Context. Classical Be stars are hot non-supergiant stars surrounded by a gaseous circumstellar disk that is responsible for the observed infrared-excess and emission lines. The phenomena involved in the disk formation still remain highly debated. Aims. To progress in the understanding of the physical process or processes responsible for the mass ejections and test the hypothesis that they depend on the stellar parameters, we initiated a survey on the circumstellar environment of the brightest Be stars. Methods. To achieve this goal, we used spectro-interferometry, the only technique that combines high spectral (R = 12 000) and high spatial (θ min = 4 mas) resolutions. Observations were carried out at the Paranal observatory with the VLTI/AMBER instrument. We concentrated our observations on the Brγ emission line to be able to study the kinematics within the circumstellar disk. Our sample is composed of eight bright classical Be stars: α Col, κ CMa, ω Car, p Car, δ Cen, μ Cen, α Ara, and o Aqr. Results. We managed to determine the disk extension in the line and the nearby continuum for most targets. We also constrained the disk kinematics, showing that it is dominated by rotation with a rotation law close to the Keplerian one. Our survey also suggests that these stars are rotating at a mean velocity of V/V c = 0.82 ± 0.08. This corresponds to a rotational rate of Ω/Ω c = 0.95 ± 0.02. Conclusions. We did not detect any correlation between the stellar parameters and the structure of the circumstellar environment. Moreover, it seems that a simple model of a geometrically thin Keplerian disk can explain most of our spectrally resolved K-band data. Nevertheless, some small departures from this model have been detected for at least two objects (i.e., κ CMa and α Col). Finally, our Be stars sample suggests that rotation is the main physical process driving the mass-ejection. Nevertheless, smaller effects from other mechanisms have to be taken into account to fully explain how the residual gravity is compensated.
Context. To progress in the understanding of evolution of massive stars one needs to constrain the mass-loss and determine the phenomenon responsible for the ejection of matter an its reorganization in the circumstellar environment Aims. In order to test various mass-ejection processes, we probed the geometry and kinematics of the dust and gas surrounding the A[e] supergiant HD 62623. Methods. We used the combined high spectral and spatial resolution offered by the VLTI/AMBER instrument. Thanks to a new multiwavelength optical/IR interferometry imaging technique, we reconstructed the first velocity-resolved images with a milliarcsecond resolution in the infrared domain. Results. We managed to disentangle the dust and gas emission in the HD 62623 circumstellar disc. We measured the dusty disc inner rim, i.e. 6 mas, constrained the inclination angle and the position angle of the major-axis of the disc. We also measured the inner gaseous disc extension (2 mas) and probed its velocity field thanks to AMBER high spectral resolution. We find that the expansion velocity is negligible, and that Keplerian rotation is a favoured velocity field. Such a velocity field is unexpected if fast rotation of the central star alone is the main mechanism of matter ejection. Conclusions. As the star itself seems to rotate below its breakup-up velocity, rotation cannot explain the formation of the dense equatorial disc. Moreover, as the expansion velocity is negligible, radiatively driven wind is also not a suitable explanation to explain the disc formation. Consequently, the most probable hypothesis is that the accumulation of matter in the equatorial plane is due to the presence of the spectroscopic low mass companion.
Context. Classical Be stars are hot non-supergiant stars surrounded by a gaseous circumstellar disk that is responsible for the observed IR-excess and emission lines. The influence of binarity on these phenomena remains controversial. Aims. δ Sco is a binary system whose primary suddenly began to exhibit the Be phenomenon at the last periastron in 2000. We want to constrain the geometry and kinematics of its circumstellar environment. Methods. We observed the star between 2007 and 2010 using spectrally-resolved interferometry with the VLTI/AMBER and CHARA/VEGA instruments. Results. We found orbital elements that are compatible with previous estimates. The next periastron should take place around July 5, 2011 (±4 days). We resolved the circumstellar disk in the Hα (FWHM = 4.8 ± 1.5 mas), Brγ (FWHM = 2.9 ± 0.5 mas), and the 2.06 μm He i (FWHM = 2.4 ± 0.3 mas) lines, as well as in the K band continuum (FWHM ≈ 2.4 mas). The disk kinematics are dominated by the rotation, with a disk expansion velocity on the order of 0.2 km s −1 . The rotation law within the disk is compatible with Keplerian rotation. Conclusions. As the star probably rotates at about 70% of its critical velocity, the ejection of matter does not seem to be dominated by rotation. However, the disk geometry and kinematics are similar to the previously studied quasi-critically rotating Be stars, α Ara, ψ Per and 48 Per.
The circumstellar disk density distributions for a sample of 63 Be southern stars from the BeSOS survey were found by modelling their Hα emission line profiles. These disk densities were used to compute disk masses and disk angular momenta for the sample. Average values for the disk mass are 3.4×10 −9 and 9.5×10 −10 M ⋆ for early (B0-B3) and late (B4-B9) spectral types, respectively. We also find that the range of disk angular momentum relative to the star are between 150-200 and 100-150 J ⋆ /M ⋆ , again for early and late-type Be stars respectively. The distributions of the disk mass and disk angular momentum are different between early and late-type Be stars at a 1% level of significance. Finally, we construct the disk mass distribution for the BeSOS sample as a function of spectral type and compare it to the predictions of stellar evolutionary models with rapid rotation. The observed disk masses are typically larger than the theoretical predictions, although the observed spread in disk masses is typically large.
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