The calibrating stars of the Mira P--L relation

Abstract

Improved light curves extracted from the MACHO data base are used to discuss the periods and amplitudes of the calibrating stars of the Mira P–L relations. Previously unpublished spectral types and positions are given for several of those that lie in the S Dor variable star field. It is found that the periods derived from the discovery observations were generally sufficiently accurate that the K- and mbol–logP relations are not significantly changed by the MACHO results. Furthermore, the periods have remained essentially constant over two to three decades. The MACHO r-band amplitudes of the oxygen-rich Large Magellanic Cloud (LMC) Miras are similar to those in galactic (Baade's window) fields at a given period but there appear to be more shorter-period stars as a proportion of the whole than in the galaxy. In O-rich stars, the distinction in amplitude between the Miras and the semiregulars is not as conspicuous at shorter periods (<200 d) as at longer ones. The LMC carbon Miras have smaller amplitudes than their O-rich counterparts. Of the six stars with P > 420 d that were found to be too luminous to fit the P–L relation, two are now known to be Li-rich. This tends to confirm the suggestion that they are hot-bottom burners.