TESS photometry is analyzed for 430 classical Be stars observed in the first year of the mission. The often complex and diverse variability of each object in this sample is classified to obtain an understanding of the behavior of this class as a population. Ninety-seven percent of the systems are variable above the noise level, with timescales spanning nearly the entire range of what is accessible with TESS, from tens of minutes to tens of days. The variability seen with TESS is summarized as follows. Nearly every system contains multiple periodic signals in the frequency regime between about 0.5 and 4 day⁻¹. One or more groups of closely spaced frequencies is the most common feature, present in 87% of the sample. Among the Be stars with brightening events that are characteristic of mass ejection episodes (18% of the full sample, or 31% of early-type stars), all have at least one frequency group, and the majority of these (83%) show a concurrent temporary amplitude enhancement in one or more frequency groups. About one-third (34%) of the sample is dominated by longer-term trends (timescales >2 day). Low-frequency stochastic signals are prominent in about 25% of the sample, with varying degrees of intensity. Higher-frequency signals (6 < f < 15 day⁻¹) are sometimes seen (in 15% of the sample) and in most cases likely reflect p-mode pulsation. In rare cases (∼3%), even higher frequencies beyond the traditional p-mode regime (f > 15 day⁻¹) are observed.

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.

We use a time-dependent hydrodynamic code and a non-LTE Monte Carlo code to model disk dissipation for the Be star 66 Ophiuchi. We compiled 63 years of observations from 1957 to 2020 to encompass the complete history of the growth and subsequent dissipation of the star’s disk. Our models are constrained by new and archival photometry, spectroscopy, and polarization observations, allowing us to model the disk dissipation event. Using Markov Chain Monte Carlo methods, we find that the properties of 66 Oph are consistent with those of a standard B2Ve star. We computed a grid of 61,568 Be star disk models to constrain the density profile of the disk before dissipation using observations of the Hα line profile and spectral energy distribution. We find at the onset of dissipation the disk has a base density of 2.5 × 10⁻¹¹ g cm⁻³ with a radial power-law index of n = 2.6. Our models indicate that after 21 yr of disk dissipation 66 Oph’s outer disk remained present and bright in the radio. We find an isothermal disk with constant viscosity with an α = 0.4 and an outer disk radius of ∼115 stellar radii best reproduces the rate of 66 Oph’s disk dissipation. We determined the interstellar polarization in the direction of the star in the V band is p = 0.63 ± 0.02% with a polarization position angle of θ _IS ≈ 857 ± 07. Using the Stokes QU diagram, we find the intrinsic polarization position angle of 66 Oph’s disk is θ _int ≈ 98° ± 3°.

The bright and understudied classical Be star HD 6226 has exhibited multiple outbursts in the last several years during which the star grew a viscous decretion disc. We analyse 659 optical spectra of the system collected from 2017 to 2020, along with a ultraviolet spectrum from the Hubble Space Telescope and high cadence photometry from both Transiting Exoplanet Survey Satellite (TESS) and the Kilodegree Extremely Little Telescope (KELT) survey. We find that the star has a spectral type of B2.5IIIe, with a rotation rate of 74 per cent of critical. The star is nearly pole-on with an inclination of 13|${_{.}^{\circ}}$|4. We confirm the spectroscopic pulsational properties previously reported, and report on three photometric oscillations from KELT photometry. The outbursting behaviour is studied with equivalent width measurements of H α and H β, and the variations in both of these can be quantitatively explained with two frequencies through a Fourier analysis. One of the frequencies for the emission outbursts is equal to the difference between two photometric oscillations, linking these pulsation modes to the mass ejection mechanism for some outbursts. During the TESS observation time period of 2019 October 7 to 2019 November 2, the star was building a disc. With a large data set of H α and H β spectroscopy, we are able to determine the time-scales of dissipation in both of these lines, similar to past work on Be stars that has been done with optical photometry. HD 6226 is an ideal target with which to study the Be disc-evolution given its apparent periodic nature, allowing for targeted observations with other facilities in the future.

Classical B emission (Be) stars are fast rotating, near-main-sequence B-type stars. The rotation and the presence of circumstellar discs profoundly modify the observables of active Be stars. Our goal is to infer stellar and disc parameters, as well as distance and interstellar extinction, using the currently most favoured physical models for these objects. We present BeAtlas, a grid of $61\, 600$ non-local thermodynamic equilibrium radiative transfer models for Be stars, calculated with the hdust code. The grid was coupled with a Monte Carlo Markov chain (MCMC) code to sample the posterior distribution. We test our method on two well-studied Be stars, α Eri and β CMi, using photometric, polarimetric, and spectroscopic data as input to the code. We recover literature determinations for most of the parameters of the targets, in particular the mass and age of α Eri, the disc parameters of β CMi, and their distances and inclinations. The main discrepancy is that we estimate lower rotational rates than previous works. We confirm previously detected signs of disc truncation in β CMi and note that its inner disc seems to have a flatter density slope than its outer disc. The correlations between the parameters are complex, further indicating that exploring the entire parameter space simultaneously is a more robust approach, statistically. The combination of BeAtlas and Bayesian-MCMC techniques proves successful, and a powerful new tool for the field: The fundamental parameters of any Be star can now be estimated in a matter of hours or days.

In photometry of γ Cas (B0.5 IVe) from the Solar Mass Ejection Imager (SMEI) and the BRIght Target Explorer (BRITE)-Constellation satellites, indications of low-order non-radial pulsation have recently been found, which would establish an important commonality with the class of classical Be stars at large. New photometry with the Transiting Exoplanet Survey Satellite (TESS) has detected three frequency groups near 1.0 (g1), 2.4 (g2), and 5.1 (g3) d−1, respectively. Some individual frequencies are nearly harmonics or combination frequencies but not exactly so. Frequency groups are known from roughly three quarters of all classical Be stars and also from pulsations of β Cep, slowly pulsating B (SPB), and γ Dor stars and, therefore, firmly establish γ Cas as a non-radial pulsator. The total power in each frequency group is variable. An isolated feature exists at 7.57 d−1 and, together with the strongest peaks in the second and third groups ordered by increasing frequency (g2 and g3), is the only one detected in all three TESS sectors. The former long-term 0.82 d−1 variability would fall into g1 and has not returned at a significant level, questioning its attribution to rotational modulation. Low-frequency stochastic variability is a dominant feature of the TESS light curve, possibly caused by internal gravity waves excited at the core–envelope interface. These are known to be efficient at transporting angular momentum outward, and may also drive the oscillations that constitute g1 and g2. The hard X-ray flux of γ Cas is the only remaining major property that distinguishes this star from the class of classical Be stars.