METAL-POOR STARS OBSERVED WITH THE MAGELLAN TELESCOPE. II. DISCOVERY OF FOUR STARS WITH [Fe/H] ⩽ –3.5

Abstract

We report on the discovery of seven low-metallicity stars selected from the Hamburg/ESO Survey, six of which are extremely metal-poor (EMP, [Fe/H] −3.0), with four having [Fe/H] −3.5. Chemical abundances or upper limits are derived for these stars based on high-resolution (R ∼ 35,000) Magellan/MIKE spectroscopy, and are in general agreement with those of other very and extremely metal-poor stars reported in the literature. Accurate metallicities and abundance patterns for stars in this metallicity range are of particular importance for studies of the shape of the metallicity distribution function of the Milky Way's halo system, in particular for probing the nature of its low-metallicity tail. In addition, taking into account suggested evolutionary mixing effects, we find that six of the program stars (with [Fe/H] −3.35) possess atmospheres that were likely originally enriched in carbon, relative to iron, during their main-sequence phases. These stars do not exhibit overabundances of their s-process elements, and hence may be, within the error bars, additional examples of the so-called CEMP-no class of objects.

High-Resolution Spectroscopic Study of Extremely Metal-Poor Star Candidates From the Skymapper Survey

“…to the heavier neutron-capture element Ba, as have been found in several studies (e.g., Honda et al 2004;Aoki et al 2005Aoki et al , 2013aLai et al 2008;Hollek et al 2011;Cohen et al 2013;Placco et al 2014a). Such stars are generally taken as evidence for an extra neutron-capture element production mechanism in addition to the main r-process as the source of the heaviest elements in the early universe (e.g., Travaglio et al 2004;Honda et al 2006;Sneden et al 2008;Jacobson & Frebel 2014).…”

“…Based on a large literature sample, Aoki et al (2013b) Placco et al (2014a) suggested this was due to the small/incomplete sample of stars in this metallicity regime; the bottom panel of Figure 13 lends support to this argument (see also Li et al 2015). Roughly half of the stars below…”

“…Objective prism observations of millions of stars, carried out by such landmark surveys as the HK Survey (Beers et al 1992) and the Hamburg-ESO Survey (Christlieb et al 2008) on Schmidt telescopes led to the medium-resolution spectroscopic followup of thousands of EMP star candidates (Norris et al 1999;Frebel et al 2006;Schörck et al 2009;Li et al 2010;Placco et al 2011). Of these, of order several hundred have been followed up with high-resolution spectroscopy and detailed element abundance analyses (e.g., McWilliam et al 1995;Norris et al 1996Norris et al , 2013Ryan et al 1996;Aoki et al 2002;François et al 2003;Cayrel et al 2004;Cohen et al 2004Cohen et al , 2013Lai et al 2008;Hollek et al 2011;Placco et al 2014a;Roederer et al 2014b).…”

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

“…Since HE0020-1741 is on the upper red giant branch, the observed carbon abundance does not reflect the chemical composition of its natal gas cloud. By using the procedure described in Placco et al (2014b), which interpolates the observed g log , [ ] Fe H , and [ ] C Fe of HE0020-1741 with a grid of theoretical stellar evolution models for low-mass stars, we determine the carbon depletion due to CN processing for HE0020-1741 to be 0.34 dex.…”

“…Because of that, we included the observed carbon and nitrogen abundances for all the syntheses, as well as the 12 C/ 13 C isotope ratio. (Placco et al 2014b).…”