Observational Constraints on First-Star Nucleosynthesis. I. Evidence for Multiple Progenitors of Cemp-No Stars

Abstract

We investigate anew the distribution of absolute carbon abundance, A(C) = log (C), for carbonenhanced metal-poor (CEMP) stars in the halo of the Milky Way, based on high-resolution spectroscopic data for a total sample of 305 CEMP stars. The sample includes 147 CEMP-s (and CEMPr/s) stars, 127 CEMP-no stars, and 31 CEMP stars that are unclassified, based on the currently employed [Ba/Fe] criterion. We confirm previous claims that the distribution of A(C) for CEMP stars is (at least) bimodal, with newly determined peaks centered on A(C)= 7.96 (the high-C region) and A(C)= 6.28 (the low-C region). A very high fraction of CEMP-s (and CEMP-r/s) stars belong to the high-C region, while the great majority of CEMP-no stars reside in the low-C region. However, there exists complexity in the morphology of the A(C)-[Fe/H] space for the CEMP-no stars, a first indication that more than one class of first-generation stellar progenitors may be required to account for their observed abundances. The two groups of CEMP-no stars we identify exhibit clearly different locations in the A(Na)-A(C) and A(Mg)-A(C) spaces, also suggesting multiple progenitors. The clear distinction in A(C) between the CEMP-s (and CEMP-r/s) stars and the CEMP-no stars appears to be as successful, and likely more astrophysically fundamental, for the separation of these sub-classes as the previously recommended criterion based on [Ba/Fe] (and [Ba/Eu]) abundance ratios. This result opens the window for its application to present and future large-scale low-and medium-resolution spectroscopic surveys.

OBSERVATIONAL CONSTRAINTS ON FIRST-STAR NUCLEOSYNTHESIS. II. SPECTROSCOPY OF AN ULTRA METAL-POOR CEMP-no STAR*

“…Recent evidence presented by Yoon et al (2016) suggests that the carbon abundance may be key to differentiating between different stellar progenitors of UMP stars. The classifications based on absolute carbon abundances, A(C), are further confirmed when looking at the sodium and magnesium abundances.…”

“…Nevertheless, the question whether one or more classes of progenitors were present (and their relative frequencies) in the primordial universe is still under discussion. In PaperI of this series, Yoon et al (2016) present evidence based on the morphology of the relationship between the absolute abundance of carbon, A(C)  = log (C), and [Fe/H], coupled with clear differences in the absolute abundances of the light elements Na and Mg among CEMP-no stars, that at least two classes of progenitors are likely to be required. It appears that one class (spinstars) dominates at the very lowest metallicities, [Fe/H] < -4.5, whereas the other class (faint SNe) dominates over the range  -4.5…”

“…Table 6 lists the abundances and references for the literature sample. The stars are divided according to the CEMP-no Groups II and III from Yoon et al (2016); two of our sample stars are classified as carbon-normal stars. The carbon abundances were corrected for the C depleted by CN processing, following the procedures described in Placco et al (2014c) and briefly summarized in Section 3.2.1.…”

Are some CEMP-s stars the daughters of spinstars?

“…Different scenarios are needed to explain CEMP stars (even in CEMP subclasses, different classes of progenitors seem to be needed, e.g. for the CEMPno category, Placco et al 2016;Yoon et al 2016;Choplin et al 2017). For CEMP-s stars, the main formation scenario is the asymptotic giant branch (AGB) scenario, suggesting that a more massive AGB companion has fed the secondary in carbon and s-elements during a mass transfer (or wind mass transfer) episode (Stancliffe & Glebbeek 2008;Lau et al 2009;Bisterzo et al 2010;Lugaro et al 2012;Abate et al 2013Abate et al , 2015bHollek et al 2015).…”

Detection of a Population of Carbon-enhanced Metal-poor Stars in the Sculptor Dwarf Spheroidal Galaxy∗

“…Generally, members of this class of carbon-rich binary stars can be identified by radial velocity monitoring or by detecting a combined enhancement in s-process elements (e.g., Ba) together with carbon that would have been produced in a companion AGB star. But recent work by Yoon et al (2016) suggests that stars with sufficiently high absolute carbon abundance (A(C)) can already be identified as CEMP-s stars just based on the [Fe/H] and A(C) measurements, as shown in Figure 10.…”