Alex C. Carciofi

Cyclic variability of the circumstellar disk of the Be star zeta Tauri II. Testing the 2D global disk oscillation model

Context. About 2/3 of the Be stars present the so-called V/R variations, a phenomenon characterized by the quasi-cyclic variation in the ratio between the violet and red emission peaks of the H i emission lines. These variations are generally explained by global oscillations in the circumstellar disk forming a one-armed spiral density pattern that precesses around the star with a period of a few years. Aims. This paper presents self-consistent models of polarimetric, photometric, spectrophotometric, and interferometric observations of the classical Be star ζ Tauri. The primary goal is to conduct a critical quantitative test of the global oscillation scenario. Methods. Detailed three-dimensional, NLTE radiative transfer calculations were carried out using the radiative transfer code HDUST. The most up-to-date research on Be stars was used as input for the code in order to include a physically realistic description for the central star and the circumstellar disk. The model adopts a rotationally deformed, gravity darkened central star, surrounded by a disk whose unperturbed state is given by a steady-state viscous decretion disk model. It is further assumed that this disk is in vertical hydrostatic equilibrium. Results. By adopting a viscous decretion disk model for ζ Tauri and a rigorous solution of the radiative transfer, a very good fit of the time-average properties of the disk was obtained. This provides strong theoretical evidence that the viscous decretion disk model is the mechanism responsible for disk formation. The global oscillation model successfully fitted spatially resolved VLTI/AMBER observations and the temporal V/R variations in the Hα and Brγ lines. This result convincingly demonstrates that the oscillation pattern in the disk is a one-armed spiral. Possible model shortcomings, as well as suggestions for future improvements, are also discussed.

The MiMeS survey of magnetism in massive stars: introduction and overview

The Magnetism in Massive Stars (MiMeS) survey represents a highprecision systematic search for magnetic fields in hot, massive OB stars. To date, MiMeS Large Programs (ESPaDOnS@CFHT, Narval@TBL, HARPSpol@ESO3.6) and associated PI programs (FORS@VLT) have yielded nearly 1200 circular spectropolarimetric observations of over 350 OB stars. Within this sample, 20 stars are detected as magnetic. Follow-up observations of new detections reveals (i) a large diversity of magnetic properties, (ii) ubiquitous evidence for magnetic wind confinement in optical spectra of all magnetic O stars, and (iii) the presence of strong, organized magnetic fields in all known Galactic Of?p stars, and iv) a complete absence of magnetic fields in classical Be stars.

Non-LTE Monte Carlo Radiative Transfer. II. Nonisothermal Solutions for Viscous Keplerian Disks

We discuss the basic hydrodynamics that determines the density structure of the disks around hot stars. Observational evidence supports the idea that these disks are Keplerian (rotationally supported) gaseous disks. A popular scenario in the literature, which naturally leads to the formation of Keplerian disks, is the viscous decretion model. According to this scenario, the disks are hydrostatically supported in the vertical direction, while the radial structure is governed by the viscous transport. This suggests that the temperature is one primary factor that governs the disk density structure. In a previous study we demonstrated, using 3-D NLTE Monte Carlo simulations, that viscous keplerian disks can be highly non-isothermal. In this paper we build upon our previous work and solve the full problem of the steady-state non-isothermal viscous diffusion and vertical hydrostatic equilibrium. We find that the self-consistent solution departs significantly from the analytic isothermal density, with potentially large effects on the emergent spectrum. This implies that non-isothermal disk models must be used for a detailed modeling of Be star disks.

THE FIRST DETERMINATION OF THE VISCOSITY PARAMETER IN THE CIRCUMSTELLAR DISK OF A Be STAR

Be stars possess gaseous circumstellar decretion disks, which are well described using standard

Properties of the δ Scorpii Circumstellar Disk from Continuum Modeling

\alpha

Dynamical Evolution of Viscous Disks around Be Stars. I: photometry

-disk theory. The Be star 28 CMa recently underwent a long outburst followed by a long period of quiescence, during which the disk dissipated. Here we present the first time-dependent models of the dissipation of a viscous decretion disk. By modeling the rate of decline of the V-band excess, we determine that the viscosity parameter

Cyclic variability of the circumstellar disk of the Be star

\alpha=1.0\pm0.2

\zeta

, corresponding to a mass injection rate

Tauri - I. Long-term monitoring observations

\dot{M}=(3.5\pm 1.3) \times 10^{-8}\ M_\sun\,\mathrm{yr}^{-1}

On the Determination of the Rotational Oblateness of Achernar

. Such a large value of