The binary Be star δ Scorpii at high spectral and spatial resolution^⋆

I. Disk geometry and kinematics before the 2011 periastron

¹ Max Planck Intitut fur Radioastronomie, Auf dem Hugel 69, 53121 Bonn, Germany
e-mail: meilland@mpifr-bonn.mpg.de
² UMR 6525 CNRS H. FIZEAU UNS, OCA, CNRS, Campus Valrose, 06108 Nice Cedex 2, France
³ Departamento de Física y Astronomía, Universidad de Valparaíso, Chile
⁴ UJF/CNRS LAOG, 414 rue de la Piscine, Domaine Universitaire, 38400 Saint-Martin d’ Hères, France
⁵ Georgia State University, PO Box 3969, Atlanta GA 30302-3969, USA
⁶ CHARA Array, Mount Wilson Observatory, 91023 Mount Wilson CA, USA
⁷ National Optical Astronomy Observatory, PO Box 26732, Tucson, AZ 85726-6732, USA

Received: 28 February 2011
Accepted: 6 June 2011

Abstract

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.