STELLAR AND CIRCUMSTELLAR ACTIVITY OF THE BE STAR MU-CENTAURI - II - MULTIPERIODIC LOW-ORDER LINE-PROFILE VARIABILITY

After the study in Paper I of the morphology of the line emission outb ursts of this bright Be star, the same database is analyzed for the te mporal characteristics of the photospheric variability. In the 1992-19 97 spectra, separate time series analyses were performed for the mean radial velocities of two groups of lines, (He I and Si III), which wer e selected for their being the least contaminated by emission componen ts. 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 ab out 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 I II lines, are near 0.28 day. Additionally the radial velocity variatio ns of the single line He I lambda 6678 were studied. This data set ext ends from 1985 to 1997 thereby permitting a refinement of the periods found in the other lines. Similar time series analyses of the flux con tained in 5 km s(-1) wide line-profile bins of the 1995-1997 spectra f ound periods that are identical to those of the radial velocities to w ithin 4 decimal places. However, not all periods are detected in all 2 1 lines studied. Within either group of periods, the line-profile vari ability patterns are indistinguishable from one another whereas the di fference between the two groups is highly significant. This implies th at there are also two different stellar surface patterns associated wi th the two groups. Their analysis is deferred to the third paper in th is series. Numerous different tests were performed to check the genuin eness of the multiperiodicity. None of them could disprove the reality of any of the periods which seems to be well founded on many observat ional facts. At times of enhanced VIR variability, an additional line- profile modulation with period similar to 0.505 day may develop. It co nsists of narrow absorption spikes at +/- similar to 0.7 upsilon sin i and, in the respective opposite wing, a very extended ramp-like ascen t of the profile to the adjacent continuum. Finally, a series of 348 l ow-resolution spectra obtained during 8 nights, which coincided with a line-emission outburst, indicates cyclic variations of the radial vel ocity of Balmer and He I lines with a time scale of 0.58 day. The same time scale was also present in the V/R ratio of the two emission peak s in H beta.