arXiv subjects

Th. Rivinius

Publications and source records attributed to Th. Rivinius.

At least 19 recordsLinked to original sources

arXiv2025

$\delta$ Circini: A massive hierarchical triple system with an eclipsing binary

$\delta$ Circini is known to be a massive multiple system containing a 3.9 d inner eclipsing binary in a slightly elliptical orbit exhibiting slow apsidal motion and a distant tertiary with a probable period of 1644 d. All three components of the system are O- or B-type stars. We carried out a comprehensive study of the system, based on light curves from TESS and other instruments, a new series of echelle spectra, older spectra from the ESO archive, and several VLTI interferometric observations. Due to the large amount of different types of data covering both orbits in the system, we obtained a more precise value of the long orbital period ($1603.24\pm0.19$ d) and fully determined all other orbital parameters. Although both orbits are eccentric, their period ratio is large enough for the system to be dynamically stable. The inner and outer orbits are in the same plane, which means that no Kozai-Lidov mechanism is acting in the system. Assuming solar metallicity in our MESA models, we found ages of $(4.4\pm 0.1)$, $(4.7\pm 0.2)$, and $(3.8\pm1.3)$ Myr for the primary, the secondary, and the tertiary, respectively. Our evolutionary scenario predicts that the inner eclipsing binary will merge within approximately 1.7 Myr and eventually evolve into a black hole. The distance to the system, estimated from the angular size of the outer orbit is $(809.9 \pm 1.8)$ pc, which implies that $\delta$ Cir might be located close to the centre of a stellar population ASCC 79, a subgroup of the young Circinus complex. With a total mass of $(53.04\pm0.29)$ M$_{\odot}$, $\delta$ Cir can contribute a significant fraction of the total mass of the population.

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arXiv2025

The mid-infrared spectrum of $\beta$ Pictoris b. First VLTI/MATISSE interferometric observations of an exoplanet

Few spectra of directly-imaged exoplanets have been obtained in the mid-infrared (> 3 $\mu$m). This region is particularly rich in molecular spectral signatures, whose measurements can help recover atmospheric parameters and provide a better understanding of giant planet formation and atmospheric dynamics. In the past years, exoplanet interferometry with the VLTI/GRAVITY instrument has provided medium-resolution spectra of a dozen substellar companions in the near infrared. The 100-meter interferometric baselines allow for the stellar and planetary signals to be efficiently disentangled at close angular separations (< 0.3''). We aim to extend this technique to the mid-infrared using MATISSE, the VLTI's mid-infrared spectro-interferometer. We take advantage of the fringe tracking and off-axis pointing capabilities recently brought by the GRA4MAT upgrade. Using this new mode, we observed the giant planet $\beta$ Pictoris b in L and M bands (2.75-5 $\mu$m) at a spectral resolution of 500. We developed a method to correct chromatic dispersion and non-common paths effects in the fringe phase and modelled the planet astrometry and stellar contamination. We obtained a high-signal-to-noise spectrum of $\beta$ Pictoris b, showing the planet continuum in L (for the first time) and M bands, which contains broad absorption features of H$_2$O and CO. In conjunction with a new GRAVITY spectrum, we modelled it with the ForMoSA nested sampling tool and the Exo-REM grid of atmospheric models, and found a solar carbon-to-oxygen ratio in the planet atmosphere. This study opens the way to the characterization of fainter and closer-in planets with MATISSE, which could complement the JWST at angular separations too close for it to obtain exoplanet spectra. Starting in 2025, the new adaptive optics system brought by the GRAVITY+ upgrade will further extend the detection limits of MATISSE.

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arXiv2025

Spatially resolved centrifugal magnetosphere caught in motion around the secondary component of $\rho$ Oph A

The recently discovered spectroscopic binary $\rho$ Oph A is a rare magnetic hot binary system composed of two early B-type stars, comprising a non-magnetic primary (Aa) and a slightly less massive magnetic secondary (Ab). Using near-IR interferometry, we aim to resolve the system's astrometric orbit. The high-spectral resolution VLTI/GRAVITY data also enables obtaining further information about the two stellar components, and about the centrifugal magnetosphere orbiting the magnetic star. We obtained a time-series of $K$-band interferometric data from VLTI/GRAVITY, and analyzed them with the geometrical model-fitting tool PMOIRED. The continuum was used to derive the relative astrometry and relative fluxes of the two components, leading to the astrometric orbital solution. Spectro-interferometry covering the Br$\gamma$ line was then used to obtain high-angular-resolution information about the dominant magnetospheric cloud in corotation with the magnetic Ab component. The combination of the astrometric orbital solution with the published radial velocities for both components led to a 3-dimensional orbital solution, dynamical masses, and a dynamical parallax, revealing a good agreement with previous estimates and confirming the previously inferred alignment of the orbital axis and the rotation axis of the Ab component, albeit at a much higher precision. Detailed analysis of the Br$\gamma$ spectrointerferometry then revealed the changing position of the magnetospheric cloud relative to the Ab component. Comparison to the location of the cloud predicted from $\rho$ Oph Ab's oblique rotator model and H$\alpha$ emission properties demonstrated excellent agreement. We are additionally able to demonstrate that the magnetic star exhibits prograde rotation. This represents the first time that the motion of a centrifugal magnetosphere has been detected in angularly resolved observations.

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arXiv2025

Discovery of the binary nature of the magnetospheric B-type star $\rho$ Oph A

The nearby B-type star $\rho$ Oph A was recently identified as a rapidly rotating magnetic B-type star with variable radio and X-ray emission consistent with a magnetospheric origin. We present a high-resolution spectropolarimetric time series obtained with ESPaDOnS, which we use to perform a magnetic analysis using least-squares deconvolution (LSD). We find that $\rho$ Oph A is a spectroscopic binary consisting of two B-type stars with masses of about 8 and 10 $M_\odot$ on a slightly eccentric 88-day orbit, with the magnetic field being associated with the smaller Ab component. This leads to $\rho$ Oph Ab's 4 kG surface magnetic dipole being approximately twice as strong as the previously reported 2 kG. The oblique rotator model derived from the longitudinal magnetic field curve agrees well with the LSD Stokes $V$ profiles, indicating that the magnetic field is likely to be very nearly dipolar. The orbital and rotational axes appear to be aligned. We report for the first time $\rho$ Oph Ab's magnetospheric H$\alpha$ emission, which is consistent with an origin in a centrifugal magnetosphere. We also demonstrate that $\rho$ Oph Ab's light curve can be recovered from the previously reported {\em Kepler-2} light curve of $\rho$ Oph C, and that it demonstrates prominent magnetospheric eclipses similar to those of $\sigma$ Ori E, and which can be reproduced using a dipolar Rigidly Rotating Magnetosphere model. Both H$\alpha$ and the light curve are indicative of a strongly asymmetric magnetosphere. All indications are that $\rho$ Oph Ab's magnetic field is essentially dipolar, meaning that contributions from higher-order multipoles probably cannot explain the strong asymmetry in the magnetosphere. Only two other stars show comparable degrees of asymmetry, both of which are also close binaries, suggesting that binarity can affect the magnetospheric plasma distributions.

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arXiv2025

VLTI/GRAVITY enables the determination of the first dynamical masses of a classical Be + stripped and bloated pre-subdwarf binary

HR~6819 is the first post-mass transfer binary system composed of a classical Be star and a bloated pre-subdwarf stripped star directly confirmed by interferometry. While the Be star is already spun up to near-critical rotation and possesses a self-ejected viscous Keplerian disk, the stripped star is found in a short-lived evolutionary stage, in which it retains the spectral appearance of a B-type main-sequence star while contracting into a faint subdwarf OB-type star. In order to understand the evolution of intermediate-mass interacting binaries, the fundamental parameters of cornerstone objects such as HR~6819 need to be known. We aim to obtain orbital parameters and model-independent dynamical masses of this binary system to quantitatively characterize this rarely observed evolutionary stage. We analyzed a time series of 12 interferometric near-IR $K$-band observations from VLTI/GRAVITY with the help of the geometrical model-fitting tool PMOIRED. We included recently published radial velocities based on FEROS high-resolution spectroscopy for the binary orbital solution. With the GRAVITY data, we obtained the astrometric orbit, relative fluxes of the components, and parameters of the circumstellar disk of the Be star; we also detected helium line signatures from the stripped star. Using the published radial velocities enabled us to obtain the dynamical masses of the components as well as the dynamical parallax. The Be star is the slightly brighter component in the $K$ band and is almost 16 times as massive as the bloated stripped star, with the individual dynamical masses being $4.24\pm0.31 {\rm M}_{\odot}$ for the Be star and $0.270\pm0.056 {\rm M}_{\odot}$ for the stripped star. The orbit is slightly eccentric, with $e=0.0289\pm0.0058$, and the semimajor axis of the orbit is $0.3800\pm0.0093$ AU. (Abstract continues but does not fit here)

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arXiv2024

Newborn Be star systems observed shortly after mass transfer

Many classical Be stars acquire their very rapid rotation by mass and angular-momentum transfer in massive binaries. Short-lived intermediate-phase objects have only been discovered recently. Data archives and the literature have been searched for additional candidates exhibiting this patterns. Thirteen candidates were identified at various confidence levels. Adding to the two known systems identified as classical Be star+pre-subdwarf binaries (LB-1 and HR6819), two more (V742Cas, HD44637) could be confirmed with interferometry, with V742Cas setting a new record for the smallest visually observed angular semi-major axis, at a=0.663mas. Two further ones (V447Sct, V1362Cyg) are not resolved interferometrically, but other evidence puts them at the same confidence level as LB-1. V2174Cyg is a candidate with very high confidence, but was not observed interferometrically. The remaining ones are either candidates with varying levels of confidence. Of a mostly magnitude complete sample of 328 Be stars, 0.5-1% are found to have recently completed the mass overflow that led to their formation. Another 5% are systems with compact subdwarf companions, i.e., further evolved after a previous overflow, and possibly two more percent harbor white dwarfs. All these systems are of early B-subtypes, however, and if the original sample is restricted to early subtypes (136 objects), these percentages increase by a factor of about 2.5, while dropping to zero for the mid and late subtypes (together 204 objects). This strongly suggests that early- vs. mid- and late-type Be stars have differently weighted channels to acquire their rapid rotation, namely binary interaction vs. evolutionary spin-up.

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arXiv2022

Dynamical masses of the primary Be star and the secondary sdB star in the single-lined binary kappa Dra (B6 IIIe)

Because many classical Be stars may owe their nature to mass and angular-momentum transfer in a close binary, the present masses, temperatures, and radii of their components are of high interest for comparison to stellar evolution models. Kappa Dra is a 61.5-day single-lined binary with a B6 IIIe primary. With the CHARA Array instruments MIRC/MIRC-X and MYSTIC, we detected the secondary at (approximately photospheric) flux ratios of 1.49 +- 0.10% and 1.63 +- 0.09% in the H and K band, respectively. From a large and diverse optical spectroscopic database only the radial velocity curve of the Be star could be extracted. However, employing the parallaxes from Hipparcos and Gaia, which agree within their nominal 1-sigma errors, we could derive the total mass and found component masses of 3.65 +- 0.48 Msun and 0.426 +- 0.043 Msun for the Be star and the companion, respectively. Previous cross-correlation of the observed far-UV spectrum with sdO spectral model templates had not detected a companion belonging to the hot O-type subdwarf (sdO) population known from ~20 earlier-type Be stars. Guided by our full 3D orbital solution, we found a strong cross-correlation signal for a stripped subdwarf B-type companion (far-UV flux ratio of 2.3 +- 0.5%), enabling the first firm characterization of such a star, and making kappa Dra the first mid- to late-type Be star with a directly-observed subdwarf companion.

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arXiv2022

The binary system of the spinning-top Be star Achernar

Achernar, the closest and brightest classical Be star, presents rotational flattening, gravity darkening, occasional emission lines due to a gaseous disk, and an extended polar wind. It is also a member of a close binary system with an early A-type dwarf companion. We aim to determine the orbital parameters of the Achernar system and to estimate the physical properties of the components. We monitored the relative position of Achernar B using a broad range of high angular resolution instruments of the VLT/VLTI (VISIR, NACO, SPHERE, AMBER, PIONIER, GRAVITY, and MATISSE) over a period of 13 years (2006-2019). These astrometric observations are complemented with a series of more than 700 optical spectra for the period from 2003 to 2016. We determine that Achernar B orbits the Be star on a seven-year period, eccentric orbit (e = 0.7255 +/- 0.0014) which brings the two stars within 2 au at periastron. The mass of the Be star is found to be mA = 6.0 +/- 0.6 Msun for a secondary mass of mB = 2.0 +/- 0.1 Msun. We find a good agreement of the parameters of Achernar A with the evolutionary model of a critically rotating star of 6.4 Msun at an age of 63 million years. We also identify a resolved comoving low-mass star, which leads us to propose that Achernar is a member of the Tucana-Horologium moving group. Achernar A is presently in a short-lived phase of its evolution following the turn-off, during which its geometrical flattening ratio is the most extreme. Considering the orbital parameters, no significant interaction occurred between the two components, demonstrating that Be stars may form through a direct, single-star evolution path without mass transfer. Since component A will enter the instability strip in a few hundred thousand years, Achernar appears to be a promising progenitor of the Cepheid binary systems.

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arXiv2022

MWC656: A Be+BH or a Be+sdO?

MWC656 has been reported as classical Be star with a black hole companion. Revisited spectral variability properties render this unlikely, with a hot subdwarf more probable.

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arXiv2022

Detecting Stripped Stars While Searching for Quiescent Black Holes

While the number of stellar-mass black holes detected in X-rays or as gravitational wave sources is steadily increasing, the known population remains orders of magnitude smaller than predicted by stellar evolution theory. A significant fraction of stellar-mass black holes is expected to hide in X-ray-quiet binaries where they are paired with a "normal" star. Although a handful of such quiescent black hole candidates have been proposed, the majority have been challenged by follow-up investigations. A confusion that emerged recently concerns binary systems that appear to contain a normal B-type star with an unseen companion, believed to be a black hole. On closer inspection, some of these seemingly normal B-type stars instead turn out to be stars stripped of most of their mass through an interaction with their binary companion, which in at least two cases is a rapidly rotating star rather than a compact object. These contaminants in the search for quiescent black holes are themselves extremely interesting objects as they represent a rare phase of binary evolution, and should be given special attention when searching for binaries hosting black holes in large spectroscopic studies.

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arXiv2022

HR 6819 is a binary system with no black hole -- revisiting the source with infrared interferometry and optical integral field spectroscopy

Two scenarios have been proposed to match the existing observational constraints of the object HR 6819. The system could consist of a close inner B-type giant plus a black hole (BH) binary with an additional Be companion in a wide orbit. Alternatively, it could be a binary composed of a stripped B star and a Be star in a close orbit. Either scenario makes HR 6819 a cornerstone object as the stellar BH closest to Earth, or as an example of an important transitional, non-equilibrium phase for Be stars with solid evidence for its nature. We aimed to distinguish between the two scenarios for HR 6819. Both models predict two luminous stars but with very different angular separations and orbital motions. Therefore, the presence of bright sources in the 1-100 milliarcsec (mas) regime is a key diagnostic for determining the nature of the HR 6819 system. We obtained new high-angular resolution data with VLT/MUSE and VLTI/GRAVITY of HR 6819. The MUSE data are sensitive to bright companions at large scales, whilst the interferometric GRAVITY data are sensitive down to separations on mas scales and large magnitude differences. The MUSE observations reveal no bright companion at large separations and the GRAVITY observations indicate the presence of a stellar companion at an angular separation of ~1.2 mas that moves on the plane of the sky over a timescale compatible with the known spectroscopic 40-day period. We conclude that HR 6819 is a binary system and that no BH is present in the system. The unique nature of HR 6819, and its proximity to Earth make it an ideal system for quantitatively characterising the immediate outcome of binary interaction and probing how Be stars form.

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arXiv2022

MOBSTER -- VI. The crucial influence of rotation on the radio magnetospheres of hot stars

Numerous magnetic hot stars exhibit gyrosynchrotron radio emission. The source electrons were previously thought to be accelerated to relativistic velocities in the current sheet formed in the middle magnetosphere by the wind opening magnetic field lines. However, a lack of dependence of radio luminosity on the wind power, and a strong dependence on rotation, has recently challenged this paradigm. We have collected all radio measurements of magnetic early-type stars available in the literature. When constraints on the magnetic field and/or the rotational period are not available, we have determined these using previously unpublished spectropolarimetric and photometric data. The result is the largest sample of magnetic stars with radio observations that has yet been analyzed: 131 stars with rotational and magnetic constraints, of which 50 are radio-bright. We confirm an obvious dependence of gyrosynchrotron radiation on rotation, and furthermore find that accounting for rotation neatly separates stars with and without detected radio emission. There is a close correlation between H$α$ emission strength and radio luminosity. These factors suggest that radio emission may be explained by the same mechanism responsible for H$α$ emission from centrifugal magnetospheres, i.e. centrifugal breakout (CBO), however, whereas the H$α$-emitting magnetosphere probes the cool plasma before breakout, radio emission is a consequence of electrons accelerated in centrifugally-driven magnetic reconnection.

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arXiv2021

Interferometric detections of sdO companions orbiting three classical Be stars

Classical Be stars are possible products of close binary evolution, in which the mass donor becomes a hot, stripped O or B-type subdwarf (sdO/sdB), and the mass gainer spins up and grows a disk to become a Be star. While several Be+sdO binaries have been identified, dynamical masses and other fundamental parameters are available only for a single Be+sdO system, limiting the confrontation with binary evolution models. In this work, we present direct interferometric detections of the sdO companions of three Be stars 28 Cyg, V2119 Cyg, and 60 Cyg, all of which were previously found in UV spectra. For two of the three Be+sdO systems, we present first orbits and preliminary dynamical masses of the components, revealing that one of them could be the first identified progenitor of a Be/X-ray binary with a neutron star companion. These results provide new sets of fundamental parameters that are crucially needed to establish the evolutionary status and origin of Be stars.

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arXiv2021

VLTI-MATISSE L- and N-band aperture-synthesis imaging of the unclassified B[e] star FS Canis Majoris

Context: FS Canis Majoris (FS CMa, HD 45677) is an unclassified B[e] star surrounded by an inclined dust disk. The evolutionary stage of FS CMa is still debated. Perpendicular to the circumstellar disk, a bipolar outflow was detected. Infrared aperture-synthesis imaging provides us with a unique opportunity to study the disk structure. Aims: Our aim is to study the intensity distribution of the disk of FS CMa in the mid-infrared L and N bands. Methods: We performed aperture-synthesis imaging of FS CMa with the MATISSE instrument (Multi AperTure mid-Infrared SpectroScopic Experiment) in the low spectral resolution mode to obtain images in the L and N bands. We computed radiative transfer models that reproduce the L- and N-band intensity distributions of the resolved disks. Results: We present L- and N-band aperture-synthesis images of FS CMa reconstructed in the wavelength bands of 3.4-3.8 and 8.6-9.0 micrometer. In the L-band image, the inner rim region of an inclined circumstellar disk and the central object can be seen with a spatial resolution of 2.7 milliarcsec (mas). An inner disk cavity with an angular diameter of 6x12mas is resolved. The L-band disk consists of a bright northwestern (NW) disk region and a much fainter southeastern (SE) region. The images suggest that we are looking at the bright inner wall of the NW disk rim, which is on the far side of the disk. In the N band, only the bright NW disk region is seen. In addition to deriving the inclination and the inner disk radius, fitting the reconstructed brightness distributions via radiative transfer modeling allows one to constrain the innermost disk structure, in particular the shape of the inner disk rim.

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arXiv2020

MOBSTER -- V: Discovery of a magnetic companion star to the magnetic $β$ Cep pulsator HD 156424

HD 156424 (B2 V) is a little-studied magnetic hot star in the Sco OB4 association, previously noted to display both high-frequency radial velocity (RV) variability and magnetospheric H$α$ emission. We have analysed the TESS light curve, and find that it is a $β$ Cep pulsator with 11 detectable frequencies, 4 of which are independent $p$-modes. The strongest frequency is also detectable in RVs from ground-based high-resolution spectroscopy. RVs also show a long-term variation, suggestive of orbital motion with a period of $\sim$years; significant differences in the frequencies determined from TESS and RV datasets are consistent with a light-time effect from orbital motion. Close examination of the star's spectrum reveals the presence of a spectroscopic companion, however as its RV is not variable it cannot be responsible for the orbital motion and we therefore infer that the system is a hierarchical triple with a so-far undetected third star. Reanalysis of LSD profiles from ESPaDOnS and HARPSpol spectropolarimetry reveals the surprising presence of a strong magnetic field in the companion star, with $\langle B_z \rangle$ about $+1.5$ kG as compared to $\langle B_z \rangle \sim -0.8$ kG for the primary. HD 156424 is thus the second hot binary with two magnetic stars. We are unable to identify a rotational period for HD 156424A. The magnetospheric H$α$ emission appears to originate around HD 156424B. Using H$α$, as well as other variable spectral lines, we determine a period of about 0.52 d, making HD 156424B one of the most rapidly rotating magnetic hot stars.

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arXiv2020

The Magnetic Early B-type Stars IV: Breakout or Leakage? H$α$ emission as a diagnostic of plasma transport in centrifugal magnetospheres

Rapidly rotating early-type stars with strong magnetic fields frequently show H$α$ emission originating in Centrifugal Magnetospheres (CMs), circumstellar structures in which centrifugal support due to magnetically enforced corotation of the magnetically confined plasma enables it to accumulate to high densities. It is not currently known whether the CM plasma escapes via Centrifugal Breakout (CB), or by an unidentified leakage mechanism. We have conducted the first comprehensive examination of the H$α$ emission properties of all stars currently known to display CM-pattern emission. We find that the onset of emission is dependent primarily on the area of the CM, which can be predicted simply by the value $B_{\rm K}$ of the magnetic field at the Kepler corotation radius $R_{\rm K}$. Emission strength is strongly sensitive to both CM area and $B_{\rm K}$. Emission onset and strength are {\em not} dependent on effective temperature, luminosity, or mass-loss rate. These results all favour a CB scenario, however the lack of intrinsic variability in any CM diagnostics indicates that CB must be an essentially continuous process, i.e.\ it effectively acts as a leakage mechanism. We also show that the emission profile shapes are approximately scale-invariant, i.e.\ they are broadly similar across a wide range of emission strengths and stellar parameters. While the radius of maximum emission correlates closely as expected to $R_{\rm K}$, it is always larger, contradicting models that predict that emission should peak at $R_{\rm K}$.

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arXiv2020

A naked-eye triple system with a nonaccreting black hole in the inner binary

Several dozen optical echelle spectra demonstrate that HR 6819 is a hierarchical triple. A classical Be star is in a wide orbit with an unconstrained period around an inner 40 d binary consisting of a B3 III star and an unseen companion in a circular orbit. The radial-velocity semi-amplitude of 61.3 km/s of the inner star and its minimum (probable) mass of 5.0 Msun (6.3 +- 0.7 Msun) imply a mass of the unseen object of >= 4.2 Msun (>= 5.0 +- 0.4 Msun), that is, a black hole (BH). The spectroscopic time series is stunningly similar to observations of LB-1. A similar triple-star architecture of LB-1 would reduce the mass of the BH in LB-1 from ~70 Msun to a level more typical of Galactic stellar remnant BHs. The BH in HR 6819 probably is the closest known BH to the Sun, and together with LB-1, suggests a population of quiet BHs. Its embedment in a hierarchical triple structure may be of interest for models of merging double BHs or BH + neutron star binaries. Other triple stars with an outer Be star but without BH are identified; through stripping, such systems may become a source of single Be stars.

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arXiv2019

gamma Cas stars: Normal Be stars with disks impacted by the wind of a helium-star companion?

$γ$ Cas stars are a $\sim$1% minority among classical Be stars with hard but only moderately strong continuous thermal X-ray flux and mostly very early-B spectral type. The X-ray flux has been suggested to originate from matter accelerated via magnetic disk-star interaction, by a rapidly rotating neutron star (NS) companion via the propeller effect, or by accretion onto a white dwarf (WD) companion. In view of the growing number of identified $γ$ Cas stars and the only imperfect matches between these suggestions and the observations, alternative models should be pursued. Two of the three best-observed $γ$ Cas stars, $γ$ Cas itself and $π$ Aqr, have a low-mass companion with low optical flux; interferometry of BZ Cru is inconclusive. Binary-evolution models are examined for their ability to produce such systems. The OB+He-star stage of post-mass transfer binaries, which is otherwise observationally unaccounted, can potentially reproduce many observed properties of $γ$ Cas stars. The interaction of the fast wind of helium stars with the disk and/or with the wind of Be stars may give rise to the production of hard X-rays. While not modelling this process, it is shown that the energy budget is favourable, and that the wind velocities may lead to hard X-rays as observed in $γ$ Cas stars. Furthermore, their observed number appears to be consistent with the evolutionary models. Within the Be+He-star binary model, the Be stars in $γ$ Cas stars are conventional classical Be stars. They are encompassed by O-star+Wolf-Rayet systems towards higher mass, where no stable Be decretion disks exist, and by Be+sdO systems at lower mass where the sdO winds may be too weak to cause the $γ$ Cas phenomenon. In decreasing order of the helium-star mass, the descendants could be Be+black-hole, Be+NS or Be+WD binaries.

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