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Multi-periodicity of the Be starη  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-noiseratio optical spectra of the Be star Centauri obtained in 1993 and 1995, respectively. Time series analysis ofthese and other data, performed using the CLEAN, CLEANEST and Fourier DopplerImaging techniques, show the presence of multiperiodic variations. They areinterpreted in terms of low and high order non-radial pulsation modes. A strongfrequency of 1.29 c/d is detected in line profile and photometric variations. It is attributed to a mode withl = 2. Other highamplitude 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 intimescales as short as∼ 1.5 hour. The dominant mode apparently alternates betweenl = 2 andl = 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 ouranalysis are compatible with theoretical unstableg modes in SPBvariables. Short time scale variability (1-3 hr), typical of high-orderp modes is also systematically present.