Analysis of the carbon-rich very metal-poor dwarf G77-61

Abstract

We present an analysis of the carbon dwarf G77-61, a known binary, based on high resolution Keck spectra. G77-61 has a very-low metallicity, although not as extreme as what was previously conjectured. This star is very carbon-enhanced, the spectra showing very strong CH, CN, and C$_2$ bands of both $^{13}$C and $^{12}$C isotopes. Atomic lines are sparse, and often blended, but we were able to derive abundances for Na, Mg, Ca, Cr, Mn, and Fe. Its [Fe/H] of -4.03$\pm$0.1 places it among the lowest metallicity stars known, and its very high [C/Fe]=+2.6, and [N/Fe]=+2.6 among the most C and N-rich metal-poor stars. The carbon isotopic ratio is $^{12}$C/$^{13}C=5\pm$1. No overabundance of s- or r-process elements is detectable, but the upper limits that can be set on these overabundances are not very constraining.