and wind parameters. For the fitting procedure, we included CMFGEN models in the iterative spectral analysis pipeline XTgrid to determine their CNO abundances. The spectral modeling was limited to changing only the effective temperature, surface gravity, CNO abundances, and mass-loss rates. Finally, we compared the derived metal abundances with prediction from Geneva stellar evolution models. The frequency spectra of all three stars show stochastic oscillations and indications of one nonradial strange mode, fr= 0.09321 d−1 in HD 42087 and a rotational splitting centred in f2= 0.36366 d−1 in HD 52089. We conclude that the rather short sectoral observing windows of TESS prevent establishing a reliable mode identification of low frequencies connected to mass-loss variabilities. The spectral analysis confirmed gradual changes in the mass-loss rates, and the derived CNO abundances comply with the values reported in the literature. We were able to achieve a quantitative match with stellar evolution models for the stellar masses and luminosities. However, the spectroscopic surface abundances turned out to be inconsistent with the theoretical predictions. The stars show N enrichment, typical for CNO cycle processed material, but the abundance ratios did not reflect the associated levels of C and O depletion. We found HD 42087 to be the most consistent with a pre-RSG evolutionary stage, HD 58350 is most likely in a post-RSG evolution and HD 52089 shows stellar parameters compatible with a star at the TAMS. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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17 pages, 2043 KiB  
Article
Infrared Spectroscopy of Be Stars: Influence of the Envelope Parameters on Brackett-Series Behaviour
by Yanina Roxana Cochetti, Anahi Granada, María Laura Arias, Andrea Fabiana Torres and Catalina Arcos
Galaxies 2023, 11(4), 90; https://doi.org/10.3390/galaxies11040090 - 17 Aug 2023
Cited by 2 | Viewed by 1423
Abstract
The IR spectra of Be stars display numerous hydrogen recombination lines, constituting a great resource for obtaining information on the physical and dynamic structures of different regions within the circumstellar envelope. Nevertheless, this spectral region has not been analysed in depth, and there [...] Read more.
The IR spectra of Be stars display numerous hydrogen recombination lines, constituting a great resource for obtaining information on the physical and dynamic structures of different regions within the circumstellar envelope. Nevertheless, this spectral region has not been analysed in depth, and there is a lack of synthetic spectra with which to compare observations. Therefore, we computed synthetic spectra with the HDUST code for different disc parameters. Here, we present our results on the spectral region that includes lines of the Brackett series. We discuss the dependence of the line series strengths on several parameters that describe the structure of the disc. We also compared model line profiles, fluxes, and EWs with observational data for two Be stars (MX Pup and π Aqr). Even though the synthetic spectra adequately fit our observations of both stars and allow us to constrain the parameters of the disc, there is a discrepancy with the observed data in the EW and flux measurements, especially in the case of MX Pup. It is possible that by including Brackett lines of higher terms or adding the analysis of other series, we may be able to better constrain the parameters of the observed disc. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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12 pages, 756 KiB  
Article
A Mini Atlas of H-Band Spectra of Southern Symbiotic Stars
by Paula Esther Marchiano, María Laura Arias, Michaela Kraus, Michalis Kourniotis, Andrea Fabiana Torres, Lydia Sonia Cidale and Marcelo Borges Fernandes
Galaxies 2023, 11(4), 80; https://doi.org/10.3390/galaxies11040080 - 22 Jun 2023
Viewed by 1347
Abstract
Symbiotic stars are interacting binary systems composed of an evolved star (generally a late-type red giant) and a degenerate or dwarf companion in orbit close enough for mass transfer to occur. Understanding the status of the late-type star is important for developing binary [...] Read more.
Symbiotic stars are interacting binary systems composed of an evolved star (generally a late-type red giant) and a degenerate or dwarf companion in orbit close enough for mass transfer to occur. Understanding the status of the late-type star is important for developing binary models for the symbiotic systems as it affects the transfer of matter needed to activate the hot component. Infrared observations have been very useful in probing the nature of late-type stars in symbiotic systems. This work presents a set of symbiotic stars observed with SOAR/OSIRIS (R∼3000) in the H-band. We aimed to search for possible molecular circumstellar emission, to characterize the cool companion in these systems, and to confront the new findings with those obtained from the previous K-band classifications. We detected molecular emission from just one object, BI Cru, which displays the second-overtone CO-bands. To fit the observed photospheric CO absorption bands, we used the MARCS atmosphere models. We present our results as a mini atlas of symbiotic stars in the near-infrared region to facilitate the comparison among different observed symbiotic systems. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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21 pages, 1509 KiB  
Article
Discovering New B[e] Supergiants and Candidate Luminous Blue Variables in Nearby Galaxies
by Grigoris Maravelias, Stephan de Wit, Alceste Z. Bonanos, Frank Tramper, Gonzalo Munoz-Sanchez and Evangelia Christodoulou
Galaxies 2023, 11(3), 79; https://doi.org/10.3390/galaxies11030079 - 19 Jun 2023
Cited by 3 | Viewed by 1619
Abstract
Mass loss is one of the key parameters that determine stellar evolution. Despite the progress we have achieved over the last decades we still cannot match the observational derived values with theoretical predictions. Even worse, there are certain phases, such as the B[e] [...] Read more.
Mass loss is one of the key parameters that determine stellar evolution. Despite the progress we have achieved over the last decades we still cannot match the observational derived values with theoretical predictions. Even worse, there are certain phases, such as the B[e] supergiants (B[e]SGs) and the Luminous Blue Variables (LBVs), where significant mass is lost through episodic or outburst activity. This leads to various structures forming around them that permit dust formation, making these objects bright IR sources. The ASSESS project aims to determine the role of episodic mass in the evolution of massive stars, by examining large numbers of cool and hot objects (such as B[e]SGs/LBVs). For this purpose, we initiated a large observation campaign to obtain spectroscopic data for ∼1000 IR-selected sources in 27 nearby galaxies. Within this project we successfully identified seven B[e] supergiants (one candidate) and four Luminous Blue Variables of which six and two, respectively, are new discoveries. We used spectroscopic, photometric, and light curve information to better constrain the nature of the reported objects. We particularly noted the presence of B[e]SGs at metallicity environments as low as 0.14 Z⊙. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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23 pages, 968 KiB  
Article
Dense Molecular Environments of B[e] Supergiants and Yellow Hypergiants
by Michaela Kraus, Michalis Kourniotis, María Laura Arias, Andrea F. Torres and Dieter H. Nickeler
Galaxies 2023, 11(3), 76; https://doi.org/10.3390/galaxies11030076 - 16 Jun 2023
Cited by 3 | Viewed by 1482
Abstract
Massive stars expel large amounts of mass during their late evolutionary phases. We aim to unveil the physical conditions within the warm molecular environments of B[e] supergiants (B[e]SGs) and yellow hypergiants (YHGs), which are known to be embedded in circumstellar shells and disks. [...] Read more.
Massive stars expel large amounts of mass during their late evolutionary phases. We aim to unveil the physical conditions within the warm molecular environments of B[e] supergiants (B[e]SGs) and yellow hypergiants (YHGs), which are known to be embedded in circumstellar shells and disks. We present K-band spectra of two B[e]SGs from the Large Magellanic Cloud and four Galactic YHGs. The CO band emission detected from the B[e]SGs LHA 120-S 12 and LHA 120-S 134 suggests that these stars are surrounded by stable rotating molecular rings. The spectra of the YHGs display a rather diverse appearance. The objects 6 Cas and V509 Cas lack any molecular features. The star [FMR2006] 15 displays blue-shifted CO bands in emission, which might be explained by a possible close to pole-on oriented bipolar outflow. In contrast, HD 179821 shows blue-shifted CO bands in absorption. While the star itself is too hot to form molecules in its outer atmosphere, we propose that it might have experienced a recent outburst. We speculate that we currently can only see the approaching part of the expelled matter because the star itself might still block the receding parts of a (possibly) expanding gas shell. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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20 pages, 936 KiB  
Article
New Insight into the FS CMa System MWC 645 from Near-Infrared and Optical Spectroscopy
by Andrea Fabiana Torres, María Laura Arias, Michaela Kraus, Lorena Verónica Mercanti and Tõnis Eenmäe
Galaxies 2023, 11(3), 72; https://doi.org/10.3390/galaxies11030072 - 10 Jun 2023
Viewed by 1513
Abstract
The B[e] phenomenon is manifested by a heterogeneous group of stars surrounded by gaseous and dusty circumstellar envelopes with similar physical conditions. Among these stars, the FS CMa-type objects are suspected to be binary systems, which could be experiencing or have undergone a [...] Read more.
The B[e] phenomenon is manifested by a heterogeneous group of stars surrounded by gaseous and dusty circumstellar envelopes with similar physical conditions. Among these stars, the FS CMa-type objects are suspected to be binary systems, which could be experiencing or have undergone a mass-transfer process that could explain the large amount of material surrounding them. We aim to contribute to the knowledge of a recently confirmed binary, MWC 645, which could be undergoing an active mass-transfer process. We present near-infrared and optical spectra, identify atomic and molecular spectral features, and derive different quantitative properties of line profiles. Based on publicly available photometric data, we search for periodicity in the light curve and model the spectral energy distribution. We have detected molecular bands of CO in absorption at 1.62 μm and 2.3 μm for the first time. We derive an upper limit for the effective temperature of the cool binary component. We found a correlation between the enhancement of the Hα emission and the decrease in optical brightness that could be associated with mass-ejection events or an increase in mass loss. We outline the global properties of the envelope, possibly responsible for brightness variations due to a variable extinction, and briefly speculate on different possible scenarios. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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40 pages, 90706 KiB  
Article
Synthetic Light Curve Design for Pulsating Binary Stars to Compare the Efficiency in the Detection of Periodicities
by Aldana Alberici Adam, Gunther F. Avila Marín, Alejandra Christen and Lydia Sonia Cidale
Galaxies 2023, 11(3), 69; https://doi.org/10.3390/galaxies11030069 - 31 May 2023
Cited by 1 | Viewed by 1332
Abstract
B supergiant stars pulsate in regular and quasi-regular oscillations resulting in intricate light variations that might conceal their binary nature. To discuss possible observational bias in a light curve, we performed a simulation design of a binary star affected by sinusoidal functions emulating [...] Read more.
B supergiant stars pulsate in regular and quasi-regular oscillations resulting in intricate light variations that might conceal their binary nature. To discuss possible observational bias in a light curve, we performed a simulation design of a binary star affected by sinusoidal functions emulating pulsation phenomena. The Period04 tool and the WaveletComp package of R were used for this purpose. Thirty-two models were analysed based on a combination of two values on each of the k = 6 variables, such as multiple pulsations, the amplitude of the pulsation, the pulsation frequency, the beating phenomenon, the light-time effect, and regular or quasi-regular periods. These synthetic models, unlike others, consider an ARMA (1, 1) statistical noise, irregular sampling, and a gap of about 4 days. Comparing Morlet wavelet with Fourier methods, we observed that the orbital period and its harmonics were well detected in most cases. Although the Fourier method provided more accurate period detection, the wavelet analysis found it more times. Periods seen with the wavelet method have a shift due to the slightly irregular time scale used. The pulsation period hitting rate depends on the wave amplitude and frequency with respect to eclipse depth and orbital period. None of the methods was able to distinguish accurate periods leading to a beating phenomenon when they were longer than the orbital period, resulting, in both cases, in an intermediate value. When the beating period was shorter, the Fourier analysis found it in all cases except for unsolved quasi-regular periods. Overall, the Morlet wavelet analysis performance was lower than the Fourier analysis. Considering the strengths and disadvantages found in these methods, we recommend using at least two diagnosis tools for a detailed time series data analysis to obtain confident results. Moreover, a fine-tuning of trial periods by applying phase diagrams would be helpful for recovering accurate values. The combined analysis could reduce observational bias in searching binaries using photometric techniques. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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14 pages, 37354 KiB  
Article
Large-Scale Ejecta of Z CMa—Proper Motion Study and New Features Discovered
by Tiina Liimets, Michaela Kraus, Lydia Cidale, Sergey Karpov and Anthony Marston
Galaxies 2023, 11(3), 64; https://doi.org/10.3390/galaxies11030064 - 4 May 2023
Cited by 1 | Viewed by 1493
Abstract
Z Canis Majoris is a fascinating early-type binary with a Herbig Be primary and a FU Orionis-type secondary. Both of the stars exhibit sub-arcsecond jet-like ejecta. In addition, the primary is associated with the extended jet as well as with the large-scale outflow. [...] Read more.
Z Canis Majoris is a fascinating early-type binary with a Herbig Be primary and a FU Orionis-type secondary. Both of the stars exhibit sub-arcsecond jet-like ejecta. In addition, the primary is associated with the extended jet as well as with the large-scale outflow. In this study, we investigate further the nature of the large-scale outflow, which has not been studied since its discovery almost three and a half decades ago. We present proper motion measurements of individual features of the large-scale outflow and determine their kinematical ages. Furthermore, with our newly acquired deep images, we have discovered additional faint arc-shaped features that can be associated with the central binary. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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15 pages, 2491 KiB  
Article
Revealing the Binarity of HD 36030—One of the Hottest Flare Stars
by Olga Maryeva, Péter Németh and Sergey Karpov
Galaxies 2023, 11(2), 55; https://doi.org/10.3390/galaxies11020055 - 12 Apr 2023
Cited by 1 | Viewed by 1756
Abstract
The Kepler and TESS space missions significantly expanded our knowledge of what types of stars display flaring activity by recording a vast amount of super-flares from solar-like stars, as well as detecting flares from hotter stars of A-F spectral types. Currently, we know [...] Read more.
The Kepler and TESS space missions significantly expanded our knowledge of what types of stars display flaring activity by recording a vast amount of super-flares from solar-like stars, as well as detecting flares from hotter stars of A-F spectral types. Currently, we know that flaring occurs in the stars as hot as B-type ones. However, the structures of atmospheres of hot B-A stars crucially differ from the ones of late types, and thus the occurrence of flaring in B-A type stars requires some extension of our theoretical views of flare formation and therefore a detailed study of individual objects. Here we present the results of our spectral and photometric study of HD 36030, which is a B9 V star with flares detected by the TESS satellite. The spectra we acquired suggest that the star is in a binary system with a low-mass secondary component, but the light curve lacks any signs of periodic variability related to orbital motion or surface magnetic fields. Because of that, we argue that the flares originate due to magnetic interaction between the components of the system. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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12 pages, 528 KiB  
Article
New Method to Detect and Characterize Active Be Star Candidates in Open Clusters
by Anahí Granada, Maziar R. Ghoreyshi, Carol E. Jones and Tõnis Eenmäe
Galaxies 2023, 11(1), 37; https://doi.org/10.3390/galaxies11010037 - 19 Feb 2023
Cited by 1 | Viewed by 1746
Abstract
With the aim of better understanding the physical conditions under which Be stars form and evolve, it is imperative to further investigate whether poorly studied young open clusters host Be stars. In this work, we explain how data from Gaia DR2 and DR3 [...] Read more.
With the aim of better understanding the physical conditions under which Be stars form and evolve, it is imperative to further investigate whether poorly studied young open clusters host Be stars. In this work, we explain how data from Gaia DR2 and DR3 can be combined to recover and characterize active Be stars in open clusters. We test our methodology in four open clusters broadly studied in the literature, known for hosting numerous Be stars. In addition, we show that the disk formation and dissipation approach that is typically used to model long term Be star variability, can explain the observed trends for Be stars in a (GDR3-GDR2) versus GDR3 plot. We propose that extending this methodology to other open clusters, and, in particular, those that are poorly studied, will help to increase the number of Be candidates. Eventually, Be stars may eclipse binary systems in open clusters. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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22 pages, 830 KiB  
Article
Reddening-Free Q Parameters to Classify B-Type Stars with Emission Lines
by Yael Aidelman and Lydia Sonia Cidale
Galaxies 2023, 11(1), 31; https://doi.org/10.3390/galaxies11010031 - 15 Feb 2023
Cited by 2 | Viewed by 2013
Abstract
The emission-line B-type stars constitute a heterogeneous group. Many of these stars show similar optical spectroscopic features and color indices, making it difficult to classify them adequately by means of photometric and spectroscopic techniques. Thus, it is relevant to deal with appropriate classification [...] Read more.
The emission-line B-type stars constitute a heterogeneous group. Many of these stars show similar optical spectroscopic features and color indices, making it difficult to classify them adequately by means of photometric and spectroscopic techniques. Thus, it is relevant to deal with appropriate classification criteria to avoid as many selection effects as possible. For this purpose, we analyzed different reddening-free Q parameters, taking advantage of the Gaia and 2MASS photometric surveys, for both main sequence and emission-line B-type stars. Along with this work, we provided various criteria to search for normal and emission-line B-type stars, using different color–color, Q–color, and Q–Q diagrams. It was also possible to identify stars in different transition phases (i.e., (Rp−J) vs. (J−Ks) diagrams) and to classify them according to their NIR radiation excesses (i.e., the (Bp−Rp) vs. (H−Ks) diagram). Other diagrams, such as the QJKHK vs. (H−Ks) or QBpJHK vs. (Bp−Ks), were very useful to search for and classify different classes of B-type stars with emission lines. These diagrams highlighted the presence of several stars, classified as CBe, with large color excesses that seemed to be caused by the presence of dust in their envelopes. Therefore, these stars would be misclassified. Three groups of HAeBe stars with different intrinsic dust properties were also distinguished. The amount of intrinsic dust emission in the diverse groups of emission-line stars was well-recognized via the QJHK vs. QBpRpHK diagram. The different selection criteria are very important tools for automated designs of machine learning and optimal search algorithms. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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20 pages, 2457 KiB  
Article
Hidden Spectra Treasures in the Foster Archive: A Pilot Study of the Be Stars α Eri, α Col, ω Car and η Cen
by Catalina Arcos, Leonardo Vanzi, Nikolaus Vogt, Stefano Garcia, Virginia Ortiz and Ester Acuña
Galaxies 2022, 10(6), 106; https://doi.org/10.3390/galaxies10060106 - 22 Nov 2022
Viewed by 1754
Abstract
We present the archive of spectroscopic photographic plates of the Universidad Católica historic observatory Manuel Foster. The archive includes more than 4800 plates covering the period of time from 1928 to 1991. The spectra present in the archive are mostly those of bright [...] Read more.
We present the archive of spectroscopic photographic plates of the Universidad Católica historic observatory Manuel Foster. The archive includes more than 4800 plates covering the period of time from 1928 to 1991. The spectra present in the archive are mostly those of bright variable or binary stars observed at different epochs. We developed a method of digitalization and data processing for the plates and verified it through the analysis of a selected sample of spectra. As an example of the potential relevance of this Foster archive we studied the variation of helium, Hβ and Hγ spectral lines over time (1980−1991), complementing with high resolution spectroscopic data from the “Be Star Observation Survey” (2012−2015), of four Be stars mainly, α Eri, α Col, ω Car and η Cen. The spectra of these stars show evidence of a circumstellar gas disk present in both periods of time. From the spectroscopic analysis, we found these stars are variable in helium and this variability presents an opposite behavior with the variability observed in the EW of the Hβ line profile. This archive represents a unique source of data from past that is available for the use of the community. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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Review

Jump to: Research

33 pages, 2909 KiB  
Review
Radiation-Driven Wind Hydrodynamics of Massive Stars: A Review
by Michel Curé and Ignacio Araya
Galaxies 2023, 11(3), 68; https://doi.org/10.3390/galaxies11030068 - 12 May 2023
Cited by 8 | Viewed by 2299
Abstract
Mass loss from massive stars plays a determining role in their evolution through the upper Hertzsprung–Russell diagram. The hydrodynamic theory that describes their steady-state winds is the line-driven wind theory (m-CAK). From this theory, the mass loss rate and the velocity profile of [...] Read more.
Mass loss from massive stars plays a determining role in their evolution through the upper Hertzsprung–Russell diagram. The hydrodynamic theory that describes their steady-state winds is the line-driven wind theory (m-CAK). From this theory, the mass loss rate and the velocity profile of the wind can be derived, and estimating these properly will have a profound impact on quantitative spectroscopy analyses from the spectra of these objects. Currently, the so-called β law, which is an approximation for the fast solution, is widely used instead of m-CAK hydrodynamics, and when the derived value is β≳1.2, there is no hydrodynamic justification for these values. This review focuses on (1) a detailed topological analysis of the equation of motion (EoM), (2) solving the EoM numerically for all three different (fast and two slow) wind solutions, (3) deriving analytical approximations for the velocity profile via the LambertW function and (4) presenting a discussion of the applicability of the slow solutions. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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103 pages, 3133 KiB  
Review
BCD Spectrophotometry and Rotation of Active B-Type Stars: Theory and Observations
by Juan Zorec
Galaxies 2023, 11(2), 54; https://doi.org/10.3390/galaxies11020054 - 10 Apr 2023
Cited by 6 | Viewed by 3684
Abstract
This review has two parts. The first one is devoted to the Barbier–Chalonge–Divan (BCD) spectrophotometric system, also known as the Paris spectral classification system. Although the BCD system has been applied and is still used for all stellar objects from O to F [...] Read more.
This review has two parts. The first one is devoted to the Barbier–Chalonge–Divan (BCD) spectrophotometric system, also known as the Paris spectral classification system. Although the BCD system has been applied and is still used for all stellar objects from O to F spectral types, the present account mainly concerns normal and ‘active’ B-type stars. The second part treats topics related to stellar rotation, considered one of the key phenomena determining the structure and evolution of stars. The first part is eminently observational. In contrast, the second part deals with observational aspects related to stellar rotation but also recalls some supporting or basic theoretical concepts that may help better understand the gains and shortcomings of today’s existent interpretation of stellar data. Full article
(This article belongs to the Special Issue Theory and Observation of Active B-type Stars)
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