A Spectroscopic Analysis of the Beta-Cephei Star Alpha-Lupi

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Stars: Variable - Lines: Profile - Stars: Individual: Lupi - Stars: Oscillations

Scientific paper

We present high spectral and time resolution observations of high signal-to-noise ratio (above 200) of the β Cephei star α Lupi, carried out for three different wavelengths during three nights. The radial velocity curves show a nice sinusoidal behaviour for all elements. However, the amplitude and the average velocity of the variation depend on the considered lines. By means of a simple least squares fit conducted on each observed night, we also point out a non-constancy of the pulsation period. A comparison between the RI and FWHM curves seems to indicate that temperature effects are present during the pulsation, while the EW varies by about 3%. The radius variation and acceleration curves have a small amplitude, indicating that the motion does not have any strong non-linear behaviour.
The moment method was used to derive the pulsation parameters. The main mode was found to be a radial one. Due to the non-constancy of the period and to the fact that two frequencies were detected in the photometric data, a search for a second period in the moments has been conducted. Therefore, the moment method has been generalized for a multiperiodic pulsation. The moments tend to indicate that a second mode indeed is present. Application of the generalized moment method to the spectra of α Lupi did not lead to a unique solution for the possible second mode, although the analysis yields the presence of a small-amplitude axisymmetric mode with degree 1 ≤ l ≤ 3.

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