Short term H-alpha variations in two Herbig PMS stars : HR 5999 and HD 52721.

Abstract

We present high spectral resolution observations of Hα as well as V photometry and IUE observations for two Herbig pre-main sequence stars, HR 5999 and HD 52721. These observations, performed in 1985, were aimed at studying short-term variations of the Hα line, within one night and from night to night. Hα spectra of comparison stars are also displayed. Only very small variations in the Hα profiles of the two targets are found within each night. On the other hand, the Hα profile in HR 5999 varies in a systematic way during the 6 nights of the observing run. The Mg II h and k profiles obtained on two consecutive days show almost no variations. The photometric data show a significant increase in the brightness of HR 5999 during the observing run, while the Hα emission also increases. In HD 52721, the night to night variations of Hα exist, but they are weak. For this star, the maximum Hα emission corresponds to the minimum brightness. The Mg II profiles of HD 52721 do not show variations over 24 hours. They are probably of interstellar origin. High resolution Hα data exist now for HR 5999 since 1983, which were obtained by Th in 1983, Faraggiana in 1983, ourselves in 1985 and Baade & Stahl in 1987. It follows that the long term behaviour of that star, as reflected by a very strong emission line, can be sketched. A tentative interpretation of the observed characteristics of the variations of `IR 5999 is given, in terms of a qualitative model involving a corotating and slowly expanding region, surrounded by slowly rotating layers with higher expansion velocities.