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Stellar and circumstellar activity of the Be star µ Centauri *
II. Multiperiodic low-order line-profile variability
Th. Rivinius 1,
D. Baade 2,
S. tefl 3,
O. Stahl 1,
B. Wolf 1 and
A. Kaufer 1
Received 23 February 1998 / Accepted 12 May 1998
After the study in Paper I of the morphology of the line emission outbursts of this bright Be star, the same database is analyzed for the temporal characteristics of the photospheric variability.
In the 1992-1997 spectra, separate time series analyses were performed for the mean radial velocities of two groups of lines, (He i and Si iii), which were selected for their being the least contaminated by emission components. A period search with iterative pre-whitening revealed six periods which maintained phase coherence over the entire time interval covered. Four of them are close to the previously reported 0.505-day period (and, in fact, constitute it). Their relative frequency spacings are about 1.8%, 0.9%, and again 1.8% and cannot be resolved in single-season data strings. The other two periods, which were not found in the Si iii lines, are near 0.28 day. Additionally, the radial velocity variations of the single line He i6678 were studied. This data set extends from 1985 to 1997 thereby permitting a refinement of the periods found in the other lines.
Similar time series analyses of the flux contained in 5 wide line-profile bins of the 1995-1997 spectra found periods that are identical to those of the radial velocities to within 4 decimal places. However, not all periods are detected in all 21 lines studied.
Within either group of periods, the line-profile variability patterns are indistinguishable from one another whereas the difference between the two groups is highly significant. This implies that there are also two different stellar surface patterns associated with the two groups. Their analysis is deferred to the third paper in this series.
Numerous different tests were performed to check the genuineness of the multiperiodicity. None of them could disprove the reality of any of the periods which seems to be well founded on many observational facts.
At times of enhanced variability, an additional line-profile modulation with period day may develop. It consists of narrow absorption spikes at and, in the respective opposite wing, a very extended ramp-like ascent of the profile to the adjacent continuum.
Finally, a series of 348 low-resolution spectra obtained during 8 nights, which coincided with a line-emission outburst, indicates cyclic variations of the radial velocity of Balmer and He i lines with a time scale of 0.58 day. The same time scale was also present in the ratio of the two emission peaks in H.
Key words: stars: emission-line, Be stars: oscillations stars: individual: Cen
Send offprint requests to: T.Rivinius@lsw.uni-heidelberg.de
Online publication: July 7, 1998