Discovery of technetium- and niobium-rich S stars: The case for bitrinsic stars ?

摘要

Context. S stars are late-type giants with overabundances of s -process elements. They come in two flavors depending on the presence or lack of presence of technetium (Tc), an element without stable isotopes. Intrinsic S stars are Tc-rich and genuine asymptotic giant branch (AGB) stars, while extrinsic S stars owe their s -process over abundances to the pollution from a former AGB companion, which is now a white dwarf (WD). In addition to Tc, another distinctive feature between intrinsic and extrinsic S stars is the overabundance of niobium (Nb) in the latter class. Indeed, since the mass transfer occurred long ago,~(93)Zr had time to decay into the only stable isotope of Nb,~(93)Nb, causing its overabundance. Aims. We discuss the case of the S stars BD+79°156 and o ~(1)Ori, whose specificity lies in sharing the distinctive features of both intrinsic and extrinsic S stars, namely the presence of Tc along with a Nb overabundance. Methods. We used high-resolution HERMES optical spectra, MARCS model atmospheres of S stars, Gaia DR2 parallaxes, and STAREVOL evolutionary tracks to determine the stellar parameters and chemical abundances of the two S stars, and to locate them in the Hertzsprung-Russell (HR) diagram. Results. BD+79°156 is the first clear case of a bitrinsic star, that is, a doubly s -process-enriched object, first through mass transfer in a binary system and then through internal nucleosynthesis that is responsible for the Tc-enrichment in BD+79°156, which must, therefore, have reached the AGB phase of its evolution. This hybrid nature of the s -process pattern in BD+79°156 is supported by its binary nature and its location in the HR diagram that is just beyond the onset of the third dredge-up on the AGB. The Tc-rich, binary S-star o ~(1)Ori with a WD companion was another long-standing candidate for a similar hybrid s -process enrichment. However, the marginal overabundance of Nb derived in o ~(1)Ori does not allow one to trace evidence of large amounts of pollution coming from the AGB progenitor of its current WD companion unambiguously. As a side product, the current study offers a new way of detecting binary AGB stars with WD companions by identifying their Tc-rich nature along with a Nb overabundance.