The binary Be star {\\delta} Sco at high spectral and spatial resolution:\n Disk geometry and kinematics before the 2011 periastron

Abstract

Classical Be stars are hot non-supergiant stars surrounded by a gaseous\ncircumstellar disk that is responsible for the observed IR-excess and emission\nlines. The influence of binarity on these phenomena remains controversial.\ndelta Sco is a binary system whose primary suddently began to exhibit the Be\nphenomenon at the last periastron in 2000. We want to constrain the geometry\nand kinematics of its circumstellar environment. We observed the star between\n2007 and 2010 using spectrally-resolved interferometry with the VLTI/AMBER and\nCHARA/VEGA instruments. We found orbital elements that are compatible with\nprevious estimates. The next periastron should take place around July 5, 2011\n(+- 4,days). We resolved the circumstellar disk in the HAlpha (FWHM =\n4.8+-1.5mas), BrGamma (FWHM = 2.9 0.,mas), and the 2.06$ \\mu$m HeI (FWHM =\n2.4+-0.3mas) lines as well as in the K band continuum (FWHM ~2.4mas). The disk\nkinematics are dominated by the rotation, with a disk expansion velocity on the\norder of 0.2km/s. The rotation law within the disk is compatible with Keplerian\nrotation. As the star probably rotates at about 70% of its critical velocity\nthe ejection of matter doesn't seems to be dominated by rotation. However, the\ndisk geometry and kinematics are similar to that of the previously studied\nquasi-critically rotating Be stars, namely Alpha Ara, Psi Per and 48 Per.\n