G77-61 - A dwarf carbon star

Abstract

Astrometric and photometric data are used to demonstrate that the cool proper-motion star G77-61, previously assumed to be an early M dwarf on the basis of astrometry and (B-V) color, is in fact a carbon star with striking C2 and CH features. An earlier parallax determination is reanalyzed, the previously derived absolute visual magnitude of roughly +9.6 is confirmed, and it is found that space-motion constraints rule out a giant or subgiant luminosity. Scanner spectrophotometry, blue and red image-tube spectra, and photoelectric colors are presented and compared with similar observations of normal carbon stars. Four evolutionary interpretations of G77-61 are considered, viz., the possibilities that the star (1) is gravitationally contracting to the white-dwarf sequence, (2) represents a rare carbon-rich halo population and is on the main sequence, (3) is undergoing a second main-sequence phase following complete mixing of its core and hydrogen envelope as well as extensive mass loss, and (4) has received carbon-rich material from a close binary companion. The binary interpretation is shown to be the most plausible explanation.