THE ATMOSPHERIC MOTION OF THE BETA-CEPHEI STAR - 12 LACERTAE

The beta-Cephei star 12 Lacertae has been studied at high spectral (R = lambda/DELTA-lambda between 15 000 and 70 000) and time resolution (around 7 min), with a high signal-to-noise ratio (around 200), using the 152-cm telescope of the Observatoire de Haute-Provence (OHP) and its spectrograph AURELIE. Two spectral ranges were considered: one around the H-alpha line and the second around the Si III 4553 angstrom line. The velocity curves obtained were all different from one to another, sometimes showing a stillstand, with various 2K-amplitudes and gamma-velocities. It is suggested that these latter changes are not the result of an interaction with a hypothetical companion, but rather due to interactions between several pulsation modes of both radial and nonradial origin. The main interference produces a beat period around 8.9 d. Except for this last phenomenon, the overall atmospheric motion of 12 Lacertae seems similar to that occurring in BW Vulpeculae and sigma-Scopii. In particular, the occurrence of two shock waves per pulsation period is supported by the observations. We suggest that the appearance of numerous iron lines, producing an opacity increase, is at the origin of the stronger shock, while the smaller one could be due to the CNO kappa-mechanism.