The role of binaries in the enrichment of the early Galactic halo

Abstract

Context. The detailed composition of most metal-poor halo stars has been found to be very uniform. However, a fraction of 20−70% (increasing with decreasing metallicity) exhibit dramatic enhancements in their abundances of carbon; these are the so-called carbonenhanced metal-poor (CEMP) stars. A key question for Galactic chemical evolution models is whether this non-standard composition reflects that of the stellar natal clouds or is due to local, post-birth mass transfer of chemically processed material from a binary companion; CEMP stars should then all be members of binary systems. Aims. Our aim is to determine the frequency and orbital parameters of binaries among CEMP stars with and without over-abundances of neutron-capture elements -CEMP-s and CEMP-no stars, respectively -as a test of this local mass-transfer scenario. This paper discusses a sample of 24 CEMP-no stars, while a subsequent paper will consider a similar sample of CEMP-s stars. Methods. High-resolution, low S/N spectra of the stars were obtained at roughly monthly intervals over a time span of up to eight years with the FIES spectrograph at the Nordic Optical Telescope. Radial velocities of ∼100 m s −1 precision were determined by cross-correlation after each observing night, allowing immediate, systematic follow-up of any variable object. Results. Most programme stars exhibit no statistically significant radial-velocity variation over this period and appear to be single, while four are found to be binaries with orbital periods of 300−2000 days and normal eccentricity; the binary frequency for the sample is 17 ± 9%. The single stars mostly belong to the recently identified low-C band, while the binaries have higher absolute carbon abundances. Conclusions. We conclude that the nucleosynthetic process responsible for the strong carbon excess in these ancient stars is unrelated to their binary status; the carbon was imprinted on their natal molecular clouds in the early Galactic interstellar medium (ISM) by an even earlier external source, strongly indicating that the CEMP-no stars are likely bona fide second-generation stars. We discuss potential production sites for carbon and its transfer across interstellar distances in the early ISM, and the implications for the composition of high-redshift DLA systems.

Abundances of carbon-enhanced metal-poor stars as constraints on their formation

“…Of the nine stars known with [Fe/H] ≤ −4.5, seven are CEMP-no stars (Placco et al 2014b;Bonifacio et al 2015;Frebel & Norris 2015), suggesting that C was already produced and enriched in the very first stellar generations. Their low binary frequency (Starkenburg et al 2014;Hansen et al 2016a), ∼17 ± 5%, consistent with that of halo stars with normal carbon content, and their lack of s-process-element abundance signatures makes a local AGB binary companion an unlikely source of their C excess, so another production site must be found.…”

“…Carollo et al (2014) presented indications that the CEMP-s stars are preferentially associated with the inner-halo population, while the CEMP-no stars are associated with the outer-halo population. Based on these, and other recent results (see, e.g., Ito et al 2013;Placco et al 2014a;Hansen et al 2016a), it is becoming increasingly clear that the CEMP-no stars may well be bona fide second-generation stars, born from an interstellar medium (ISM) polluted with the nucleosynthetic products of the very first stars.…”

“…In contrast, the normal binary frequency of the CEMP-no stars (Starkenburg et al 2014;Bonifacio et al 2015;Hansen et al 2016a) suggests that their abundances reflect the composition of the gas from which they were born.…”

The role of binaries in the enrichment of the early Galactic halo

“…Hansen et al (2011) first showed that the enhancement of r-process elements observed in a small fraction (3-5%) of VMP and EMP stars is not causally connected to membership in a binary system, this conclusion was confirmed and further strengthened in Paper I of this series (Hansen et al 2015b). Paper II (Hansen et al 2016) examined the same question for the class of CEMP-no stars and found that only 17 ± 9% (4 of 24) of their programme stars were binaries; this percentage is identical to the binary frequency found in metal-poor red giants. The present Paper III addresses the extent to which binaries may play a role in the origin of CEMP-s and CEMP-r/s stars, using the same approach.…”

“…The great majority of the former can be accounted for by scenarios involving transfer of enriched material from a binary companion that has passed through the asymptotic giantbranch (AGB) stage of evolution. The origin of the latter has still not been identified with certainty, but their binary frequency is not higher than among metal-poor giants in general (see Paper II of this series, Hansen et al 2016). As discussed there, a number of lines of evidence strongly suggest that the CEMP-no stars contain the nucleosynthesis products of the very first stars born in the Universe, i.e., that they are bona-fide second-generation stars.…”