New activity in the large circumstellar disk of the Be-shell star 48 Librae

S. Štefl , J. -B. Le Bouquin (1) , A. C. Carciofi , T. Rivinius , D. Baade , F. Rantakyrö

1 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble

Aims: Spectroscopic, polarimetric, and high spectral resolution interferometric data covering the period 1995-2011 are analyzed to document the transition into a new phase of circumstellar disk activity in the classical Be-shell star 48 Lib. The objective is to use this broad data set to additionally test disk oscillations as the basic underlying dynamical process.
Methods: The long-term disk evolution is described using the V/R ratio of the violet and red emission components of Hα and Brγ, radial velocities and profiles of He i and optical metal shell lines, as well as multi-band BVRI polarimetry. Single-epoch broad-band and high-resolution interferometric visibilities and phases are discussed with respect to a classical disk model and the given baseline orientations.
Results: Spectroscopic signatures of disk asymmetries in 48 Lib vanished in the late nineties but recovered some time between 2004 and 2007, as shown by a new large-amplitude and long-duration V/R cycle. Variations in the radial velocity and line profile of conventional shell lines correlate with the V/R behavior. They are shared by narrow absorption cores superimposed on otherwise seemingly photospheric He i lines, which may form in high-density gas at the inner disk close to the photosphere. Large radial velocity variations continued also during the V/R-quiet years, suggesting that V/R may not always be a good indicator of global density waves in the disk. The comparison of the polarization after the recovery of the V/R activity shows a slight increase, while the polarization angle has been constant for more than 20 years, placing tight limits on any 3-D precession or warping of the disk. The broad H-band interferometry gives a disk diameter of (1.72 ± 0.2) mas (equivalent to 15 stellar radii), position angle of the disk (50 ± 9)° and a relatively low disk flattening of 1.66 ± 0.3. Within the errors the same disk position angle is derived from polarimetric observations and from photocenter shifts across Brγ. The high-resolution interferometric visibility and phase profiles show a double or even multiple-component structure. A preliminary estimate based on the size of the Brγ emitting region indicates a large diameter for the disk (tens of stellar radii). Overall, no serious contradiction between the observations and the disk-oscillation model could be construed.

Based partly on observations collected at the European Southern Observatory, Chile (Prop. Nos. 383.D-0522, 266.D-5655, 081.C-0475).

Mots clés: techniques: spectroscopic; circumstellar matter; techniques: interferometric; stars: mass-loss; techniques: polarimetric; stars: emission-line

Domaines: Planète et Univers [physics]; Astrophysique [astro-ph]

S. Štefl, J. -B. Le Bouquin, A. C. Carciofi, T. Rivinius, D. Baade, et al.. New activity in the large circumstellar disk of the Be-shell star 48 Librae. Astronomy and Astrophysics - A&A, 2012, 540, ⟨10.1051/0004-6361/201118054⟩. ⟨insu-03612443⟩