Abundances In Very Metal-Poor Dwarf Stars

By J. Cohen, N. Christlieb, A. McWilliam, S. Shectman, I. Thompson, G. Wasserburg, I. Ivans, M. Dehn, T. Karlsson, Jorge Meléndez. 2004

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We discuss the detailed composition of 28 extremely metal-poor (EMP) dwarfs, 22 of which are from the Hamburg/ESO Survey (HES), based on Keck echelle spectra. Our sample has a median [Fe/H] of -2.7 dex, extends to -3.5 dex, and is somewhat less metal-poor than was expected from [Fe/H](HK, HES) determined from low-resolution spectra. Our analysis supports the existence of a sharp decline in the distribution of halo stars with metallicity below [Fe/H] = -3.0 dex. So far no additional turnoff stars with [Fe/H] < -3.5 have been identified in our follow-up efforts. For the best-observed elements between Mg and Ni, we find that the abundance ratios appear to have reached a plateau, i.e., [X/Fe] is approximately constant as a function of [Fe/H], except for Cr, Mn, and Co, which show trends of abundance ratios varying with [Fe/H]. These abundance ratios at low metallicity correspond approximately to the yield expected from Type II supernovae (SNe) with a narrow range in mass and explosion parameters; high-mass Type II SN progenitors are required. The dispersion of [X/Fe] about this plateau level is surprisingly small and is still dominated by measurement errors rather than intrinsic scatter. These results place strong constraints on the characteristics of the contributing SNe. The dispersion in neutron-capture elements and the abundance trends for Cr, Mn, and Co are consistent with previous studies of evolved EMP stars. We find halo-like enhancements for the α-elements Mg, Ca, and Ti, but solar Si/Fe ratios for these dwarfs. This contrasts with studies of EMP giant stars, which show Si enhancements similar to other α-elements. Sc/Fe is another case where the results from EMP dwarfs and from EMP giants disagree; our Sc/Fe ratios are enhanced compared to the solar value by ~0.2 dex. Although this conflicts with the solar Sc/Fe values seen in EMP giants, we note that α-like Sc/Fe ratios have been claimed for dwarfs at higher metallicity. Two dwarfs in the sample are carbon stars, while two others have significant C enhancements, all with 12C/13C ~7 and with C/N between 10 and 150. Three of these C-rich stars have large enhancements of the heavy neutron capture elements, including lead, which implies a strong s-process contribution, presumably from binary mass transfer; the fourth shows no excess of Sr or Ba.

Subfields: Astronomy and Astrophysics, Instrumentation

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THE ABUNDANCES OF NEUTRON-CAPTURE SPECIES IN THE VERY METAL-POOR GLOBULAR CLUSTER M15: A UNIFORM ANALYSIS OF RED GIANT BRANCH AND RED HORIZONTAL BRANCH STARS (2011, methodology): “For this figure, halo and disk star data have been taken from these surveys: Fulbright (2000)(34), Reddy et al. (2003)(76), Cayrel et al. (2004)(20), Cohen et al. (2004)(22), Simmerer et al. (2004)(80), Barklem et al. (2005)(8), Reddy, Lambert & Allende Prieto (2006)(75), François et al.…”
THE DISCOVERY AND ANALYSIS OF VERY METAL-POOR STARS IN THE GALAXY (2005, background): “F or p er so na l u se o nl y. recently also Magellan/MIKE), albeit with slightly lower S/N and less complete wavelength coverage (see Cohen et al. 2002, 2004; Carretta et al. 2002).”
Chemical Evolution in Hierarchical Models of Cosmic Structure. I. Constraints on the Early Stellar Initial Mass Function (2005, background): “…galaxies for comparison to HST and JWST data at z > 6, and (2) a model distribution of metal-poor Galactic stars for comparison to the existing samples (Cayrel et al. 2004; Cohen et al. 2004) and eventually to the thousands of metal-poor halo stars expected from large future surveys (SEGUE, WFMOS).”
CHEMICAL EVOLUTION IN HIERARCHICAL MODELS OF COSMIC STRUCTURE. II. THE FORMATION OF THE MILKY WAY STELLAR HALO AND THE DISTRIBUTION OF THE OLDEST STARS (2009, background): “Without exact knowledge of how HES stars are distributed kinematically into the halo or disk, and without a more detailed model of disk formation in the theoretical MDF, no more exact results can be obtained.”
Spectroscopic Studies of Very Metal-poor Stars with the Subaru High Dispersion Spectrograph. III. Light Neutron-Capture Elements (2005, result): “These values are similar to the majority of metal-poor dwarf stars reported by Cohen et al. (2004) and to the giants studied in Paper II.”
Stellar Abundances for the Galactic Archeology (SAGA) Database — Compilation of the Characteristics of Known Extremely Metal-Poor Stars (2008, unknown): “…& Kovtyukh 2001; Aoki et al. 2002; Cohen et al. 2002; Carretta et al. 2002; Johnson 2002; Nissen et al. 2002; Cayrel et al. 2004; Honda et al. 2004; Cohen et al. 2004; Simmerer et al. 2004; Spite et al. 2005; Barklem et al. 2005; Jonsell et al. 2005; Aoki et al. 2005; Garćıa Pérez & Primas 2006;…”
First results from the IllustrisTNG simulations: a tale of two elements - chemical evolution of magnesium and europium (2017, background): “While some sites correspond to large regions of stars, many are more uniformly distributed throughout the galaxies, agreeing qualitatively with observations of the MW showing highly enhanced europium stars in the halo (Cohen et al. 2004; Venn et al. 2004; Barklem et al. 2005).”
The stellar halo of the Galaxy (2008, background): “Cayrel et al. (2004) find that the scatter in several [α/Fe] and [Fe-peak/Fe] ratios is consistent with being only due to observational errors, as shown in Figure 11 (see also Cohen et al. 2004).”
Neutron-Capture Elements in the Early Galaxy (2008, methodology): “Sources of the C-rich stars are from a few large-sample surveys (Van Eck et al. 2003; Cohen et al. 2004, 2006; Barklem et al. 2005; Aoki et al. 2007a, and references therein), supplemented by many papers on individual stars.”
A SEARCH FOR STARS OF VERY LOW METAL ABUNDANCE. VI. DETAILED ABUNDANCES OF 313 METAL-POOR STARS (2014, background): “While observers are generally aware of this effect (e.g., Cohen et al. 2004 discuss it in detail for the case of Mg i lines7), limitations in the size of the stellar sample of- 7 The line-by-line differences for five Mg i lines listed in Table 12 of Cohen et al. (2004) agree in sign but not…”
HE 1327–2326, an Unevolved Star with [Fe/H] < –5.0. I. A Comprehensive Abundance Analysis (2005, methodology): “In selecting stars for this exercise, we take into account that Cohen et al. (2004) adopt the Teff scale of Houdashelt et al. (2000), which is 200 K higher than the scale we adopt here, and accept stars with 6280 6480 K from their sample.”
First stars VIII. Enrichment of the neutron-capture elements in the early Galaxy (2007, unknown): “The great uniformity in the [α/Fe] ratios of metal-poor stars has recently been demonstrated in more limited samples of turnoff (dwarf) stars also by Cohen et al. (2004), Arnone et al. (2005), and Spite et al. (2005) and will be further discussed in the next paper of this series (Bonifacio et al.,…”
The Dawes Review 2: Nucleosynthesis and Stellar Yields of Low- and Intermediate-Mass Single Stars (2014, unknown): “10 (e.g., Cohen et al. 2004; Sivarani et al. 2006; Jonsell et al. 2006; Aoki et al. 2007; Beers et al. 2007; Lucatello et al. 2011).”
First stars XII. Abundances in extremely metal-poor turnoff stars,and comparison with the giants (2009, methodology): “Cohen et al. (2004) analysed a set of dwarf stars that is directly comparable to those analysed in the present paper.”