First spectro-interferometric survey of Be stars

Abstract

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.

Populations of rotating stars

“…Howarth 2007). This idea is also supported by many interferometric observations, which have allowed to determine the oblateness of some Be stars (Domiciano de Souza et al 2003;Zhao et al 2011;Touhami et al 2011;Meilland et al 2012;Kraus et al 2012), suggesting that some of them are close to their critical rotation.…”

“…Rivinius et al (2006) showed that for the brightest shell stars of different spectral types, the mean fraction of the critical velocity is 81%, which corresponds to a mean ω higher than 0.92. Meilland et al (2012) found a mean value of ω = 0.95 for a sample of eight Be stars, considering V sin i and V crit from Frémat et al (2005) but i measurements derived from interferometry. Delaa et al (2011) deduced from interferometric observations of Be stars 48 Per and Ψ Per that both stars rotate at nearly critical rotation.…”

Spectral and spatial imaging of the Be+sdO binaryϕPersei

“…This nonlinearity in the photometric response of the detector does not generate bias in the derived complex visibilities as explained by Delaa et al (2011). To model the spectrally resolved visibility and differential phase in the Hα emission line, we used the simple kinematic model developed for fast model-fitting of a geometrically and optically thin equatorial disk in rotation described in detail by Delaa et al (2011) and Meilland et al (2012).…”

Critical study of the distribution of rotational velocities of Be stars

“…Studies of individual stars, based on nonradial pulsation modes (Cameron et al 2008;Saio 2013) and rotational rates inferred from interferometric data (Meilland et al 2012;Domiciano de Souza et al 2014), produce estimates of Ω/Ω c ranging from 0.60 to 0.98, which correspond to linear velocities ratios 0.44 V/V c 0.88 (V c is the critical equatorial linear velocity) or to ratios of centrifugal to gravity accelerations 0.14 a c /a g 0.70. These rates suggest that the surface rotational velocities of Be stars are undoubtedly very high, but they are not necessarily critical.…”