Astrometry and Photometry for Two Dwarf Carbon Stars

Abstract

Preliminary trigonometric parallaxes and BVI photometry are presented for two dwarf carbon stars, LP765-18 (= LHS1075) and LP328-57 (= CLS96). The data are combined with the literature values for a third dwarf carbon star, G77-61 (= LHS1555). All three stars have very similar luminosities (9.6<M_V<10.0) and very similar broadband colors across the entire visual-to-near IR (BVIJHK) wavelength range. Their visual (BVI) colors differ from all known red dwarfs, subdwarfs, and white dwarfs. In the M_V versus V-I color-magnitude diagram they are approximately 2 magnitudes subluminous compared with normal disk dwarfs with solar-like metallicities, occupying a region also populated by O-rich subdwarfs with -1.5<[m/H]<-1.0. The kinematics indicate that they are members of the Galactic spheroid population. The subluminosity of all three stars is due to an as-yet-unknown combination of (undoubtedly low) metallicity, possibly enhanced helium abundance, and unusual line-blanketing in the bandpasses considered. The properties of the stars are compared with models for the production of dwarf carbon stars.