CS 22964−161: A Double‐Lined Carbon‐ ands‐Process–Enhanced Metal‐Poor Binary Star

Abstract

A detailed high-resolution spectroscopic analysis is presented for the carbon-rich low-metallicity Galactic halo object CS 22964À161. We have discovered that CS 22964À161 is a double-lined spectroscopic binary and have derived accurate orbital components for the system. From a model atmosphere analysis we show that both components are near the metal-poor main-sequence turnoff. Both stars are very enriched in carbon and in neutron-capture elements that can be created in the s-process, including lead. The primary star also possesses an abundance of lithium close to the value of the ''Spite plateau.'' The simplest interpretation is that the binary members seen today were the recipients of these anomalous abundances from a third star that was losing mass as part of its AGB evolution. We compare the observed CS 22964À161 abundance set with nucleosynthesis predictions of AGB stars, discuss issues of envelope stability in the observed stars under mass transfer conditions, and consider the dynamical stability of the alleged original triple star. Finally, we consider the circumstances that permit survival of lithium, whatever its origin, in the spectrum of this extraordinary system.

TOPoS

“…This implies that NLTE and granulation effects have opposite directions, and they may tend to cancel; however, for iron it is not possible to simply add the 3D Thompson et al 2008;Aoki et al 2008;Behara et al 2010;Placco et al 2011;Carollo et al 2012;Masseron et al 2010Masseron et al , 2012Yong et al 2013;Cohen et al 2013;Spite et al 2013;Caffau et al 2013a). The bimodal distribution is clear in this plot.…”

“…6 we have included only unevolved stars since giant stars may be internally "mixed" (Spite et al 2005(Spite et al , 2006 and the C abundance may thus be decreased by an amount that is generally difficult to estimate. We have included in the plot only stars from the literature that have a measured Ba abundance or a significant upper limit (Sivarani et al 2006;Frebel et al 2005Frebel et al , 2006Thompson et al 2008;Aoki et al 2008;Behara et al 2010;Masseron et al 2010Masseron et al , 2012Yong et al 2013;Cohen et al 2013;Li et al 2015), so as to have some indication on their classification as CEMP-no, CEMP-s, or CEMP-rs. In order to clearly define the behaviour of the carbon abundance at low metallicity, we have also included the ultra-iron-poor subgiant HE 1327-2326 (Frebel et al 2005Cohen et al 2013) and SDSS J1619+1705 (Caffau et al 2013a), even though their Ba abundance is undetermined.…”

Three carbon-enhanced metal-poor dwarf stars from the SDSS

“…A few stars show Li at the Spite plateau level, while others show a lower Li abundance. A remarkable example is the double lined spectroscopic binary CS 22964-161 (Thompson et al 2008). The two stars are both CEMP, the primary is a warm sub-giant, while the secondary is a main sequence star.…”

Thermohaline Convection in Main Sequence Stars

“…Stancliffe et al (2007) pointed out that in case of accretion of metal-rich matter, this material would subsequently fall down inside the star due to thermohaline convection. In a more recent paper (Thompson et al 2008), we suggested that, between the stellar birth and the time when the AGB sends its processed material onto it, the main sequence star had time to suffer helium and heavy element diffusion below its convective zone, thereby creating a stabilizing µ-gradient. In the presence of this diffusion-induced µ-gradient, outside matter may accumulate in the convection zone until the overall µ-gradient becomes flat.…”

“…This may correspond to the internal structure of a main sequence star just before the accretion of heavy material from an AGB companion. Helium depletion due to gravitational settling leads to a pre-existing stabilizing µ-gradient, which then allows a large part of the accreted matter to remain in the convective zone (after Thompson et al 2008) In summary, thermohaline convection, which has long been forgotten in astrophysics, has to be taken into account in several important cases during stellar evolution. Its competition with other processes like meridional circulation, magnetic fields, etc.…”