Detection of Enhanced Germanium in a New Cool Extreme Helium Star A 980: Insights and Implications

Abstract

A fine abundance analysis of a recently discovered hydrogen-deficient carbon (HdC) star, A 980, is presented. Based on the observed high-resolution optical spectrum, we ascertain that A 980 is a cool extreme helium (EHe) star and not an HdC. Singly-ionized germanium Ge II lines are identified in A 980's optical spectrum. These are the first-ever detections of germanium lines in an EHe's observed spectrum, and provide the first measurements of germanium abundance in an EHe star. The overabundance of germanium in A 980's atmosphere provides us with evidence for the synthesis of germanium in EHe stars. Among the known cool EHe stars, A 980 exhibits a maximum enhancement of the s-process elements based on significant number of transitions. The measured elemental abundances reveal signs of H-burning, He-burning, and specifically the nucleosyntheses of the key elements: Ge, Sr, Y, Zr, and Ba. The nucleosyntheses of these key elements are discussed in light of asymptotic giant branch evolution and the expectation from the accretion of an He white dwarf by a C-O white dwarf or by a neutron star.