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Astron. Astrophys. 333, 125-140 (1998)


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Stellar and circumstellar activity of the Be star µ Centauri *

I. I. Line emission outbursts

Th. Rivinius 1, D. Baade 2, S. Stefl 3, O. Stahl 1, B. Wolf 1 and A. Kaufer 1

1 Landessternwarte Königstuhl, D-69117 Heidelberg, Germany
2 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany
3 Astronomical Institute, Academy of Sciences, CZ-251 65 Ondrejov, Czech Republic

Received 10 November 1997 / Accepted 26 January 1998

Abstract

With a total of 408 echelle spectra obtained with HEROS (Heidelberg Extended Range Optical Spectrograph) during 6 observing runs covering 355 nights in 4 years and the spectral range from 3450 to 8620 Å at a resolving power of 20 000, this study is based on one of the most extensive homogeneous observational records of the short-, medium-, and long-term variability of any Be star. One week worth of low-resolution spectra with very high temporal sampling was additionally obtained. Three dense series of very high-resolution low-noise profiles of He >[FORMULA] 6678 observed in 1985-1987 (309 spectra), one of Si iii [FORMULA] 4553 obtained in 1995 (27 spectra) simultaneously to the low-resolution data and several shorter datasets of different observing dates and wavelengths could also be relied upon for verification purposes.

The star was found to be in the process of continued gradual recovery of the [FORMULA] emitting disk which had been lost from 1977-1989. During the monitoring period numerous line emission outbursts were observed. A detailed generalized pattern of an outburst cycle is derived from observations of different circumstellar lines at times of various levels of emission from the disk. Relative quiescence, precursor, outburst, and subsequent relaxation can be distinguished as the main constituting phases, even though there are distinct differences between different groups of spectral lines. The actual appearance of outbursts depends also quite noticeably on the strength of the already present circumstellar emission.

Outbursts are preceded by a significant decline in the peak height of all circumstellar emission lines, which only lasts a few days. The outbursts proper are characterized by the occurrence within 1-3 days of (i) broad emission wings, (ii) rapid cyclic variability of the violet-to-red ([FORMULA]) emission peak ratio, (iii) temporary high velocity absorptions, (iv) transient sharp absorption spikes at the edges of photospheric lines, (v) an increase in the separation of emission peaks, and (vi) an increase of emission strength as the burst goes on. Particularly the [FORMULA] activity may be accompanied by relatively short-lived narrow, blueshifted high-velocity absorption components. The relaxation phase following the outburst exhibits a slow but steady decline from these sudden enhancements and ultimately returns to the pre-outburst state of relative quiescence. Outbursts can differ in amplitude and duration by a factor of a few, and the larger an outburst is, the more time it takes relative quiescence to be reached. At times the frequency of outbursts is so high, or the amplitude of an outburst is so large, that the next outburst takes place before the previous pre-outburst state has been re-established.

Based on this empirical phenomenology, a schematic picture of the associated ejection of matter into a near-stellar orbit is sketched.

Key words: stars: emission-line, Be – stars: mass loss – circumstellar matter – stars: individual: µ Cen

* Based on observations collected at the European Southern Observatory at La Silla, Chile, ESO proposal Nos. 55.D-0502, 56.D-0381, 58.D-0697

Send offprint requests to: T.Rivinius@lsw.uni-heidelberg.de

© European Southern Observatory (ESO) 1998

Online publication: April 15, 1998
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