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

Abstract

Context. It has been noted that, in classical extremely metal-poor (EMP) stars, the abundance ratio of two well-observed neutroncapture elements, Sr and Ba, is always higher than [Sr/Ba] = -0.5, which is 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. Aims. 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. Methods. For a rigorous comparison with previous data, four stars with very low Sr/Ba ratios were observed and analyzed in the same way as in the First Stars program: analysis within LTE approximation through 1D (hydrostatic) model atmosphere, providing homogeneous abundances of nine neutron-capture elements.Results. 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, but in evolved giants C is partly burned into N, and owing to their high N abundance, they could still have initially been carbon-rich EMP stars (CEMP). The abundances of Na to Mg present similar anomalies to those in CEMP stars. 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.

The Pristine survey XI: the FORS2 sample

“…This clear upward trend of [Sr/Ba] with decreasing [Ba/Fe] well visible in the figure was investigated for high-resolution observations (see e.g. Spite et al 2005Spite et al , 2014 and recently also for low-resolution observations (Caffau et al 2018). This finding confirms once more the earlier conclusions by Mashonkina et al (2017), who stated that [Sr/Ba] of the VMP stars in the Milky Way and classical dSphs reveal two branches in the [Sr/Ba] versus [Ba/H] plane, but not simply a scatter of data.…”

New Neutron-capture Site in Massive Pop III and Pop II Stars as a Source for Heavy Elements in the Early Galaxy

“…In addition, there should be "low-s" stars with similar patterns of heavy elements to CEMPs and CEMP-r/s stars but with low enrichment of these elements (0 < [Ba/Fe] < 1). CS 29493-090 (Spite et al 2014) is such an example. In general, the low-s stars should have similar C enhancement to CEMP-no stars and be observed both as single stars and with a lower binary frequency than CEMP-s stars.…”

TOPoS

“…Here we redefine the class of CEMP-no stars as [Ba/Fe] ≤ 1.2. The limit of [Ba/Fe] = 1.2 was chosen because it is the highest value reached (LTE computation) by the normal metal-poor stars (not C-rich) François et al 2007;Spite et al 2014). This new definition also has the virtue of allowing us to classify SDSS J1036+1212 and CS 29528-41 as CEMP-no; they had previously been classified as CEMP-no/s and CEMP-s by Behara et al (2010) and Sivarani et al (2006), respectively.…”