An Elemental Assay of Very, Extremely, and Ultra-Metal-Poor Stars

Abstract

We present a high-resolution elemental-abundance analysis for a sample of 23 very metal-poor (VMP; [Fe/H] < −2.0) stars, 12 of which are extremely metal-poor (EMP; [Fe/H] < −3.0), and 4 of which are ultra metal-poor (UMP; [Fe/H] < −4.0). These stars were targeted to explore differences in the abundance ratios for elements that constrain the possible astrophysical sites of element production, including Li, C, N, O, the α-elements, the iron-peak elements, and a number of neutron-capture elements. This sample substantially increases the number of known carbon-enhanced metal-poor (CEMP) and nitrogen-enhanced metal-poor (NEMP) stars -our program stars include eight that are considered "normal" metal-poor stars, six CEMP-no stars, five CEMP-s stars, two CEMP-r stars, and two CEMP-r/s stars. One of the CEMP-r stars and one of the CEMP-r/s stars are possible NEMP stars. We detect lithium for three of the six CEMP-no stars, all of which are Li-depleted with respect to the Spite plateau. The majority of the CEMP stars have [C/N] > 0. The stars with [C/N] < 0 suggest a larger degree of mixing; the few CEMP-no stars that exhibit this signature are only found at [Fe/H] < −3.4, a metallicity below which we also find the CEMP-no stars with large enhancements in Na, Mg, and Al. We confirm the existence of two plateaus in the absolute carbon abundances of CEMP stars, as suggested by Spite et al. We also present evidence for a "floor" in the absolute Ba abundances of CEMP-no stars at A(Ba) ∼ −2.0. 1 Based on observations made with the European Southern Observatory telescopes.enhanced metal-poor (CEMP) stars (Beers et al. 1992; Beers & Christlieb 2005; Norris et al. 2013b). This class comprises a number of sub-classes (originally defined by Beers & Christlieb 2005), based on the behavior of their neutron-capture elements:(1) CEMP-no stars, which exhibit no over-abundances of n-capture elements, (2) CEMP-s stars, which show n-capture over-abundances consistent with the slow neutron-capture process, (3) CEMP-r stars, with n-capture over-abundances associated with the rapid neutron-capture process, and (4) CEMP-r/s stars, which exhibit n-capture overabundances that suggest contribution from both the slow and rapid neutron-capture processes. Each of these subclasses appear to be associated with different elementproduction histories, thus their study provides insight into the variety of astrophysical sites in the early Galaxy that were primarily responsible for their origin. The CEMP-no stars are of special importance, as the preponderance of evidence points to their being associated with elemental-abundance patterns that were produced by the very first generation of massive stars (Norris et al. 2013b;Hansen et al. 2014;Maeder et al. 2014), thus they potentially provide a unique probe of the first mass function in the early universe along with providing information on the nucleosynthesis and properties of the first stars.In a previous paper, Hansen et al. (2014) (hereafter paper I) provided a detailed study of the elemental abundances f...

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

“…This scatter has been found in numerous previous studies, and points towards differences in the formation sites and/or processes involved, if these processes are assumed to be robust and produce similar amounts of heavy elements in each event (see, e.g., Hansen et al 2014). Figure 7 compares the X-Shooter measurements of the [Sr/Fe], [Ba/Fe], and [Ba/Sr] abundance ratios for our programme stars to larger samples with high-resolution determinations from Hansen et al (2012Hansen et al ( , 2015, and Placco et al (2014b). As seen from these figures, the CEMP-no stars follow the general trend of stars without C-enhancement predicted by standard Galactic chemical evolution (GCE) models, while the CEMP-s stars increase the star-to-star scatter not just at extremely low metallicities, but over the entire range of [Fe/H].…”

“…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.…”

“…All our programme stars have been checked against the data of Placco et al (2014b) and Hansen et al (2015), and the comparison samples used therein (Fig. 2).…”

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

“…Combined with the observed low 12 C/ 13 C ratios (see e.g. Ryan et al 2005 andHansen et al 2015a, and references therein), this points towards a large amount of mixing that is not included in their models. Recent work by Abate et al (2015a,b), employing models of AGB nucleosynthesis production from Karakas (2010), reaches better agreement for some stars, while for others they have the same problem as Bisterzo et al (2012).…”

Deep SDSS optical spectroscopy of distant halo stars

“…We present stellar atmospheric parameters and abundance estimates of [Fe/H] < λ < 5070 Å, 5220 < λ < 5280 Å and 5295 < λ < 5500 Å (to determine Fe); 5160 < λ < 5190 Å (to determine Mg abundances); and 3910 < λ < 3990 Å (for Ca abundances Yong et al 2013;Hansen et al 2015. Two scenarios were proposed for this chemical pattern: i) the results of mixing and processing of material due to stellar rotation, or ii) nucleosynthesis in mixing and fallback supernova models.…”