High and intermediate-resolution spectroscopy of Be stars

Abstract

We present an atlas of H ,H ei 4471 and Mg ii 4481 line proles obtained in a 10 year observation period of 116 Be stars, which enabled many of them to be observed at quite dierent emission epochs. From the best t of the observed He i 4471 line proles with non-LTE, uniform (Te; logg) and full limb-darkened model line proles, we determined the V sini of the program stars. To account, to some degree, for the line formation peculiarities related to the rapid rotation-induced non-uniform distributions of temperature and gravity on the stellar surface, the t was achieved by considering (Te; logg) as free parameters. This method produced V sini estimations that correlate with the rotational velocities determined by Slettebak (1982) within a dispersion 30 km s 1 and without any systematic deviation. They can be considered as given in the new Slettebak's et al. (1975) system. Only 13 program stars have discrepant V sini values. In some objects, this discrepancy could be attributed to binary eects. Using the newly determined V sini parameters, we found that the ratio of true rotational velocities V=Vc of the program Be stars has a very low dispersion around the mean value. Assuming then that all the stars are rigid rotators with the same ratio V (!)=Vc, we looked for the value of ! that better represents the distribution of V sini=Vc for randomly oriented rotational axes. We obtained ! =0 :795. This value enabled us to determine the probable inclination angle of the stellar rotation axis of the program stars. In the observed line proles of H ,H ei 4471, Mg ii 4481 and Fe ii 4351 we measured several parameters related to the absorption and/or emission components, such as: equivalent width, residual emission and/or absorption intensity, FWHM, emission peak separations, etc. The parameters related to the H line emission proles were used to investigate the structure of the nearby environment of the central star. From the characteristics of the correlations between these quantities and the inferred inclination angle, we concluded that in most of cases the H line emission forming regions may not be strongly flattened. Using a simple representation of the radiation flux emitted by the star+envelope system, we derived rst order estimates of physical parameters characterizing the H line emission formation region. Thus, we obtained that the total extent of the H region is Rf' 2:5 1:0 R and that the density distribution in these layers can be mimicked with a power law R ,w here =2 :5 +2:2 0:6 . The same approach enabled us to estimate the optical depth of the H line emission formation region. From its dependence with the aspect angle, we concluded that these regions are caracterized by a modest flattening and that the (equator)=(pole) density contrast of the circumstellar envelope near the star should be two orders of magnitude lower than predicted by models based on a priori disc-shaped circumstellar envelopes. We found that the separation between the emission peaks, p, and the full width at half maximum, 1=2 ,o f the H line emission are not only sensitive to kinematic eects, but to line optical depth as well. This nding agrees with previous theoretical predictions and conrms that Huang's (1972) relation overestimates the extent of the H line emission formation region.