Multi-periodicity of the Be star η  Centauri from spectroscopic and photometric observations

¹ Instituto Astronômico e Geofísico, Universidade de São Paulo C.P. 3386, 01060-970, São Paulo, SP, Brazil
² Observatoire de Paris, Section d'Astrophysique de Meudon, URA 335 du CNRS/DASGAL, F-92195 Meudon Cedex, France

Send offprint request to: E. Janot-Pacheco

Received: 25 May 1998
Accepted: 1 December 1998

Abstract

We present photometric data and 209 high-resolution, high signal-to-noise ratio optical spectra of the Be star Centauri obtained in 1993 and 1995, respectively. Time series analysis of these and other data, performed using the CLEAN, CLEANEST and Fourier Doppler Imaging techniques, show the presence of multiperiodic variations. They are interpreted in terms of low and high order non-radial pulsation modes. A strong frequency of 1.29 c/d is detected in line profile and photometric variations. It is attributed to a mode with l = 2. Other high amplitude signals present in spectroscopic data are 1.78 c/d, 3.82 c/d and 4.51 c/d. A frequency of 1.48 c/d appearing in spectroscopic data is compatible with the 1.56 c/d periodicity largely dominant in observations taken prior to 1993. The corresponding "superperiods" for four of the frequencies are commensurate at the 8% level. The star showed period and non-radial pulsation degree variations in timescales as short as ∼ 1.5 hour. The dominant mode apparently alternates between l = 2 and l = 4 every other day during the eight day time span. This could be a modulation linked to the superperiod or to the stellar rotation. The main periodicities detected in our analysis are compatible with theoretical unstable g modes in SPB variables. Short time scale variability (1-3 hr), typical of high-order p modes is also systematically present.