Observational Properties of Nonradial Oscillations in Early-Type Stars and their Possible Effect on Mass Loss: the Example of ζ Puppis (04 If)

Abstract

Thanks to the fast rotation which via the Doppler effect permits one to indirectly resolve the stellar disk in one dimension and amplitudes often reaching 10 km/s or more, nonradial oscillations with degrees up to m = 20 are easily detected in high S/N (better than 200) observations of early type stars. Recent work by Smith (1981, 1985), Vogt and Penrod (1983), Penrod and Smith (1985), Penrod (1985), Baade (1982, 1984, and unpublished data), and Baade and Ferlet (1984) show the ubiquity of nonradial oscillations in the upper left part of the HRD. With the increasing amount of data some general properties of these pulsations are emerging, and there are even first indications of systematic differences between different groups of stars. The latter is encouraging in the context of this workshop because it could mean that from the analysis of the oscillations additional insight can be gained into the stars’ internal structure, i.e. seismology of early-type stars may in fact become a reality. From the above references the following can be extracted as the most important results obtained thus far:

• Virtually all “normal” stars with spectral type B6 or earlier are nonradial oscillators, at least occasionally (for supergiants the spectroscopic evidence is still less complete).

• Observations have not so far succeeded in restricting the range of possible excitation mechanisms suggested by various theoretical models. But the rotation rate may play an important role.

• Simultaneously excited modes are are often coupled: P_m x m = const (m = mode degree; the constant is different for different stars).

• The modes with the longest wavelengths have typical periods between 0.5 and 2 days. If they are g-modes, very high radial overtones are implied. On the other hand, the nonradial modes observed in some of the radially pulsating ß Cephei stars all fall into the range 0.13 to 0.3 day and thus are almost identical to those of the radial modes.

• For modes with periods much longer than the radial fundamental mode, in some slow to moderately fast rotators a horizontal-to-vertical amplitude ratio, K, of only 0.15 has been deduced. This is in conflict with the characteristics of g and r modes as the only ones with so long periods. At least in most Be stars, K appears to be much larger.

• The mode spectra of Be and Bn stars seem to differ in that symmetry variations of the overall profiles have been found only in Be stars - but also in the narrow lined 53 Per stars (however, the modes causing these variations are probably different in 53 Per and Be stars).

• Mass loss episodes from Be stars are correlated with temporarily decreased (often vanishing) pulsation amplitudes.

• The V/R ratio of the double emission in the Hel λ6678 profile of many Be stars varies with the period of the slowest pulsation mode.

• In the case of multi-mode oscillations amplitude ratios can be variable. In some stars also a mode switching has been diagnosed.

• In many stars pulsation cycles are not always all alike. Various deviations from the mean occur. At least in some cases these are very unlikely to be due to multi-mode beating.