Reprocessing the Hipparcos data of evolved stars

We analyze the K band luminosities of a sample of galactic long-period variables using parallaxes measured by the Hipparcos mission. The parallaxes are in most cases re-computed from the Hipparcos Intermediate Astrometric Data using improved astrometric fits and chromaticity corrections. The K band magnitudes are taken from the literature and from measurements by COBE, and are corrected for interstellar and circumstellar extinction. The sample contains stars of several spectral types: M, S and C, and of several variability classes: Mira, semiregular SRa, and SRb. We find that the distribution of stars in the period-luminosity plane is independent of circumstellar chemistry, but that the different variability types have different P-L distributions. Both the Mira variables and the SRb variables have reasonably well-defined period-luminosity relationships, but with very different slopes. The SRa variables are distributed between the two classes, suggesting that they are a mixture of Miras and SRb, rather than a separate class of stars. Using the flux-weighted mean parallax method, we derive the mean luminosity as a function of period. The Miras show a steeper period-luminosity relationship than found for Large Magellanic Cloud Miras by Feast et al. (1989), although the relationship found here fits the longest-period (P > 400 d) LMC Miras well. The maximum absolute K magnitude of the sample is about −8.2 for both Miras and semi-regular stars, only a little fainter than the expected AGB limit. We show that the stars with the longest periods (P > 400 − 500 d) have high mass loss rates and are almost all Mira variables.