The low Sr/Ba ratio on some extremely metal-poor stars

TL;DR

This study investigates extremely metal-poor stars with unusually low Sr/Ba ratios, analyzing their neutron-capture element patterns to understand their origins, and finds some stars exhibit s-process signatures without clear pollution sources.

Contribution

It provides homogeneous LTE abundance measurements of neutron-capture elements in four EMP stars, revealing s-process-like patterns in stars without typical pollution indicators.

Findings

Three stars show s-process element patterns similar to theoretical models.

One star's Sr/Ba ratio exceeds the solar r-only value, so it is excluded.

Stars lack evidence of binary companions or internal pollution mechanisms.

Abstract

It has been noted that, in classical extremely metal-poor (EMP) stars, the abundance ratio of Sr and Ba, is always higher than [Sr/Ba] = -0.5, the value of the solar r-only process; however, a handful of EMP stars have recently been found with a very low Sr/Ba ratio. We try to understand the origin of this anomaly by comparing the abundance pattern of the elements in these stars and in the classical EMP stars. Four stars with very low Sr/Ba ratios were observed and analyzed within LTE approximation through 1D (hydrostatic) model atmosphere, providing homogeneous abundances of nine neutron-capture elements. In CS 22950-173, the only turnoff star of the sample, the Sr/Ba ratio is, in fact, found to be higher than the r-only solar ratio, so the star is discarded. The remaining stars (CS 29493-090, CS 30322-023, HE 305-4520) are cool evolved giants. They do not present a clear carbon enrichment. The abundance patterns of the neutron-capture elements in the three stars are strikingly similar to a theoretical s-process pattern. This pattern could at first be attributed to pollution by a nearby AGB, but none of the stars presents a clear variation in the radial velocity indicating the presence of a companion. The stellar parameters seem to exclude any internal pollution in a TP-AGB phase for at least two of these stars. The possibility that the stars are early-AGB stars polluted during the core He flash does not seem compatible with the theory.