Tajan H. de Amorim

Context. Classical Be stars exhibit mass ejection events that feed their viscous decretion disks. Recent TESS space photometry and simultaneous spectroscopy have revealed that these flickers are localised, short-lived, and associated with near-Keplerian rotating material close to the stellar surface. Aims. We constrain the geometrical and dynamical conditions required for a localised surface ejection to generate a Keplerian decretion disk, and we predict the corresponding photometric, spectroscopic, and polarimetric observables. Methods. We performed 3D smoothed particle hydrodynamic (SPH) simulations of asymmetric, eruption-like mass ejections from a rotating equatorial sector of a B-type star. We systematically varied the size and angular velocity of the injection volume, the viscosity of the material, and the injection radius. The SPH outputs were post-processed with the radiative transfer code HDUST to obtain synthetic observables. We scaled the density of each model to approximately match the observed properties of a reference flicker for the Be star f Car (B3Vne). Results. A mildly super-Keplerian rotation of the injection volume, a high viscosity, and a mass-loss rate of the order 10 ⁻⁶ M _⊙ yr ⁻¹ str ⁻¹ are required for the ejected material to remain in orbit and form a small Keplerian disk. The resulting photometric, polarimetric, and H α variations reproduce the observed behaviour of the reference flicker. The simulations confirm that during mass ejection the disk is strongly asymmetric and dynamically evolving, and circularises within a few days after the end of the flicker, as observed. Models with a mass ejection angle that is too wide or an angular velocity that is too high fail to reproduce the observed light curve and line profile behaviour. The models are consistent with mass ejection happening very close to the stellar equator. Conclusions. Localised, short-duration, mildly super-Keplerian ejections combined with high viscosity and high mass-loss rates can account for the short-timescale variability of the circumstellar environment of Be stars. Be disks can be formed from such outbursts, and realistic 3D injection geometries are essential to connect surface dynamics to disk build-up within the framework of the viscous decretion disk model.

We present a detailed analysis of the early post-mass-transfer binary system HD 698 (V742 Cas), combining high-resolution optical spectroscopy, long-baseline interferometry, and radiative transfer modeling. Counter-phased RV curves reveal a circular orbit with a period of 55.927 ± 0.001 d and component masses of M _Be = 7.48 ± 0.07 M _⊙ and M _comp = 1.23 ± 0.02 M _⊙ . The Be primary is traced via broad H α wings, while narrow metallic absorption lines originate from a slowly rotating companion. The angular separation measured via interferometry implies a dynamical distance of 888 ± 5 pc. The spectral energy distribution is best reproduced with a color excess E ( B − V ) = 0.321 ± 0.016 due to interstellar reddening and a moderately dense viscous decretion disk with base density ρ ₀ ≃ 5 × 10 ⁻¹² g cm ⁻³ at r = R _eq , declining radially as ρ ( r ) ∝ r ^{− n} with n = 3.0. The companion is found to be a luminous and inflated star with T eff , comp = 10 . 0 − 0.1 + 0.2 kK, R comp = 13 . 1 − 0.2 + 0.2 R ⊙ , and log L / L ⊙ = 3.19 , contributing significantly to the flux ( L _comp / L _Be ∼ 0.3). Spectral line mismatches provide further circumstantial evidence that the companion is hydrogen poor, consistent with a stripped-envelope star enriched by CNO processing. HD 698 thus belongs to the emerging class of Be + bloated O/B binaries, representing a short-lived, high-luminosity post-mass-transfer phase, when the stripped donor is still spectroscopically detectable before reaching the subdwarf phase.

With its two stellar eclipses and two disk attenuations per binary orbit, V658 Carinae stands out as the first clear eclipsing Be + sdOB system. This rare alignment offers a unique opportunity to probe the structure and dynamics of a Be star disk with unprecedented detail. In this study, we present the most comprehensive observational data set and modeling effort for this system to date, including optical, near-infrared, and ultraviolet spectroscopy, space-based photometry, and optical polarization. Using a new ray-tracing code, we apply a three-component model, consisting of an oblate, rapidly rotating Be star, a symmetric circumstellar disk, and a compact stripped companion, to reproduce the system’s light curve, polarization, and spectral features. Our analysis yields precise constraints on the stellar and disk parameters, determining its status as the second-known late-type Be + stripped star, and also provides strong spectroscopic evidence for a tenuous circumsecondary envelope. Despite the model’s overall success, several key observables, such as the H _α equivalent width and the secondary attenuation, remain poorly reproduced, pointing to the need for more sophisticated modeling. In particular, future improvements should incorporate the companion’s radiative feedback on the disk and account for asymmetric disk structures expected by the gravitational interaction with the companion. Owing to its unique geometry and rich diagnostics, V658 Car establishes itself as a benchmark system for Be stars (and rapid rotators in general), stripped stars, post-RLOF massive binaries, and circumstellar disk structures.

Multicolor OGLE survey light curves of about 20 yr duration are analyzed for about 3000 classical Be stars in the Large and Small Magellanic Clouds (LMC, SMC) in order to study their properties and variability. Each light curve was manually analyzed to distinguish between different scenarios, such as photospheric baseline levels and disk buildup and dissipation phases. This analysis was aided by dynamical disk models and photospheric models to coarsely determine the inclination angle and mass. Measured quantities such as the fraction of time spent actively ejecting mass (the duty cycle (DC)), the fraction of time spent with a detectable disk (the disk duty cycle (DDC)), the buildup and dissipation time of isolated disk events, and the number of mass outbursts per year allow us to characterize and compare the behavior of the two populations. There is a wide spread in the DC, with median values of 0.44 (LMC) and 0.60 (SMC). The DDC is high for both populations, with median values of 0.99 (LMC) and 1.0 (SMC), indicating that disks are almost always present for these stars. The occurrence rate of outbursts ranges from zero to about two per year, with median values of 0.31 (LMC) and 0.26 (SMC). There are strong statistical differences in the behavior of the LMC and SMC populations, with the lower metallicity stars being more active in terms of their DC and DDC, and with less frequent but longer-lasting outbursts.

V658 Car is the first known eclipsing binary system involving a classical Be star and an sdOB companion, offering a unique opportunity to study disk physics and binary interactions in unprecedented detail. From TESS data and multi-color observations from the comissão para a colaboração entre profissionais e amadores collaboration, we analyze the system’s color–magnitude diagram and compare it with radiative transfer models that include the Be star, its circumstellar disk, and the sdOB companion. While the stellar eclipses are well reproduced, two features observed in the multi-color photometry challenge the current modeling paradigm: the discrepancy between the observed reddening and the modeled blueing during the first attenuation phase and the complete lack of modeled attenuation around the second stellar eclipse. These issues highlight the need for more sophisticated modeling approaches to capture the complex interplay between disk opacity and binary dynamics.

Be stars are characterized by the presence of a circumstellar Keplerian disk formed from material ejected from the rapidly rotating stellar surface. This article presents recent observational and theoretical progress on two central aspects of this phenomenon: the mechanisms driving mass loss, and the fate of the ejected material. Using simultaneous TESS photometry and ground-based spectroscopy, we examine the short-term variability associated with discrete mass ejection events, or “flickers”, and review strong evidence linking them to pulsational activity near the stellar surface. Complementary 3D hydrodynamic simulations reproduce key observational signatures and establish that disk formation requires compact and asymmetric ejection sites with sufficient angular momentum to overcome re-accretion. In systems with binary companions, new high-resolution simulations resolve the outer disk for the first time and identify five dynamically distinct regions, including a circumsecondary disk and a circumbinary spiral outflow. Together, these results provide a coherent framework that traces the full life cycle of disk material from pulsation-driven ejection near the stellar surface to its final destination, whether re-accreted by the companion or lost from the system entirely.

Context. In recent decades, binarity among O and B stars has been shown to be key in our understanding of their birth, evolution, and death. For the particular case of Be stars, binarity might be the cause for their spinup, which comprises a piece to the puzzling mechanism behind their mass outbursts, leading to the formation of their viscous decretion disks. Detecting companions in systems with Be stars can be challenging, making it difficult to obtain observational constraints on the binary fraction of Be stars. Aims. We explore the effects of a binary companion in a system with a Be star, from the moment when the disk first begins to form until it reaches quasi steady-state. The tidal forces considerably affect the Be disk, leading to the formation of distinct regions in the system. These effects bring on observational consequences that can be used to infer the presence of a otherwise undetectable companion. Methods. We used smoothed particle hydrodynamics (SPH) simulations of coplanar, circular binary systems with fixed Be parameters (star mass of 12.9 M_⊙, equatorial radius of 5.5 R_⊙, and effective temperature of 26 000 K). We also varied the orbital periods (30, 50 and 84 days), disk viscosities (α = 0.1, 0.5, and 1), and mass ratios (q = 0.16, 0.33, and 0.5). A high spatial resolution was achieved by adopting particle splitting in the SPH code, along with a more realistic description of the secondary star and the disk viscosity. Results. With the upgraded code, we were able to probe a region approximately four times larger than previously possible. Our models show that the disk can be divided into five regions of interest: an inner Be disk, spiral dominated disk, and bridge, as well as the previously unseen circumsecondary and circumbinary regions. These features were revealed thanks to the increased resolution of the simulation. In this work, we describe the configuration and kinematics of each region and provide a summary of their expected observational signals. In all simulations, mass transfer takes place from the Be disk into the Roche lobe of the companion via the bridge. In other words, the disk is not sharply truncated at a given radius; rather, it suffers a strong decrease in density in a region spanning several stellar radii. This truncation region is azimuthally variable, elongated along the Roche potentials of the binary system. Material that enters the Roche lobe of the companion is partially captured by it, effectively forming a rotationally supported, disk-like structure. Part of the material is accreted by the companion in all simulations, but the expected X-ray emission of this accretion is faint. Material that does not get accreted then escapes the Roche lobe of the companion and forms a circumbinary, one-armed spiral around the system. This is the first work to describe the region beyond the truncation region of the Be disk and its observational consequences in detail. Conclusions. All five regions are present for all models explored in this work. The effects of orbital period, viscosity, and mass ratio on the structure of Be binary systems are significant and have an anticipated impact the observables. Based on our models, we argue that observational features of previously unclear origin, such as the intermittent shell features and emission features of HR 2142 and HD 55606, originate in areas beyond the truncation region. This new understanding of the behavior of disks in Be binaries will enable not only an improved interpretation of the existing data, but also aid in the planning of future observations.

Multi-color OGLE survey light curves of about 20 years duration are analyzed for about 3000 classical Be stars in the Large and Small Magellanic Clouds (LMC, SMC) in order to study the properties and variability. Each light curve was manually analyzed to distinguish between different scenarios, such as photospheric baseline levels, and disk build-up and dissipation phases. This analysis was aided by dynamical disk models and photospheric models to coarsely determine inclination angle and mass. Measured quantities such as the fraction of time spent actively ejecting mass (the duty cycle), the fraction of time spent with a detectable disk (the disk duty cycle), the build-up and dissipation time of isolated disk events, and the number of mass outbursts per year allow us to characterize and compare the behavior of the two populations. There is a wide spread in the duty cycle, with median values of 0.44 (LMC) and 0.60 (SMC). The disk duty cycle is high for both populations, with median values of 0.99 (LMC) and 1.0 (SMC), indicating that disks are almost always present for these stars. The occurrence rate of outbursts ranges from zero to about two per year, with median values of 0.31 (LMC) and 0.26 (SMC). There are strong statistical differences in the behavior of the LMC and SMC populations, with the lower metallicity stars being more active in terms of their duty cycle and disk duty cycle, and with less frequent but longer lasting outbursts.

Context. Classical Be stars are rapidly rotating B-type stars exhibiting Balmer emission lines originating from circumstellar Keplerian gaseous disks, which likely form through episodic mass-ejection events. The currently favored model for Be star disks is the viscous decretion disk (VDD) model. However, the mechanism behind the mass ejection process during the formation of a VDD is a mystery. Aims. We present peculiar behavior of the Be star KIC 9715425, which exhibited several minor outbursts (MIBs) and one major outburst (MAB) observed as brightenings in broadband photometry, as well as transient Hα line emission. The variability may provide valuable keys to understanding the nature of Be outbursts. Methods. Based on Kepler, the All-Sky Automated Survey for Supernovae (ASAS-SN) and the Transiting Exoplanet Survey Satellite time-series photometry, the Large Sky Area Multi-Object Fiber Spectroscopic Telescope spectroscopy, multi-wavelength observations in the near-ultraviolet (NUV) from the Galaxy Evolution Explorer (GALEX) and optical from Gaia, Xinglong 2.16 m telescope and Isaac Newton Telescope of KIC 9715425 covering its outbursts, we determined fundamental parameters of the central star and the circumstellar disk. A frequency analysis and spectral modeling were carried out to characterize the events. Results. During the major outburst, we find a 36% flux increase in the GALEX NUV band compared to 25% in the Kepler band, suggesting that whatever produced the flux increase should be hotter than the central B-type star. This is contradictory to the conventional scenario that the VDD should be cooler. In addition, such a high flux increase can only be accounted for by a luminous disk, which should produce a much stronger Hα emission than observed. The origin of this anomalously hot component remains unexplained. Conclusions. Except for the bluing, the available observations of KIC 9715425 are perfectly compatible with being a normal Be star. If the bluing was physically related to the MAB, the most economical effort to identify the nature of the underlying process could be adaptive NUV and far-ultraviolet monitoring of Be stars with cyclically repeating MABs.