The warm circumstellar envelope and wind of the G9 IIb star HR 6902

IUE observations of the eclipsing binary system HR 6902 obtained at various epochs spread over four years indicate the presence of warm circumstellar material enveloping the G9 IIb primary. The spectra show Si IV and C IV absorption up to a distance of 3.3 giant radii (R_g}. Line ratio diagnostics yields an electron temperature of ~ 78000 K which appears to be constant over the observed height range. Applying a least square fit absorption line analysis we derive column densities as a function of height. We find that the inner envelope (< 3 R_g) of the bright giant is consistent with a hydrostatic density distribution. The derived line broadening velocity of ~ 70 kms^{-1} is sufficient to provide turbulent pressure support for the required scale height. However, an improved agreement with observations over the whole height regime including the emission line region is obtained with an outflow model. We demonstrate that the common beta power-law as well as a P \propto rho wind yield appropriate fit models. Adopting a continuous mass outflow we obtain a mass-loss rate of M_loss= (0.8 - 3.4)*10^{-11} M_{sun}yr^{-1} depending on the particular wind model.Comment: 11 pages, 8 figures, submitted to Astronomy Astrophysics main Journa

Evolution and appearance of Be stars in SMC clusters

Star clusters are privileged laboratories for studying the evolution of massive stars (OB stars). One particularly interesting question concerns the phases, during which the classical Be stars occur, which unlike HAe/Be stars, are not pre-main sequence objects, nor supergiants. Rather, they are extremely rapidly rotating B-type stars with a circumstellar decretion disk formed by episodic ejections of matter from the central star. To study the impact of mass, metallicity, and age on the Be phase, we observed SMC open clusters with two different techniques: 1) with the ESO-WFI in its slitless mode, which allowed us to find the brighter Be and other emission-line stars in 84 SMC open clusters 2) with the VLT-FLAMES multi-fiber spectrograph in order to determine accurately the evolutionary phases of Be stars in the Be-star rich SMC open cluster NGC 330. Based on a comparison to the Milky Way, a model of Be stellar evolution / appearance as a function of metallicity and mass / spectral type is developed, involving the fractional critical rotation rate as a key parameter.Comment: Proceedings of the IAUS266 of the GA200

A hydrodynamic study of the circumstellar envelope of alpha Scorpii

Context: Both the absolute mass-loss rates and the mechanisms that drive the mass loss of late-type supergiants are still not well known. Binaries such as alpha Sco provide the most detailed empirical information about the winds of these stars. Aims: The goal was to improve the binary technique for the determination of the mass-loss rate of alpha Sco A by including a realistic density distribution and velocity field from hydrodynamic and plasma simulations. Methods: We performed 3D hydrodynamic simulations of the circumstellar envelope of alpha Sco in combination with plasma simulations accounting for the heating, ionization, and excitation of the wind by the radiation of alpha Sco B. These simulations served as the basis for an examination of circumstellar absorption lines in the spectrum of alpha Sco B as well as of emission lines from the Antares nebula. Results: The present model of the extended envelope of alpha Sco reproduces some of the structures that were observed in the circumstellar absorption lines in the spectrum of alpha Sco B. Our theoretical density and velocity distributions of the outflow deviate considerably from a spherically expanding model, which was used in previous studies. This results in a higher mass-loss rate of (2 +/- 0.5) x 10^-6 M_sun/yr. The hot H II region around the secondary star induces an additional acceleration of the wind at large distances from the primary, which is seen in absorption lines of Ti II and Cr II at -30 km/s.Comment: 12 pages, 14 figures, accepted for publication in A&

Magnetic field detection in the B2Vn star HR 7355

The B2Vn star HR 7355 is found to be a He-rich magnetic star. Spectropolarimetric data were obtained with FORS1 at UT2 on Paranal observatory to measure the disk-averaged longitudinal magnetic field at various phases of the presumed 0.52 d cycle. A variable magnetic field with strengths between B_z = -2200 and +3200G was found, with confidence limits of 100 to 130G. The field topology is that of an oblique dipole, while the star itself is seen about equator-on. In the intensity spectra the HeI-lines show the typical equivalent width variability of He-strong stars, usually attributed to surface abundance spots. The amplitudes of the equivalent width variability of the HeI lines are extraordinarily strong compared to other cases. These results not only put HR 7355 unambiguously among the early-type magnetic stars, but confirm its outstanding nature: With v sin i = 320 km/s the parameter space in which He-strong stars are known to exist has doubled in terms of rotational velocity.Comment: 6 pages, 5 figures, 1 Table. Accepted for publication in MNRAS Letter

The (B0+?)+O6 system FN CMa: A case for tidal-pulsational interaction?

FN CMa is visually double with a separation of about 0.6arcsec. Sixty high-cadence VLT/UVES spectra permit the A and B components to be disentangled, as the relative contribution of each star to the total light entering the spectrograph fluctuates between exposures due to changes in seeing. Component A exhibits rapid line-profile variations, leading us to attribute the photometric variability seen by HIPPARCOS (with a derived P=0.08866d) to this component. From a total of 122 archival and new echelle spectra it is shown that component A is an SB1 binary with an orbital period of 117.55 days. The eccentricity of 0.6 may result in tidal modulation of the pulsation(s) of component Aa.Comment: 2 pages, 1 figure, IAUS 272 - Active OB Stars: Structure, Evolution, Mass Loss and Critical Limit

Differential interferometric phases at high spectral resolution as a sensitive physical diagnostic of circumstellar disks

Context. The circumstellar disks ejected by many rapidly rotating B stars (so-called Be stars) offer the rare opportunity of studying the structure and dynamics of gaseous disks at high spectral as well as angular resolution. Aims. This paper explores a newly identified effect in spectro-interferometric phase that can be used for probing the inner regions of gaseous edge-on disks on a scale of a few stellar radii. Methods. The origin of this effect (dubbed central quasi-emission phase signature, CQE-PS) lies in the velocity-dependent line absorption of photospheric radiation by the circumstellar disk. At high spectral and marginal interferometric resolution, photocenter displacements between star and isovelocity regions in the Keplerian disk reveal themselves through small interferometric phase shifts. To investigate the diagnostic potential of this effect, a series of models are presented, based on detailed radiative transfer calculations in a viscous decretion disk. Results. Amplitude and detailed shape of the CQE-PS depend sensitively on disk density and size and on the radial distribution of the material with characteristic shapes in differential phase diagrams. In addition, useful lower limits to the angular size of the central stars can be derived even when the system is almost unresolved. Conclusions. The full power of this diagnostic tool can be expected if it can be applied to observations over a full life-cycle of a disk from first ejection through final dispersal, over a full cycle of disk oscillations, or over a full orbital period in a binary system