Non-radially pulsating Be stars

Based on more than 3000 high-resolution echelle spectra of 27 early-type Be stars, taken over six years, it is shown that the short-term periodic line profile variability of these objects is due to non-radial pulsation. The appearance of the line profile variability depends mostly on the projected rotational velocity v sin i and thus, since all Be stars rotate rapidly, on the inclination i. The observed variability of the investigated stars is described, and for some of them line profile variability periods are given for the first time. For two of the investigated stars the line profile variability was successfully modeled as non-radial pulsation with l=m=+2 already in previous works. Since Be stars with similarly low v sin i share the same variability properties, these are in general explainable under the same model assumptions. The line profile variability of stars with higher v sin i is different from the one observed in low v sin i stars, but can be reproduced by the same model, if only the model inclination is modified to more equatorial values. Only for a few stars with periodic line profile variability the l=m=2 non-radial pulsation mode is not able to provide a satisfying explanation. These objects might pulsate in different modes (e.g. tesseral ones, l != |m|). Almost all stars in the sample show traces of outburst-like variability, pointing to an ephemeral nature of the mass-loss phenomenon responsible for the formation of the circumstellar disk of early-type Be stars, rather than a steady star-to-disk mass transfer. In addition to the variability due to non-radial pulsation present in most stars, several objects were found to show other periods residing in the immediate circumstellar environment. The presence of these secondary periods is enhanced in the outburst phases. Short-lived aperiodic phenomena were clearly seen in two stars. But, given the unfavourable sampling of our database to follow rapid variability of transient nature, they might be more common. Only in two out of 27 stars short-term spectroscopic variability was not detected at all.

Based on observations collected at the European Southern Observatory at La Silla, Chile, 55.D-0502, 56.D-0381, 58.D-0697, 62.H-0319, 63.H-0080, 64.H-0548, and 267.D-5702, the German-Spanish Astronomical Centre, Calar Alto, operated by the Max-Planck-Institute for Astronomy, Heidelberg, jointly with the Spanish National Commission for Astronomy, and on observations with the Wendelstein 80-cm and the Ondřejov 2-m telescopes, both equipped with the HEROS spectrograph provided by the Landessternwarte Heidelberg.