Reprocessing the Hipparcos Observations of C stars: Improved Parallaxes and Luminosity Calibration

Among 318 C stars present in the Hipparcos Catalogue, 89 have parallaxes which are either negative or whose uncertainty exceeds by large amounts what one expects for stars of similar Hp magnitudes. The results of a careful reprocessing of the Hipparcos Intermediate Astrometric Data for these stars are reported, with emphasis on parallaxes and absolute magnitude calibrations. An improvement in both the precision and the consistency of the astrometric parameters based on the FAST, NDAC or FAST+NDAC datasets (as prescribed in the Hipparcos Catalogue) can indeed be achieved. Similar problems are present for the Hipparcos Catalogue parallaxes of other classes of giant stars, for instance the S stars and oxygen-rich AGB stars. We plan to reprocess their data also to provide consistent parallaxes and luminosities for these high luminosity stars.