Amanda C. Rubio

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.

The effectiveness and stability of mass transfer in a binary system are crucial for determining the final product of its evolution. Rapid binary population synthesis codes simplify the complex physics of mass transfer and common-envelope evolution by adopting parameterized prescriptions for the stability of mass transfer, accretion efficiency in stable mass transfer, and the efficiency of common-envelope ejection. Our goal is to calibrate these uncertain parameters by comparing binary population synthesis models with observational data. Binary systems composed of a white dwarf and main-sequence star are ideal for studying the effects of binary interaction, as they can be formed through stable or unstable mass transfer, or without any interaction. These different evolutionary paths affect the orbital period and masses of the present-day population. The APOGEE-GALEX-Gaia catalog (AGGC) provides a homogeneous sample of over 500 systems with well-measured radial velocities that can be used as a comparison baseline for population synthesis simulations of white dwarf–main-sequence binaries. We compare the distribution of the observed maximum radial velocity variation (Δ RV _max ) in the AGGC to binary population models simulated with COSMIC, a publicly developed binary population synthesis code. Within these synthetic populations, we vary the mass transfer and common-envelope ejection efficiency, and the criteria for mass transfer stability at the first ascent, asymptotic, and thermally pulsing giant branch phases. We also compare our models to systems with orbital solutions and estimated stellar masses. The Δ RV _max comparison shows clear preference for models with a higher fraction of stars undergoing stable mass transfer during the first ascent giant branch phase, and for highly effective envelope ejection during common-envelope phases. For the few systems with estimated WD masses, a comparison to models shows a slight preference for nonconservative stable mass transfer. In COSMIC and similar codes, the envelope ejection efficiency and the envelope binding energy are degenerate parameters. Therefore, our result of a high ejection efficiency may indicate either that additional sources of energy (such as recombination energy from the expansion of the envelope) are required to eject the envelope, or that its binding energy is lower than traditionally assumed. Future comparisons to population synthesis simulations of WD binaries can be drawn for other datasets as they become available, such as upcoming Gaia data releases and the future LISA mission, and for binary systems in other evolutionary stages.

Binary stars and their interactions shape the formation of compact binaries, supernovae, and gravitational-wave sources. The efficiency of mass transfer—the fraction of mass retained by the accretor during binary interaction—is a critical parameter that significantly impacts the final fates of these systems. However, this parameter is observationally poorly constrained, due to the scarcity of well-characterized post-mass-transfer binaries. Be+sdOB binaries, consisting of a rapidly rotating Be star and a stripped hot subdwarf companion, are particularly valuable for studying mass transfer, since they represent clear examples of past binary interaction. Recently, a significantly expanded observational sample of 16 Be+sdOB binaries with well-constrained masses was obtained through combined spectroscopic and interferometric observations. In this work, we compile and analyze this sample, to provide robust constraints on the mass-transfer efficiency in binaries that underwent stable mass transfer during the donor’s hydrogen-shell-burning phase. Our analysis reveals that the mass transfer was predominantly conservative: half of the systems require mass-transfer efficiencies above 50%. This challenges the commonly adopted assumptions of highly nonconservative mass transfer in binary evolution modeling. Our findings are inconsistent with models that account for spinup and limit accretion due to a centrifugal barrier. We also find tension with a commonly used mass-transfer model in rapid population synthesis that limits accretion based on the thermal timescale of the accretor. These results have strong implications for almost all products of binary evolution, including a variety of supernovae, white dwarfs, blue stragglers, runaway stars, X-ray binaries, and gravitational-wave sources.

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. Classical Be stars are well known to eject mass to build up a disk, but the details governing the initial distribution and subsequent evolution of this matter into a disk are in general poorly constrained through observations. Aims. By combining high-cadence time-series spectroscopy with contemporaneous space photometry from the Transiting Exoplanet Survey Satellite (TESS), we have sampled about 30 mass ejection events in 13 Be stars. Our goal is to constrain the geometrical and kinematic properties of the ejecta as early as possible, facilitating the investigation into the material's initial conditions and evolution, and understanding its interactions with preexisting material. Methods. The photometric variability is analyzed together with measurements of the at-times rapidly changing emission features in order to identify the onset of outburst events and obtain information about the geometry of the ejecta and how it changes over time. Short-lived line asymmetries display oscillation cycles (Štefl frequencies), which are compared to photometric and stable spectroscopic frequencies. Results. All Be stars observed with sufficiently high cadence during an outburst are found to exhibit rapid oscillations of line asymmetry with a single frequency in the days following the start of the event. For a given star this circumstellar frequency may differ only slightly from event to event even when the outbursts they are associated with have different properties. These circumstellar frequencies are typically between 0.5 to 2 d⁻¹, and are generally near photometric frequencies. They are slightly below prominent (generally stable) spectroscopic frequencies seen in photospheric absorption lines. The emission asymmetry cycles break down after roughly 5–10 cycles, with the emission line profile converging toward approximate symmetry shortly thereafter. In photometry, several frequencies typically emerge at relatively high amplitude at some point during the mass ejection process. Conclusions. In all observed cases, freshly ejected material was initially constrained within a narrow azimuthal range, indicating it was launched from a localized region on the stellar surface. The material orbits the star with a frequency consistent with the near-surface Keplerian orbital frequency. This material circularizes into a disk configuration after several orbital timescales. This is true whether or not there was a preexisting disk at the time of the observed outburst. We find no evidence for precursor phases prior to the ejection of mass in our sample. The several photometric frequencies that emerge during outburst are at least partially stellar in origin.

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.