TOPoS

Abstract

Context. In the course of the Turn Off Primordial Stars (TOPoS) survey, aimed at discovering the lowest metallicity stars, we have found several carbon-enhanced metal-poor (CEMP) stars. These stars are very common among the stars of extremely low metallicity and provide important clues to the star formation processes. We here present our analysis of six CEMP stars. Aims. We want to provide the most complete chemical inventory for these six stars in order to constrain the nucleosynthesis processes responsible for the abundance patterns. Methods. We analyse both X-Shooter and UVES spectra acquired at the VLT. We used a traditional abundance analysis based on OSMARCS 1D local thermodynamic equilibrium (LTE) model atmospheres and the turbospectrum line formation code. were not able to detect any iron lines, yet we could place a robust (3σ) upper limit of [Fe/H] < −5.0 and measure the Ca abundance, with [Ca/H] = −5.0, and carbon, A(C) = 6.90, suggesting that this star could be even more metal-poor than SDSS J1742+2531. This makes these two stars the seventh and eighth stars known so far with [Fe/H] < −4.5, usually termed ultra-iron-poor (UIP) stars. No lithium is detected in the spectrum of SDSS J1742+2531 or SDSS J1035+0641, which implies a robust upper limit of A(Li) < 1.8 for both stars. Conclusions. Our measured carbon abundances confirm the bimodal distribution of carbon in CEMP stars, identifying a high-carbon band and a low-carbon band. We propose an interpretation of this bimodality according to which the stars on the high-carbon band are the result of mass transfer from an AGB companion, while the stars on the low-carbon band are genuine fossil records of a gas Article published by EDP Sciences A28, page 1 of 20 A&A 579, A28 (2015) cloud that has also been enriched by a faint supernova (SN) providing carbon and the lighter elements. The abundance pattern of the UIP stars shows a large star-to-star scatter in the [X/Ca] ratios for all elements up to aluminium (up to 1 dex), but this scatter drops for heavier elements and is at most of the order of a factor of two. We propose that this can be explained if these stars are formed from gas that has been chemically enriched by several SNe, that produce the roughly constant [X/Ca] ratios for the heavier elements, and in some cases the gas has also been polluted by the ejecta of a faint SN that contributes the lighter elements in variable amounts. The absence of lithium in four of the five known unevolved UIP stars can be explained by a dominant role of fragmentation in the formation of these stars. This would result either in a destruction of lithium in the pre-main-sequence phase, through rotational mixing or to a lack of late accretion from a reservoir of fresh gas. The phenomenon should have varying degrees of efficiency.

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

“…Several studies have confirmed the existence of the carbon bands, and have populated them with more stars (e.g., Bonifacio et al 2015;Hansen et al 2015). Here we discuss the behaviour of the heavy elements and how they associate with these bands in order to better constrain the astrophysical sites and processes that enriched the different subclasses of CEMP 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.…”

“…A few exceptions to this empirical rule have been presented in Bonifacio et al (2015) and Caffau et al (2011). In the former study, a star with an upper limit of [Fe/H] < −5.0 was found to have a low absolute C abundance, while the latter study presented a truly metal-poor star (SDSS J102915+172927) with metallicity slightly above [Fe/H] = −5.0, but without large carbonicity ([C/Fe] ≤ 0.9).…”

“…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.…”

An in-depth spectroscopic examination of molecular bands from 3D hydrodynamical model atmospheres

“…Spite et al (2013) found that when the absolute carbon abundances, A(C), of CEMP stars are presented as a function of [Fe/H], two distinct bands are found; the high and low carbon bands. This work was subsequently extended in Bonifacio et al (2015). The high carbon band contains the most iron-rich CEMP stars, and is populated by all four subclasses of CEMP star.…”