Detection of enhanced germanium in a new cool extreme helium star A980: insights and implications

Abstract

A fine abundance analysis of a recently discovered hydrogen-deficient carbon (HdC) star, A980, is presented. Based on the observed high-resolution optical spectrum, we ascertain that A980 is a cool extreme helium (EHe) star and not an HdC star. Singly ionized germanium Ge II lines are identified in A980’s optical spectrum. These are the first-ever detections of germanium lines in an EHe star's observed spectrum and provide the first measurements of germanium abundance in an EHe star. The overabundance of germanium in A980’s atmosphere provides us with evidence for the synthesis of germanium in EHe stars. Among the known cool EHe stars, A980 exhibits a maximum enhancement of the s-process elements based on a 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