First results from HIPPARCOS trigonometrical parallaxes of Mira-type variables

Abstract

HIPPARCOS trigonometrical parallaxes are given for 16 pre-selected Mira variables. Linear diameters are derived for eight oxygen-rich Miras with known angular diameters. Comparison with pulsation theory shows that two of them (both with periods over 400d) are fundamental pulsators, while the others (all with periods of less than 400d) pulsate in an overtone. The Mira PL relations in M_K and M_bol are calibrated for oxygen-rich overtone pulsators, adopting slopes for these relations from LMC data. A mean LMC distance modulus of 18.54 is derived; this is very close to that of 18.57 derived from the Cepheids. The uncertainty in the value derived from the Miras is estimated to be less than 0.2 mag. The absolute magnitude of the only carbon-rich Mira in the sample, R Lep (period of 427d), indicates that it is a fundamental pulsator. Other stars discussed individually are: the symbiotic Mira, R Aqr; the double-period Mira, R Cen; and two Miras with decreasing periods, R Aql and R Hya.