Kinematics and geometrical study of the Be stars 48 Persei andψPersei with the VEGA/CHARA interferometer

Abstract

Context. Five different physical processes might be responsible for the formation of decretion disks around Be stars: fast rotation of the star, stellar pulsations, binarity, stellar winds, and magnetic fields. Our observations indicate that fast rotation seems to produce a disk in Keplerian rotation, at least in the specific case of the two stars observed. We do not know if this observational result is a generality or not. Aims. We measure the size, orientation, shape, and kinematics of the disks around 2 Be stars, namely 48 Per and ψ Per. Methods. We used the VEGA/CHARA interferometer with a spectral resolution of 5000 to obtain spectrally dispersed visibility modulus and phases within the Hα emission line. Results. We were able to estimate the disk extension in the continuum and in the Hα line, as well as flattening, for both stars. Both stars rotate at nearly a critical rotation, but while the disk of 48 Per seems to be in Keplerian rotation, our preliminary data suggest that the disk of ψ Per is possibly faster than Keplerian, similarly to what has been found for κ CMa with observations carried out in the near-IR. However, more data is needed to confirm the fast rotation of the disk. Conclusions. Assuming a simple uniform disk model for the stellar photosphere in the continuum and a Gaussian brightness distribution in the line emission region, we obtain a ratio of the disk diameter over the photospheric diameter of 8 for 48 Per and 11 for and ψ Per. We also found that the major axis of 48 Per is parallel to the polarization angle and not perpendicular to it as previously observed for many Be stars, including ψ Per. This might be due to the optical thickness of the disk, which is also responsible for the incoherent scattering of a non negligible part of the Hα line emission. To our knowledge, this is the first time that this effect has been measured in a Be star.

A large Hαline forming region for the massive interacting binaries β Lyrae and υ Sagitarii

“…The difference mainly originates from a saturation effect of the photon-counting camera in the emission line. After careful testing, we found that the saturation does not affect the interferometric measurements (Delaa et al 2011).…”

Spectral and spatial imaging of the Be+sdO binaryϕPersei

“…Some examples of differential visibilities and phases for the best-fit model for various baselines between 32 and 278 m are plotted in Fig 5, bottom part. As already explained in Delaa et al (2011), phase variations for baselines overresolving the object (i.e., 60 m in the major-axis orientation and 200 m in the minor-axis one in the case of ϕ Persei) are only poorly fitted. The jumps seen for some baselines might be either due to inhomogeneity in the disk or to VEGA data reduction biases at very low visibility.…”

“…The photoncounting detectors of VEGA suffer from a local saturation effect in presence of bright spectral lines that attenuates the line intensity. 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).…”

Populations of rotating stars

“…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. Therefore, the question to which extent Be stars are really critically rotating stars is not settled yet.…”