First Panel discussion
International Astronomical Union Colloquium
On the last day of this meeting two general discussions were held, each beginning with comments by members of a discussion panel. The first panel and discussion concentrated on reviewing the theoretical problem of the LBV eruption mechanism. Members of this panel were I. acting as moderator or chairman. Remarks by panel members and other participants are given here in condensed form, somewhat reworded for brevity and clarity. It will be noted that the most generally accepted consensus here was
... hat the basic cause of LBV instability is not yet known!] Appenzeller: Having the privilege of opening this discussion, I shall start with a brief overview and a personal assessment of the different physical processes that have been discussed during this meeting as possible causes of the peculiar properties and dramatic variabilities of the LBV's. To follow some logical order, stellar interior mechanisms will be dealt with first, followed by atmospheric and circumstellar phenomena. Among the interior mechanisms, we first have the suggestion of multi-mode pulsations induced and powered by interior differential rotation. Sreenivasan has outlined the details of this scenario in his contribution to this volume. They are complex and depend upon the interior structure. Therefore it is difficult to derive a reliable observational signature that could be used to prove the presence of this mechanism. Another potential deep-interior process involves thermal oscillations as described by Norbert Langer for hydrogen shell burning models. Of course this suggestion depends critically on the preceding evolutionary phases and a star's ability to reach the stage in question. As Maeder pointed out, a density inversion observed in computed stellar models near the red boundary of the LBV region in the H-R diagram may indicate the onset of violent events at an earlier evolutionary stage. Because of Rayleigh-Taylor instabilities it is very unlikely that density inversions will occur in real stars; but it is not clear how a real star will react to a density gradient trying to change its sign. Strongly enhanced mass loss as observed in LBV eruptions could well result, but a milder reaction (e.g., modified convection properties) also seems possible. Obviously this phenomenon deserves further studies. From the discussions at this meeting it seems that radiation-pressure-induced massloss instabilities are still the most popular concept for explaining LBV eruptions. The new, sophisticated model computations reported by Kudritzki et al., Leitherer et al., and by Owocki seem to provide further support for this type of mechanism. However, only time-dependent computations could actually prove that this mechanism is indeed the cause of the observed variations. As described in de Jager's review talk, turbulent pressure effects must be present in practically all extremely luminous stars. However, at least in the hotter LBV's (and during minimum phases) radiation pressure probably dominates and turbulent or wave pressure probably can become important only in conjunction with radiation effects. 241 K. Davidson et al. (eds.). Physics of Luminous Blue Variables, 241-248.