Observational Evidence Points at AGB Stars as Possible Progenitors of CEMP-s and CEMP-r/s Stars

The origin of enhanced abundance of heavy elements observed in the surface chemical composition of carbon-enhanced metal-poor (CEMP) stars still remains poorly understood. Here, we present detailed abundance analysis of seven CEMP stars based on high-resolution (R ~ 50,000) spectra that reveal enough evidence of asymptotic giant branch (AGB) stars being possible progenitors for these objects. For the objects HE 0110-0406, HE 1425-2052, and HE 1428-1950, we present for the first time a detailed abundance analysis. Our sample is found to consist of one metal-poor ([Fe/H] < -1.0) and six very metal-poor ([Fe/H] < -2.0) stars with enhanced carbon and neutron-capture elements. We have critically analyzed the observed abundance ratios of [O/Fe], [Sr/Ba], and [hs/ls] and examined the possibility of AGB stars being possible progenitors. The abundance of oxygen estimated in the program stars is characteristic of AGB progenitors except for HE 1429-0551 and HE 1447+0102. The estimated values of [Sr/Ba] and [hs/ls] ratios also support AGB stars as possible progenitors. The locations of the program stars in the absolute carbon abundance A(C) versus [Fe/H] diagram, along with the Group I objects, hint at the binary nature of the object. We have studied the chemical enrichment histories of the program stars based on abundance ratios [Mg/C], [Sc/Mn], and [C/Cr]. Using [C/N] and ¹²C/¹³C ratios, we have examined whether any internal mixing had modified their surface chemical compositions. Kinematic analysis shows that the objects HE 0110-0406 and HE 1447+0102 are thick-disk objects and the remaining five objects belong to the halo population of the Galaxy. ^* Based on data collected at Himalayan Chandra Telescope, IAO, Hanle, India, and at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. The Subaru data are retrieved from the JVO portal (http://jvo.nao.ac.jp/portal/v2/) operated by the NAOJ.