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We examine the evolution of the disk surrounding the Be star in the highly eccentric binary system $\delta$ Scorpii over its three most recent periastron passages. $V$-band and $B-V$ photometry, along with H$\alpha$ spectroscopy are combined with a new set of extensive multi-band polarimetry data to produce a detailed comparison of the disk's physical conditions during the time periods surrounding each closest approach of the secondary star. We use the three-dimensional Monte Carlo radiative transfer code \textsc{HDUST} and smoothed particle hydrodynamics (\textsc{SPH}) code to support our observations with models of disk evolution, discussing the behaviour of the H$\alpha$ and He\,\textsc{i} 6678 lines, $V$-band magnitude, and polarization degree. We compare the characteristics of the disk immediately before each periastron passage to create a baseline for the unperturbed disk. We find that the extent of the H$\alpha$ emitting region increased between each periastron passage, and that transient asymmetries in the disk become more pronounced with each successive encounter. Asymmetries of the H$\alpha$ and He\,\textsc{i} 6678 lines in 2011 indicate that perturbations propagate inward through the disk near periastron. When the disk's direction of orbit is opposite to that of the secondary, the parameters used in our models do not produce spiral density enhancements in the H$\alpha$ emitting region because the tidal interaction time is short due to the relative velocities of the disk particles with the secondary. The effects of the secondary star on the disk are short-lived and the disk shows independent evolution between each periastron event., Comment: 20 pages, 13 figures, accepted for publication in ApJ

We build on our previous work involving smoothed particle hydrodynamic simulations of Be stars, by using the model that exhibited disc tearing as input into the three-dimensional Monte Carlo radiative transfer code HDUST to predict observables from a variety of viewing angles throughout the disc tearing process. We run one simulation at the start of each orbital period from 20 to 72 orbital periods, which covers two complete disc tearing events. The resulting trends in observables are found to be dependent on the relative position of the observer and the tearing disc. The $\rm H\alpha$ equivalent width, $V$ magnitude, and polarization can all increase or decrease in any combination depending on the viewpoint of the observer. The $\rm H\alpha$ line profile also displays changes in strength and peak separation throughout the tearing process. We show how the outer disc of the torn system can have a large effect on the $\rm H\alpha$ line profile, and also contributes to a wavelength-dependent polarization position angle, resulting in a similar sawtooth shape to the polarization percentage. Finally, we compare our predictions to Pleione (28 Tau) where evidence has suggested that a disc tearing event has occurred in the past. We find that our tearing disc model can broadly match the trends seen in Pleione's observables, as well as produce the two-component $\rm H\alpha$ lines observed in Pleione. This is the strongest evidence, thus far, of Pleione's disc having indeed experienced a tearing event., Comment: 8 pages, 12 figures

Classical 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 NLTE radiative transfer models for Be stars, calculated with the HDUST code. The grid was coupled with a Monte Carlo Markov chain code to sample the posterior distribution. We test our method on two well-studied Be stars, $\alpha$ Eri and $\beta$ 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 $\alpha$ Eri, the disc parameters of $\beta$ 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 $\beta$ 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.

We use the three-dimensional Monte Carlo radiative transfer code HDUST to model Be stars where the disc is tilted from the equatorial plane of the star. We compute 128 models across 4 spectral types, B0, B2, B5 and B8, tilting the disc by $0^o$, $10^o$, $20^o$, and $40^o$, while varying disc density according to spectral type. We also compute every model for an average and high stellar rotation rate. We first discuss non-tilted disc temperatures and show its non-linear dependence on stellar and disc parameters. We find that tilting the disc minimally affects the density-weighted average disc temperature, but tilting does create a temperature asymmetry in disc cross sections, which is more pronounced for a faster rotation rate. We also investigate the effect tilting has on $V$-band magnitude, polarization, and the H$\alpha$ line. Tilting the disc does affect these observables, but the changes are entirely dependent on the position of the observer relative to the direction of tilt. We find the observables that distinguish tilting from a change in density or geometry are the H$\alpha$ line shape, where it can transition between single-peaked and double-peaked, and the polarization position angle, whose value is dependent on the projected major elongation axis of the disc on the sky. We also present one early and one late-type model with warped discs. We find their temperature structure varies a small amount from the uniformly tilted models, and the different observables correspond to different tilt angles, consistent with their expected volume of origin within the disc., Comment: 20 pages, 23 figures

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., Comment: Accepted to ApJ. Figure sets available on request

We acquired H$\alpha$ spectroscopic observations from 2005 to 2019 showing Pleione has transitioned from a Be phase to a Be-shell phase during this period. Using the radiative transfer code \hdust\ we created a grid of $\sim100,000$ disk models for Pleione. We successfully reproduced the observed transition with a disk model that varies in inclination while maintaining an equatorial density of $\rho_0(r) = 3\times 10^{-11} (r/R_{eq})^{-2.7}~\rm{g~cm^{-3}}$, and an H$\alpha$ emitting region extending to $15~\rm{R_{eq}}$. We use a precessing disk model to extrapolate the changing disk inclination over $120$ years and follow the variability in archival observations. The best-fit disk model precesses over a line of sight inclination between $\sim25\rm{^{\circ}}$ and $\sim144\rm{^{\circ}}$ with a precessional period of $\sim80.5$ years. Our precessing models match some of the observed variability but fail to reproduce all of the historical data available. Therefore, we propose an ad-hoc model based on our precessing disk model inspired by recent SPH simulations of similar systems, where the disk tears due to the tidal influence of a companion star. In this model, a single disk is slowly tilted to an angle of $30^{\circ}$ from the stellar equator over $34$ years. Then, the disk is torn by the companion's tidal torque, with the outer region separating from the innermost disk. The small inner disk returns to the stellar equator as mass injection remains constant. The outer disk precesses for $\sim15$ years before gradually dissipating. The process repeats every $34$ years and reproduces all trends in Pleione's variability., Comment: 19 pages, 15 figures

Light scattered off particles can become linearly polarized. Stars surrounded by oblique, co-rotating envelopes are therefore expected to manifest periodic linear polarimetric variations. The electron scattering magnetospheres of magnetic massive stars are expected to be suitable candidates to observe this effect. In this paper, we present the first semi-analytical model capable of synthesizing the continuum polarimetric signatures of magnetic O-type stars in an optically thin, single electron scattering limit. The purpose of this investigation is to improve our general understanding of magnetic hot stars by characterizing their polarimetric behaviour. Our linear polarization model is constructed by combining the analytical expressions for the polarimetric variations of an obliquely rotating envelope with the Analytic Dynamical Magnetosphere model to represent a physical model for the envelope density structure. We compute grids of model Stokes $Q$ and $U$ curves and show that their shapes are unique to the choice of inclination and obliquity angles. We apply our model to HD 191612, a prototypical Of?p-type star, having both polarimetric and photometric observations. We find that the polarimetric modulations are best reproduced with $i=19^{+12}_{-3}$$^\circ$, $\beta=71^{+3}_{-9}$$^\circ$, and $\log \dot{M}_{B=0}=-6.11^{+0.12}_{-0.06}$ [M$_{\odot}$ yr$^{-1}$]. These results agree with previous investigations of this star. By combining both polarimetric and photometric synthesis tools, we simultaneously model the observations thus adding further refinement of the wind and magnetic properties of HD 191612., Comment: Accepted by MNRAS

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., Comment: accepted to ApJ

UV spectroscopy and spectropolarimetry hold the key to understanding certain aspects of massive stars that are largely inaccessible with optical or longer wavelength observations. This is especially true for the rapidly-rotating Be and Bn stars, owing to their high temperatures, geometric asymmetries, binary properties, and evolutionary history. UV spectropolarimetric observations are extremely sensitive to the photospheric consequences of rapid rotation (i.e. oblateness, temperature, and surface gravity gradients). Our polarized radiative-transfer modelling predicts that with low-resolution UV spectropolarimetry covering 120 -- 300 nm the inclination angle of a rapid rotator can be determined to within 5 degrees, and the rotation rate to within 1%. The origin of rapid rotation in Be/n stars can be explained by either single-star or binary evolution, but their relative importance is largely unknown. Some Be stars have hot sub-luminous (sdO) companions, which at an earlier phase transferred their envelope (and with it mass and angular momentum) to the present-day rapid rotator. Through spectral modelling of a wide range of simulated Be/n+sdO configurations, we demonstrate that high-resolution high-SNR UV spectroscopy can detect an sdO star even when $\sim$1,000 times fainter in the UV than its Be/n star companion. This degree of sensitivity is needed to more fully explore the parameter space of Be/n+sdO binaries, which so far has been limited to about a dozen systems with relatively luminous sdO stars. We suggest that a UV spectropolarimetric survey of Be/n stars is the next step forward in understanding this population. Such a dataset would, when combined with population synthesis models, allow for the determination of the relative importance of the possible evolutionary pathways traversed by these stars, which is also crucial for understanding their future evolution and fate., Comment: 38 pages, 16 figures, 1 table, topical collection in Astrophysics and Space Science: UV Spectropolarimetry for Stellar, Interstellar, and Exoplanetary Astrophysics with Polstar

We use a three-dimensional smoothed particle hydrodynamics code to simulate growth and dissipation of Be star discs in systems where the binary orbit is misaligned with respect to the spin axis of the primary star. We investigate six different scenarios of varying orbital period and misalignment angle, feeding the disc at a constant rate for 100 orbital periods, and then letting the disc dissipate for 100 orbital periods. During the disc growth phase, we find that the binary companion tilts the disc away from its initial plane at the equator of the primary star before settling to a constant orientation after 40 to 50 orbital periods. While the mass-injection into the disc is ongoing, the tilting of the disc can cause material to reaccrete onto the primary star prematurely. Once disc dissipation begins, usually the disc precesses about the binary companion's orbital axis with precession periods ranging from 20 to 50 orbital periods. In special cases we detect phenomena of disc tearing, as well as Kozai-Lidov oscillations of the disc. These oscillations reach a maximum eccentricity of about 0.6, and a minimum inclination of about \ang{20} with respect to the binary's orbit. We also find the disc material to have highly eccentric orbits beyond the transition radius, where the disc changes from being dominated by viscous forces, to heavily controlled by the companion star, in contrast to its nearly circular motion inward of the transition radius. Finally, we offer predictions to how these changes will affect Be star observables., Comment: 14 pages, 14 figures

We use a smoothed particle hydrodynamics (SPH) code to examine the effects of a binary companion on a Be star disk for a range of disk viscosities and misalignment angles, i.e. the angle between the orbital plane and the primary's spin axis. The density structures in the disk due to the tidal interaction with the binary companion are investigated. Expanding on our previous work, the shape and density structure of density enhancements due to the binary companion are analyzed and the changes in observed interferometric features due to these orbiting enhancements are also predicted. We find that larger misalignment angles and viscosity values result in more tightly wound spiral arms with densities that fall-off more slowly with radial distance from the central star. We show that the orbital phase has very little effect on the structure of the spiral density enhancements. We demonstrate that these spiral features can be detected with an interferometer in H$\alpha$ and K-band emission. We also show that the spiral features affect the axis ratios determined by interferometry depending on the orientation of these features and the observer. For example, our simulations show that the axis ratios can vary by 20% for our co-planar binary disk system depending on the location of the disk density enhancements., Comment: 13 pages, 14 figures

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 66 Oph is consistent with standard B2Ve stellar properties. We computed a grid of 61568 Be star disk models to constrain the density profile of the disk before dissipation using observations of the H$\alpha$ line profile and SED. We find at the onset of dissipation the disk has a base density of $2.5\times10^{-11}\ \rm{g\ cm^{-3}}$ with a radial power-law index of $n=2.6$. Our models indicate that after 21 years 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 $\alpha = 0.4$ and an outer disk radius of $\sim$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 $p=0.63 \pm 0.02\%$ with a polarization position angle of $\theta_{IS}\approx85.7 \pm 0.7^\circ$. Using the Stokes QU diagram, we find the intrinsic polarization position angle of 66 Oph's disk is $\theta_{int}\approx98 \pm 3^\circ$., Comment: 20 pages, 16 figures

The observed emission lines of Be stars originate from a circumstellar Keplerian disk that are generally well explained by the Viscous Decretion Disk model. In an earlier work we performed the modeling of the full light curve of the bright Be star $\omega$ CMa (Ghoreyshi et al. 2018) with the 1-D time-dependent hydrodynamics code SINGLEBE and the Monte Carlo radiative-transfer code HDUST. We used the V -band light curve that probes the inner disk through four disk formation and dissipation cycles. This new study compares predictions of the same set of model parameters with time-resolved photometry from the near UV through the mid-infrared, comprehensive series of optical spectra, and optical broad-band polarimetry, that overall represent a larger volume of the disk. Qualitatively, the models reproduce the trends in the observed data due to the growth and decay of the disk. However, quantitative differences exist, e.g., an overprediction of the flux increasing with wavelength, too slow decreases in Balmer emission-line strength that are too slow during disk dissipation, and the discrepancy between the range of polarimetric data and the model. We find that a larger value of the viscosity parameter alone, or a truncated disk by a companion star, reduces these discrepancies by increasing the dissipation rate in the outer regions of the disk., Comment: 25 pages, 19 figures

In photometry of $\gamma$ Cas (B0.5 IVe) from the SMEI and 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 TESS satellite 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 $\beta$ Cep, SPB, and $\gamma$ Dor stars and, therefore, firmly establish $\gamma$ 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 $\gamma$ Cas is the only remaining major property that distinguishes this star from the class of classical Be stars., Comment: 19 pages, 11 figures. To be published in MNRAS

TESS photometry is analyzed for 432 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. 98\% 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 -- 4 d$^{-1}$. One or more groups of closely-spaced frequencies is the most common feature, present in 85\% of the sample. Among the Be stars with brightening events that are characteristic of mass ejection episodes (17\% of the full sample, or 30\% 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 of the sample is dominated by low frequency ($f < 0.5$ d$^{-1}$, and often much lower) variability. Stochastic signals are prominent in about 26\% of the sample, with varying degrees of intensity. Higher frequency signals ($6 < f < 15$ d$^{-1}$) are sometimes seen (in 14\% of the sample) and in most cases likely reflect p mode pulsation. In rare cases ($\sim$3\%), even higher frequencies beyond the traditional p mode regime ($f > 15$ d$^{-1}$) are observed., Comment: Submitted to AAS Journals. 52 pgs, 12 figures, 2 tables

We present a detailed visible and near-IR spectro-interferometric analysis of the Be-shell star $\omicron$ Aquarii from quasi-contemporaneous CHARA/VEGA and VLTI/AMBER observations. We measured the stellar radius of $\omicron$ Aquarii as 4.0 $\pm$ 0.3 $\mathrm{R_{\odot}}$. We constrained the disk geometry and kinematics using a kinematic model and a MCMC fitting procedure. The disk sizes in H$\alpha$ and Br$\gamma$ were found to be similar, at $\sim$10-12 $\mathrm{D_{\star}}$, which is uncommon since most results for Be stars show a larger extension in H$\alpha$ than in Br$\gamma$. We found that the inclination angle $i$ derived from H$\alpha$ is significantly lower ($\sim$15 deg) than the one derived from Br$\gamma$. The disk kinematics were found to be near to the Keplerian rotation in Br$\gamma$, but not in H$\alpha$. After analyzing all our data using a grid of HDUST models (BeAtlas), we found a common physical description for the disk in both lines: $\Sigma_{0}$ = 0.12 g cm\textsuperscript{-2} and $m$ = 3.0. The stellar rotational rate was found to be very close ($\sim$96\%) to the critical value. Our analysis of multi-epoch H$\alpha$ profiles and imaging polarimetry indicates that the disk has been stable for at least 20 years. Compared to Br$\gamma$, the data in H$\alpha$ shows a substantially different picture that cannot fully be understood using the current physical models of Be star disks. $\omicron$ Aquarii presents a stable disk, but the measured $m$ is lower than the standard value in the VDD model for steady-state. Such long-term stability can be understood in terms of the high rotational rate for this star, the rate being a main source for the mass injection in the disk. Our results on the stellar rotation and disk stability are consistent with results in the literature showing that late-type Be stars are more likely to be fast rotators and have stable disks., Comment: 24 pages, 17 figures, accepted for publication in Astronomy & Astrophysics on 06/02/2020. Abstract shortened to fit within arXiv character limit

We model the circumstellar disk of $\delta$ Sco using the 3-dimensional Monte Carlo radiative transfer code HDUST in order to quantify the large scale changes in the disk through the years 2000 to 2018, and to see if these changes can be attributed to the secondary star affecting the disk throughout its orbit. We determine our best-fitting models through matching simulated observations to actual H$\rm \alpha$ spectroscopy and V-band photometric observations. Our modelling results confirm previous findings that the disk of $\delta$ Sco was forming early in the century. We also find a period of disk dissipation when the companion is at apastron, as well as a significant growth of the disk between 2009 and 2011, prior to the periastron of 2011. Due to the steady-state nature of the disk after 2011, it is difficult to say whether the variations seen are due to the effect of the close passage of the binary companion., Comment: 16 pages, 12 figures, 4 tables, accepted for publishing in ApJ

Rapid rotation is a fundamental characteristic of classical Be stars and a crucial property allowing for the formation of their circumstellar disks. Past evolution in a mass and angular momentum transferring binary system offers a plausible solution to how Be stars attained their fast rotation. Although the subdwarf remnants of mass donors in such systems should exist in abundance, only a few have been confirmed due to tight observational constraints. An indirect method of detecting otherwise hidden companions is offered by their effect on the outer parts of Be star disks, which are expected to be disrupted or truncated. In the context of the IR and radio continuum excess radiation originating in the disk, the disk truncation can be revealed by a turndown in the spectral energy distribution due to reduced radio flux levels. In this work we search for signs of spectral turndown in a sample of 57 classical Be stars with radio data, which include new data for 23 stars and the longest wavelength detections so far ($\lambda \approx$ 10\,cm) for 2 stars. We confidently detect the turndown for all 26 stars with sufficient data coverage (20 of which are not known to have close binary companions). For the remaining 31 stars, data are inconclusive as to whether the turndown is present or not. The analysis suggests that many if not all Be stars have close companions influencing their outer disks. If confirmed to be subdwarf companions, the mass transfer spin-up scenario might explain the existence of the vast majority of classical Be stars.

The tidal interaction of a Be star with a binary companion forms two spiral arms that cause orbital modulation of the Be disc structure. The aim of this work is to identify observables in which this modulation is apparent. The structure of a Be disc in a coplanar circular binary system is computed with a smoothed-particle hydrodynamics code, and a radiation transfer code calculates the spectral energy distribution. Line depolarisation was confirmed, with polarisation profiles nearly reverse to emission-line profiles. The continuum flux maximizes for pole-on discs, but photometric variability maximizes for edge-on discs. The linear polarisation exhibits one or two maxima per orbital cycle. While polarisation variability in visible passbands is important only at low inclinations, infrared bands may demonstrate high orbital variability even at large inclinations. More evident is the modulation in the polarisation angle (PA) for low inclinations. The latter can be used to track azimuthal asymmetries for pole-on discs, where the spectroscopic variability in the violet-to-red (V/R) emission-component ratio disappears. PA reversals coincide with phases where V/R=1, tracking lines of sight directed towards regions where the approaching and receding arms overlap. Continuum flux and polarisation are mostly in phase for neighbouring wavelength regions. It is suggested that studies of non-symmetric discs distorted by tidal forces from a secondary star may be used to study disc variabilities of other origins., Comment: 20 pages, 19 figures; accepted for publication in MNRAS

In early-type Be stars, groups of nonradial pulsation (NRP) modes with numerically related frequencies may be instrumental for the release of excess angular momentum through mass-ejection events. Difference and sum/harmonic frequencies often form additional groups. The goal of this study is to find out whether a similar frequency pattern occurs in the cooler third-magnitude B7-8\,IIIe shell star $\nu$ Pup. Time-series analyses are performed of space photometry with BRITE-Constellation (2015, 2016/17, and 2017/18), SMEI (2003--011), and Hipparcos (1989-1993). Two IUE SWP and 27 optical echelle spectra spanning 20 years were retrieved from various archives. The optical spectra exhibit no anomalies or well-defined variabilities. A magnetic field was not detected. All three photometry satellites recorded variability near 0.656 c/d which is resolved into three features separated by ~0.0021 c/d. First harmonics form a second frequency group, also spaced by ~0.0021 c/d. The frequency spacing is very nearly but not exactly equidistant. Variability near 0.0021 c/d was not detected. The long-term frequency stability could be used to derive meaningful constraints on the properties of a putative companion star. The IUE spectra do not reveal the presence of a hot subluminous secondary. $\nu$\,Pup is another Be star exhibiting an NRP variability pattern with long-term constancy and underlining the importance of combination frequencies and frequency groups. The star is a good target for efforts to identify an effectively single Be star., Comment: 16 pages, 12 figures, 5 tables, submitted to Astronomy & Astrophysics

We analyzed V-band photometry of the Be star {\omega} CMa, obtained during the last four decades, during which the star went through four complete cycles of disc formation and dissipation. The data were simulated by hydrodynamic models based on a time-dependent implementation of the viscous decretion disc (VDD) paradigm, in which a disc around a fast-spinning Be star is formed by material ejected by the star and driven to progressively larger orbits by means of viscous torques. Our simulations offer a good description of the photometric variability during phases of disc formation and dissipation, which suggests that the VDD model adequately describes the structural evolution of the disc. Furthermore, our analysis allowed us to determine the viscosity parameter {\alpha}, as well as the net mass and angular momentum (AM) loss rates. We find that {\alpha} is variable, ranging from 0.1 to 1.0, not only from cycle to cycle but also within a given cycle. Additionally, build-up phases usually have larger values of {\alpha} than the dissipation phases. Furthermore, during dissipation the outward AM flux is not necessarily zero, meaning that {\omega} CMa does not experience a true quiescence but, instead, switches between a high to a low AM loss rate during which the disc quickly assumes an overall lower density but never zero. We confront the average AM loss rate with predictions from stellar evolution models for fast-rotating stars, and find that our measurements are smaller by more than one order of magnitude., Comment: 16 pages, 16 figures

High-dispersion spectroscopic monitoring of HESS J0632+057 has been carried out over four orbital cycles in order to search for orbital modulation, covering the entire orbital phase. We have measured radial velocity of H$\alpha$ emission line with the method introduced by Shafter et al. (1986), which has been successfully applied to some Be stars. The velocity is seen to increase much earlier than expected for the orbital period of 315 days, and much more steeply than expected at around "apastron". The period of the H$\alpha$ modulation is found to be as $308^{+26}_{-23}$ days. We have also analyzed Swift/XRT data from 2009 to 2015 to study the orbital modulation, selecting the data with good statistics ($\geq$ 30 counts). With additional two-year data to the previous works, the orbital period has been updated to $313^{+11}_{-8}$ days, which is consistent with the previous X-ray periods and the spectroscopic one. The past XMM-Newton and Chandra observations prefer the period of 313 days. With the new period, assuming that H$\alpha$ velocities accurately trace the motion of the Be star, we have derived a new set of the orbital parameters. In the new orbit, which is less eccentric ($e \sim 0.6$), two outbursts occur after apastron, and just after periastron. Besides, the column density in bright phase ($4.7^{+0.9}_{-08}\times10^{21}\;\mathrm{cm^{-2}}$) is higher than in faint phase ($2.2\pm0.5\times10^{21}\;\mathrm{cm^{-2}}$). These facts suggest that outbursts occur when the compact object passes nearby/through the Be disk. The mass function implies that mass of the compact object is less than 2.5 $\mathrm{M_{sun}}$ assuming that the mass of the Be star is 13.2--18.2 $\mathrm{M_{sun}}$ (Aragona at al. 2010) unless the inclination is extremely small. The photon index indicates that the spectra becomes softer when the system is bright. These suggest that the compact object is a pulsar., Comment: 15 pages, 7 figures, accepted to PASJ

The BRITE Constellation of nanosatellites obtained mmag photometry of 28 Cygni for 11 months in 2014-2016. Observations with the Solar Mass Ejection Imager in 2003-2010 and 118 H$\alpha$ line profiles were added. For decades, 28 Cyg has exhibited four large-amplitude frequencies: two closely spaced frequencies of spectroscopically confirmed $g$ modes near 1.5 c/d, one slightly lower exophotospheric (Stefl) frequency, and at 0.05 c/d the difference frequency between the two g modes. This top-level framework is indistinguishable from eta Cen (Paper I), which is also very similar in spectral type, rotation rate, and viewing angle. The Stefl frequency is the only one that does not seem to be affected by the difference frequency. The amplitude of the latter undergoes large variations; around maximum the amount of near-circumstellar matter is increased, and the amplitude of the Stefl frequency grows by some factor. During such brightenings dozens of transient spikes appear in the frequency spectrum, concentrated in three groups. Only eleven frequencies were common to all years of BRITE observations. Be stars seem to be controlled by several coupled clocks, most of which are not very regular on timescales of weeks to months but function for decades. The combination of g modes to the low difference frequency and/or the atmospheric response to it appears significantly nonlinear. Like in eta Cen, the difference-frequency variability seems the main responsible for the modulation of the star-to-disc mass transfer in 28 Cyg. A hierarchical set of difference frequencies may reach the longest timescales known of the Be phenomenon., Comment: 17 pages, 21 figures, submitted to Astronomy & Astrophysics

In order to study the growth and evolution of circumstellar disks around classical Be stars, we analyze optical time-series photometry from the KELT survey with simultaneous infrared and visible spectroscopy from the APOGEE survey and BeSS database for a sample of 160 Galactic classical Be stars. The systems studied here show variability including transitions from a diskless to a disk-possessing state (and vice versa), and persistent disks that vary in strength, being replenished at either regularly or irregularly occurring intervals. We detect disk-building events (outbursts) in the light curves of 28\% of our sample. Outbursts are more commonly observed in early- (57\%), compared to mid- (27\%) and late-type (8\%) systems. A given system may show anywhere between 0 -- 40 individual outbursts in its light curve, with amplitudes ranging up to $\sim$0.5 mag and event durations between $\sim$2 -- 1000 days. We study how both the photometry and spectroscopy change together during active episodes of disk growth or dissipation, revealing details about the evolution of the circumstellar environment. We demonstrate that photometric activity is linked to changes in the inner disk, and show that, at least in some cases, the disk growth process is asymmetrical. Observational evidence of Be star disks both growing and clearing from the inside out is presented. The duration of disk buildup and dissipation phases are measured for 70 outbursts, and we find that the average outburst takes about twice as long to dissipate as it does to build up in optical photometry. Our analysis hints that dissipation of the inner disk occurs relatively slowly for late-type Be stars., Comment: Published in The Astronomical Journal. 23 pages, 25 figures, 2 tables

We apply the viscous decretion disc (VDD) model to interpret the infrared disc continuum emission of 80 Be stars observed in different epochs. In this way, we determined 169 specific disc structures, namely their density scale, $\rho_0$, and exponent, $n$. We found that the $n$ values range mainly between $1.5$ and $3.5$, and $\rho_0$ varies between $10^{-12}$ and $10^{-10}\,\mathrm{g\,cm^{-3}}$, with a peak close to the lower value. Our large sample also allowed us to firmly establish that the discs around early-type stars are denser than in late-type stars. Additionally, we estimated the disc mass decretion rates and found that they range between $10^{-12}$ and $10^{-9}\,\mathrm{M_{\odot}\,yr^{-1}}$. These values are compatible with recent stellar evolution models of fast-rotating stars. One of the main findings of this work is a correlation between the $\rho_0$ and $n$ values. In order to find out whether these relations can be traced back to the evolution of discs or have some other origin, we used the VDD model to calculate temporal sequences under different assumptions for the time profile of the disc mass injection. The results support the hypothesis that the observed distribution of disc properties is due to a common evolutionary path. In particular, our results suggest that the timescale for disc growth, during which the disc is being actively fed by mass injection episodes, is shorter than the timescale for disc dissipation, when the disc is no longer fed by the star and dissipates as a result of the viscous diffusion of the disc material., Comment: 21 pages, 15 figures, 4 tables, 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society

We investigated the properties of $80$ Be star disks, using the time dependent viscous decretion disk (VDD) model to interpret their mid-infrared continuum fluxes. Besides estimating the typical parameter ranges for such disks, this approach allowed us to relate the disk structure to its evolutionary status. In this contribution, we present a brief summary of the recently published results, and discuss some properties and time-scales of the VDD model solution., Comment: 6 pages, 3 figures

We use a smoothed particle hydrodynamics (SPH) code to examine the effects of misaligned binary companions on Be star discs. We systematically vary the degree of misalignment between the disc and the binary orbit, as well as the disc viscosity and orbital period to study their effects on the density in the inner and outer parts of the disc. We find that varying the degree of misalignment, the viscosity, and the orbital period affects both the truncation radius and the density structure of the outer disc, while the inner disc remains mostly unaffected. We also investigate the tilting of the disc in the innermost part of the disc and find the tilt increases with radius until reaching a maximum around 5 stellar radii. The direction of the line of nodes, with respect to the equator of the central star, is found to be offset compared to the orbital line of nodes, and to vary periodically in time, with a period of half a orbital phase. We also compare the scale height of our discs with the analytical scale height of an isothermal disc, which increases with radius as $r^{1.5}$. We find that this formula reproduces the scale height well for both aligned and misaligned systems but underestimates the scale height in regions of the disc where density enhancements develop., Comment: 11 pages, 10 figures, to be published in MNRAS

The first results of radiative transfer calculations on decretion discs of binary Be stars are presented. A smoothed particle hydrodynamics code computes the structure of Be discs in coplanar circular binary systems for a range of orbital and disc parameters. The resulting disc configuration consists of two spiral arms, and can be given as input into a Monte Carlo code, which calculates the radiative transfer along the line of sight for various observational coordinates. Making use of the property of steady disc structure in coplanar circular binaries, observables are computed as functions of the orbital phase. Orbital-phase series of line profiles are given for selected parameter sets under various viewing angles, to allow comparison with observations. Flat-topped profiles with and without superimposed multiple structures are reproduced, showing, for example, that triple-peaked profiles do not have to be necessarily associated with warped discs and misaligned binaries. It is demonstrated that binary tidal effects give rise to phase-locked variability of the violet-to-red (V/R) ratio of hydrogen emission lines. The V/R ratio exhibits two maxima per cycle; in certain cases those maxima are equal, leading to a clear new V/R cycle every half orbital period. This study opens a way in identifying binaries and in constraining the parameters of binary systems that exhibit phase-locked variations induced by tidal interaction with a companion star., Comment: 12 pages, 16 figures; accepted for publication by MNRAS

Context. The structure of the inner parts of Be star disks (20 stellar radii) is well explained by the viscous decretion disk (VDD) model, which is able to reproduce the observable properties of most of the objects studied so far. The outer parts, on the ther hand, are not observationally well-explored, as they are observable only at radio wavelengths. A steepening of the spectral slope somewhere between infrared and radio wavelengths was reported for several Be stars that were previously detected in the radio, but a convincing physical explanation for this trend has not yet been provided. Aims. We test the VDD model predictions for the extended parts of a sample of six Be disks that have been observed in the radio to address the question of whether the observed turndown in the spectral energy distribution (SED) can be explained in the framework of the VDD model, including recent theoretical development for truncated Be disks in binary systems. Methods. We combine new multi-wavelength radio observations from the Karl. G. Jansky Very Large Array (JVLA) and Atacama Pathfinder Experiment (APEX) with previously published radio data and archival SED measurements at ultraviolet, visual, and infrared wavelengths. The density structure of the disks, including their outer parts, is constrained by radiative transfer modeling of the observed spectrum using VDD model predictions. In the VDD model we include the presumed effects of possible tidal influence from faint binary companions. Results. For 5 out of 6 studied stars, the observed SED shows strong signs of SED turndown between far-IR and radio wavelengths. A VDD model that extends to large distances closely reproduces the observed SEDs up to far IR wavelengths, but fails to reproduce the radio SED. ... (abstract continues but did not fit here), Comment: 20 pages, 8 figures

Context. The mechanism of disk formation around fast-rotating Be stars is not well understood. In particular, it is not clear which mechanisms operate, in addition to fast rotation, to produce the observed variable ejection of matter. The star Achernar is a privileged laboratory to probe these additional mechanisms because it is close, presents B-Be phase variations on timescales ranging from 6 yr to 15 yr, a companion star was discovered around it, and probably presents a polar wind or jet. Aims. Despite all these previous studies, the disk around Achernar was never directly imaged. Therefore we seek to produce an image of the photosphere and close environment of the star. Methods. We used infrared long-baseline interferometry with the PIONIER/VLTI instrument to produce reconstructed images of the photosphere and close environment of the star over four years of observations. To study the disk formation, we compared the observations and reconstructed images to previously computed models of both the stellar photosphere alone (normal B phase) and the star presenting a circumstellar disk (Be phase). Results. The observations taken in 2011 and 2012, during the quiescent phase of Achernar, do not exhibit a disk at the detection limit of the instrument. In 2014, on the other hand, a disk was already formed and our reconstructed image reveals an extended H-band continuum excess flux. Our results from interferometric imaging are also supported by several H-alpha line profiles showing that Achernar started an emission-line phase sometime in the beginning of 2013. The analysis of our reconstructed images shows that the 2014 near-IR flux extends to 1.7 - 2.3 equatorial radii. Our model-independent size estimation of the H-band continuum contribution is compatible with the presence of a circumstellar disk, which is in good agreement with predictions from Be-disk models.

The late-type Be star $\beta$ CMi is remarkably stable compared to other Be stars that have been studied. This has led to a realistic model of the outflowing Be disk by Klement et al. These results showed that the disk is likely truncated at a finite radius from the star, which Klement et al.~suggest is evidence for an unseen binary companion in orbit. Here we report on an analysis of the Ritter Observatory spectroscopic archive of $\beta$ CMi to search for evidence of the elusive companion. We detect periodic Doppler shifts in the wings of the H$\alpha$ line with a period of 170 d and an amplitude of 2.25 km s$^{-1}$, consistent with a low-mass binary companion ($M\approx 0.42 M_\odot$). We then compared the small changes in the violet-to-red peak height changes ($V/R$) with the orbital motion. We find weak evidence that it does follow the orbital motion, as suggested by recent Be binary models by Panoglou et al. Our results, which are similar to those for several other Be stars, suggest that $\beta$ CMi may be a product of binary evolution where Roche lobe overflow has spun up the current Be star, likely leaving a hot subdwarf or white dwarf in orbit around the star. Unfortunately, no direct sign of this companion star is found in the very limited archive of {\it International Ultraviolet Explorer} spectra., Comment: Accepted to AAS Journals, 2 tables, 3 figures, 11 pages

In this work we explore the effect of binarity in the decretion disc of Be stars, in order to explain their variability. To this aim, we performed smoothed particle hydrodynamics (SPH) simulations on Be binary systems, following the matter ejected isotropically from the equator of the Be star towards the base of an isothermal decretion disc. We let the system evolve for time long enough to be considered at steady state, and focus on the effect of viscosity for coplanar prograde binary orbits. The disc structure is found to be locked to the orbital phase, exhibiting also a dependence on the azimuthal angle. Additionally, we present the first results from detailed non-local thermodynamic equilibrium (non-LTE) radiative transfer calculations of the disc structure computed with the SPH code. This is achieved by the use of the three-dimensional (3D) Monte Carlo code HDUST, which can produce predictions with respect to a series of observables., Comment: 6 pages, 3 figures; to appear in the proceedings of the conference "The B[e] phenomenon: Forty years of studies" held in Prague, 2016

Classical Be stars are introduced as object class and their particular potential for space based photometry is highlighted. A brief summary of the various types of variability observed in Be stars makes clear that an interpretation of every single frequency as a pulsation mode falls short, instead there are as well purely circumstellar variations and those that originate in the immediate stellar to circumstellar interaction region. In particular the latter offer great potential, as they are linked to one of the few remaining great riddles of Be stars, namely how they feed their disks., Comment: 8 pages, 4 figures, invited talk to 2nd BRITE-Constellation Science Conference held in Innsbruck, to be published in the Proceedings of the Polish Astronomical Society, eds: G. Stachowski, E. Poretti, and J. Matthews

We analyze 45 spectropolarimetric observations of the eclipsing, interacting binary star V356 Sgr, obtained over a period of 21 years, to characterize the geometry of the system's circumstellar material. After removing interstellar polarization from these data, we find the system exhibits a large intrinsic polarization signature arising from electron scattering. In addition, the lack of repeatable eclipses in the polarization phase curves indicates the presence of a substantial pool of scatterers not occulted by either star. We suggest that these scatterers form either a circumbinary disk coplanar with the gainer's accretion disk or an elongated structure perpendicular to the orbital plane of V356 Sgr, possibly formed by bipolar outflows. We also observe small-scale, cycle-to-cycle variations in the magnitude of intrinsic polarization at individual phases, which we interpret as evidence of variability in the amount of scattering material present within and around the system. This may indicate a mass transfer or mass loss rate that varies on the time-scale of the system's orbital period. Finally, we compare the basic polarimetric properties of V356 Sgr with those of the well studied beta Lyr system; the significant differences observed between the two systems suggests diversity in the basic circumstellar geometry of Roche-lobe overflow systems, Comment: 14 pages, 6 figures, 5 tables, accepted to MNRAS, Tables 1 and 3 will be available in full at CDS after publication

A global disk oscillation implemented in the viscous decretion disk (VDD) model has been used to reproduce most of the observed properties of the well known Be star $\zeta$ Tau. 48 Librae shares several similarities with $\zeta$ Tau -- they are both early-type Be stars, they display shell characteristics in their spectra, and they exhibit cyclic $V/R$ variations -- but has some marked differences as well, such as a much denser and more extended disk, a much longer $V/R$ cycle, and the absence of the so-called triple-peak features. We aim to reproduce the photometric, polarimetric, and spectroscopic observables of 48 Librae with a self-consistent model, and to test the global oscillation scenario for this target. Our calculations are carried out with the three-dimensional NLTE radiative transfer code HDUST. We employ a rotationally deformed, gravity-darkened central star, surrounded by a disk whose unperturbed state is given by the VDD model. A two-dimensional global oscillation code is then used to calculate the disk perturbation, and superimpose it on the unperturbed disk. A very good, self-consistent fit to the time-averaged properties of the disk is obtained with the VDD. The calculated perturbation has a period $P = 12$ yr, which agrees with the observed period, and the behaviour of the $V/R$ cycle is well reproduced by the perturbed model. The perturbed model improves the fit to the photometric data and reproduces some features of the observed spectroscopic data. Some suggestions to improve the synthesized spectroscopy in a future work are given.

Be stars are surrounded by outflowing circumstellar matter structured in the form of decretion discs. They are often members of binary systems, where it is expected that the decretion disc interacts both radiatively and gravitationally with the companion. In this work we study how various orbital (period, mass ratio and eccentricity) and disc (viscosity) parameters affect the disc structure in coplanar systems. We simulate such binaries with the use of a smoothed particle hydrodynamics code. The main effects of the secondary on the disc are its truncation and the accumulation of material inwards of truncation. We find two cases with respect to the effects of eccentricity: (i) In circular or nearly circular prograde orbits, the disc maintains a rotating, constant in shape, configuration, which is locked to the orbital phase. The disc is smaller in size, more elongated and more massive for low viscosity parameter, small orbital separation and/or high mass ratio. (ii) Highly eccentric orbits are more complex, with the disc structure and total mass strongly dependent on the orbital phase and the distance to the secondary. We also study the effects of binarity in the disc continuum emission. Since the infrared and radio SED are sensitive to the disc size and density slope, the truncation and matter accumulation result in considerable modifications in the emergent spectrum. We conclude that binarity can serve as an explanation for the variability exhibited in observations of Be stars, and that our model can be used to detect invisible companions., Comment: 15 pages, 30 figures

In the last decade, the viscous decretion disk model has emerged as the new paradigm for Be star disks. In this contribution, we propose a simple analytical model to estimate the continuum infrared excess arising from these circumstellar disks, in the light of the currently accepted scenario. We demonstrate that the disk can be satisfactorily described by a two component system: an inner optically thick region, which we call the pseudo-photosphere, and a diffuse outer part. In particular, a direct connexion between the disk brightness profile and the thermal structure is derived, and then confronted to realistic numerical simulations. This result quantifies how the non-isothermality of the disk ultimately affects both infrared measured fluxes and visibilities., Comment: To appear in conference proceedings "Bright Emissaries: Be Stars as Messengers of Star-Disk Physics", eds. A. Sigut and C. Jones (11 Aug 2014 - 13 Aug 2014, London, Ontario, Canada)

Spectroscopic observations of Achernar over the past decades, have shown the photospheric line width, as measured by the rotational parameter $v \sin i$, to vary in correlation with the emission activity. Here we present new observations, covering the most recent activity phase, and further archival data collected from the archives. The $v \sin i$ variation is confirmed. On the basis of the available data it cannot be decided with certainty whether the increased line width precedes the emission activity, i.e. is a signature of the ejection mechanism, or postdates is, which would make it a signature of re-accretion of some of the disk-material. However, the observed evidence leans towards the re-accretion hypothesis. Two further stars showing the effect of variable line width in correlation with emission activity, namely 66 Oph and $\pi$ Aqr, are presented as well., Comment: To appear in conference proceedings "Bright Emissaries: Be Stars as Messengers of Star-Disk Physics", eds. A. Sigut and C. Jones (11 Aug 2014 - 13 Aug 2014, London, Ontario, Canada)

Empirical evidence for the involvement of nonradial pulsations (NRP's) in the mass loss from Be stars ranges from (i) a singular case (\object{$\mu$ Cen}) of repetitive mass ejections triggered by multi-mode beating to (ii) several photometric reports about enormous numbers of pulsation modes popping up during outbursts and on to (iii) effective single-mode pulsators. The BRITE Constellation of nanosatellites was used to obtain mmag photometry of the Be stars $\eta$ and \object{$\mu$ Cen}. In the low-inclination star \object{$\mu$ Cen}, light pollution by variable amounts of near-stellar matter prevented any new insights into the variability and other properties of the central star. In the equator-on star \object{$\eta$ Cen}, BRITE photometry and {\sc Heros} echelle spectroscopy from the 1990s reveal an intricate clockwork of star-disk interactions. The mass transfer is modulated with the frequency difference of two NRP modes and an amplitude three times as large as the amplitude sum of the two NRP modes. This process feeds a high-amplitude circumstellar activity running with the incoherent and slightly lower so-called \v{S}tefl frequency. The mass loss-modulation cycles are tightly coupled to variations in the value of the \v{S}tefl frequency and in its amplitude, albeit with strongly drifting phase differences. The observations are well described by the decomposition of the mass loss into a pulsation-related engine in the star and a viscosity-dominated engine in the circumstellar disk. Arguments are developed that large-scale gas-circulation flows occur at the interface. The propagation rates of these eddies manifest themselves as \v{S}tefl frequencies. Bursts in power spectra during mass-loss events can be understood as the noise inherent to these gas flows., Comment: Recommended for publication in Astronomy & Astrophysics

The MiMeS project is a large-scale, high resolution, sensitive spectropolarimetric investigation of the magnetic properties of O and early B type stars. Initiated in 2008 and completed in 2013, the project was supported by 3 Large Program allocations, as well as various programs initiated by independent PIs and archival resources. Ultimately, over 4800 circularly polarized spectra of 560 O and B stars were collected with the instruments ESPaDOnS at the Canada-France-Hawaii Telescope, Narval at the T\'elescope Bernard Lyot, and HARPSpol at the European Southern Observatory La Silla 3.6m telescope, making MiMeS by far the largest systematic investigation of massive star magnetism ever undertaken. In this paper, the first in a series reporting the general results of the survey, we introduce the scientific motivation and goals, describe the sample of targets, review the instrumentation and observational techniques used, explain the exposure time calculation designed to provide sensitivity to surface dipole fields above approximately 100 G, discuss the polarimetric performance, stability and uncertainty of the instrumentation, and summarize the previous and forthcoming publications., Comment: Accepted by MNRAS

We investigate the continuum emission of viscous decretion discs around Be stars in this paper. The results obtained from non-LTE (local thermodynamic equilibrium) radiative transfer models show two regimes in the disc surface brightness profile: an inner optically thick region, which behaves as a pseudo-photosphere with a wavelength-dependent size, and an optically thin tenuous outer part, which contributes with about a third of the total flux. The isophotal shape of the surface brightness is well described by elliptical contours with an axial ratio $b/a=\cos i$ for inclinations $i<75^{\circ}$. Based on these properties, a semi-analytical model was developed to describe the continuum emission of gaseous discs. It provides fluxes and spectral slopes at the infrared within an accuracy of $10\%$ and $5\%$, respectively, when compared to the numerical results. The model indicates that the infrared spectral slope is mainly determined by both the density radial slope and the disc flaring exponent, being practically independent of disc inclination and base density. As a first application, the density structure of 15 Be stars was investigated, based on the infrared flux excess, and the results compared to previous determinations in the literature. Our results indicate that the decretion rates are in the range of $10^{-12}$ to $10^{-9}\,{\rm M_{\odot}\,yr^{-1}}$, which is at least two orders of magnitude smaller than the previous outflowing disc model predictions., Comment: 13 pages, 19 figures

The viscous decretion disk (VDD) model is able to explain most of the currently observable properties of the circumstellar disks of Be stars. However, more stringent tests, focusing on reproducing multitechnique observations of individual targets via physical modeling, are needed to study the predictions of the VDD model under specific circumstances. In the case of nearby, bright Be star $\beta$ CMi, these circumstances are a very stable low-density disk and a late-type (B8Ve) central star. The aim is to test the VDD model thoroughly, exploiting the full diagnostic potential of individual types of observations, in particular, to constrain the poorly known structure of the outer disk if possible, and to test truncation effects caused by a possible binary companion using radio observations. We use the Monte Carlo radiative transfer code HDUST to produce model observables, which we compare with a very large set of multitechnique and multiwavelength observations that include ultraviolet and optical spectra, photometry covering the interval between optical and radio wavelengths, optical polarimetry, and optical and near-IR (spectro)interferometry. Due to the absence of large scale variability, data from different epochs can be combined into a single dataset. A parametric VDD model with radial density exponent of $n$ = 3.5, which is the canonical value for isothermal flaring disks, is found to explain observables typically formed in the inner disk, while observables originating in the more extended parts favor a shallower, $n$ = 3.0, density falloff. Modeling of radio observations allowed for the first determination of the physical extent of a Be disk (35$^{+10}_{-5}$ stellar radii), which might be caused by a binary companion. Finally, polarization data allowed for an indirect measurement of the rotation rate of the star, which was found to be $W \gtrsim 0.98$, i.e., very close to critical., Comment: 19 pages (35 including online material), 17 figures, 2 online figures, 2 online tables with data

We present spatially and spectrally resolved Br-gamma emission around the planet-hosting, transitional Herbig Ae/Be star HD 100546. Aiming to gain insight into the physical origin of the line in possible relation to accretion processes, we carried out Br-gamma spectro-interferometry using AMBER/VLTI from three different baselines achieving spatial and spectral resolutions of 2-4 mas and 12000. The Br-gamma visibility is larger than that of the continuum for all baselines. Differential phases reveal a shift between the photocentre of the Br-gamma line -displaced 0.6 mas (0.06 au at 100 pc) NE from the star- and that of the K-band continuum emission -displaced 0.3 mas NE from the star. The photocentres of the redshifted and blueshifted components of the Br-gamma line are located NW and SE from the photocentre of the peak line emission, respectively. Moreover, the photocentre of the fastest velocity bins within the spectral line tends to be closer to that of the peak emission than the photocentre of the slowest velocity bins. Our results are consistent with a Br-gamma emitting region inside the dust inner rim (<0.25 au) and extending very close to the central star, with a Keplerian, disc-like structure rotating counter-clockwise, and most probably flared (25 deg). Even though the main contribution to the Br-gamma line does not come from gas magnetically channelled on to the star, accretion on to HD 100546 could be magnetospheric, implying a mass accretion rate of a few 10^-7 Msun/yr. This value indicates that the observed gas has to be replenished on time-scales of a few months to years, perhaps by planet-induced flows from the outer to the inner disc as has been reported for similar systems., Comment: 5 Figures. Accepted in MNRAS

We have used the radiative transfer code HDUST to analyze and interpret the long-term photometric behavior of the Be star omega CMa, considering four complete cycles of disk formation and dissipation. This is the first time in which a full lightcurve of a Be star was investigated and modeled including both disk build-up and dissipation phases. Based on the quite good fit of the observed data we were able to derive the history of stellar mass decretion rates (including long- and short-term changes) during the disk formation and dissipation phases in all four cycles., Comment: 4 pages, 4 figures