Envelope structure of the cyclic V/R variable shell stars.

Abstract

The radial velocity measurement in the far ultraviolet and visual spectra of two V/R variable shell stars, ζ Tau and 48 Lib, has been performed. The radial velocities of once and twice ionized elements in the far ultraviolet region coincide with those of the visual shell lines, while the radial velocities of more ionized elements, such as Si IV and C IV, are always negative. The co-existence of the positive and negative radial velocities rules out the simple superposition of hot and cold regions towards the observer, and the authors suggest a two-zone structure consisting of a cool equatorial disk and a hot expanding atmosphere which locates at higher latitude, present independently of the cool equatorial disk; this expanding atmosphere is also observed in other Be stars when the cool disk is temporarily not seen.