A&A 459, 137-145 (2006)
DOI: 10.1051/0004-6361:20053008

Bright Be-shell stars

Th. Rivinius¹, S. Stefl¹ and D. Baade<sup>2

1</sup>  European Southern Observatory, Casilla 19001, Santiago 19, Chile
    e-mail: T.Rivinius@eso.org
²  European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany

(Received 25 March 2003 / Accepted 28 June 2006)

Abstract
Echelle observations are presented and discussed for 23 of the 27 known "normal" shell stars brighter than about 6.5 mag. In addition to those typical cases, three stars with known transitions between emission & shell and pure emission line appearance, and three rapidly rotating B stars without records of line emission (Bn stars) are added to the sample. Long-term V/R emission-line variability and central quasi emission bumps (CQEs) in photospheric lines were found in 75% of all normal shell stars. This strongly suggests that the velocity law in most, if not all, disks of Be stars is roughly Keplerian. Both phenomena may occur in the same star but not at the same time. This is in agreement with the previous conclusion that CQEs only form in the presence of negligible line-of-sight velocities while long-term V/R variations are due to non-circular gas particle orbits caused by global disk oscillations. V/R variations associated with binary orbits are much less pronounced. Similarly, phase lags between different lines were detected in long-term V/R variable stars only. A binary fraction of only one-third is too low to support binary hypotheses as an explanation of the Be phenomenon. CQEs detected in 3 out of 19 Bn stars reveal the presence of disk-like equatorial concentrations of matter in B stars without emission lines. Accordingly, there seem to be intermediate cases between disk-free B stars and Be stars. Previous claims of the existence of shell stars with low v sin i could not be confirmed. Shell stars are Be stars viewed equator-on, and their observed rotational velocities are indistinguishable from the equatorial ones which are the same as in Be stars. The mean fraction of the critical rotation velocity is %. The standard deviation is comparable to, or even less than, the observational uncertainties. Since this would require star-to-star differences to be negligible, which is unrealistic, the correlation between the widths of strong spectral lines and the stellar rotation velocities may be truncated or severely distorted at its extreme end. A number of not previously known facts about individual stars is also reported.

Key words: stars: emission line, Be -- stars: circumstellar matter -- stars: rotation -- stars: statistics

© ESO 2006

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