Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999iaus..192..348z&link_type=abstract
The stellar content of Local Group galaxies, Proceedings of the 192nd symposium of the International Astronomical Union held in
Astronomy and Astrophysics
Astronomy
Scientific paper
Mira variables in the LMC define a narrow PL relation, for periods between 200 and 400 days. At longer periods many LMC Miras are over-luminous with respect to the relation and the relation appears to break down. Mass loss in Galactic Miras becomes important at periods longer than 300 days: it is therefore possible that mass loss and the break down of the PL relation are related. The pulsation mode of Miras has not yet been established with certainty. We show that Miras and Semi-regulars are in the same evolutionary phase. One nearby star shows evidence for mode switching. Hipparcos data on Galactic Miras and Semi-regulars stars is used to show that two sequences exists, one corresponding to fundamental mode and one to the first-overtone: the first overtone sequences precisely corresponds to the LMC Mira PL relation. First overtone pulsators are found dominant in the LMC and in Galactic globular clusters. At the Galactic Centre fundamental mode Miras are dominant. In the solar neighbourhood both sequences occur. The Mira pulsation mode therefore appears metallicity dependent. The first overtone Mira PL relation appears a very good distance indicator, being independent of metallicity. This is tested for the metal-rich globular clusters: the Mira PL relation has the same slope as for the LMC, and the zeropoint agrees well with the long globular cluster distance scale. There is however significant disagreement with the RR Lyrae distance scale. Stars on the PL relation show evidence for mass loss, so mass loss is not limited to fundamental mode pulsators as had been suggested. However, mass-loss rates may be higher for fundamental mode pulsators at the same stellar mass. We suggest that at low metallicity, stars may reach higher final core masses due to less effective mass loss in the first overtone Mira mode. This would lead to a higher final mass function in old populations.
Bedding Tim R.
Zijlstra Albert A.
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