The pulsation sequences

Wood and Sebo (1996) and Wood et al. (1999) have compared the observed PL relations with theoretical relations expected for radially pulsating asymptotic giant branch (AGB) stars. The comparisons show that sequence C (the Mira sequence) corresponds to pulsation in the fundamental mode while sequences B and C correspond to pulsation in the first, second and third overtones. A problem with these comparisons is that the effective temperatures (radii) of LPVs are poorly known so that it may be possible to fit a given pulsation mode to a different observed sequence by varying the assumed theoretical T_eff (this will change the radius and hence the pulsation period). Wood et al. (1999) overcame this problem by using the fact that many of the LPVs in the MACHO sample are multimode pulsators so that period ratios in these stars could be plotted against the longer period (the so-called Petersen diagram) and then compared to theory. The only possible combination of pulsation mode and observed sequence which fitted both the PL laws and the Petersen diagram was the combination described above. It therefore seems that the identification of the fundamental pulsation mode as the mode of the Mira variables is now secure. This then raises the question of why the directly observed radii of Miras (e.g. Haniff et al. 1995; van Belle et al. 1996) are so large. The answer to this question is not currently known.

© Copyright Astronomical Society of Australia 1997