Extremely Metal-Poor Stars. VIII. High-Resolution, High Signal-to-Noise Ratio Analysis of Five Stars with [Fe/H] ≲ –3.5

By J. Norris, S. Ryan, Timothy C. Beers. 2001

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High-resolution, high signal-to-noise ratio (⟨S/N⟩ = 85) spectra have been obtained for five stars, CD -24°17504, CD -38°245, CS 22172-002, CS 22885-096, and CS 22949-037, having [Fe/H] ≲ -3.5 according to previous lower S/N material. LTE model atmosphere techniques are used to determine [Fe/H] and relative abundances, or their limits, for some 18 elements, and to constrain more tightly the early enrichment history of the Galaxy than is possible based on previous analyses. We compare our results with high-quality higher abundance literature data for other metal-poor stars and with the canonical Galactic chemical enrichment results of Timmes and colleagues and obtain the following basic results. (1) Large supersolar values of [C/Fe] and [N/Fe], not predicted by the canonical models, exist at lowest abundance. For C at least, the result is difficult to attribute to internal mixing effects. (2) We confirm that there is no upward trend in [α/Fe] as a function of [Fe/H], in contradistinction to some reports of the behavior of [O/Fe]. (3) The abundances of aluminum, after correction for non-LTE effects, are in fair accord with theoretical prediction. (4) We confirm earlier results concerning the Fe peak elements that [Cr/Fe] and [Mn/Fe] decrease at lowest abundance while [Co/Fe] increases, behaviors that had not been predicted. We find, however, that [Ni/Fe] does not vary with [Fe/H], and at [Fe/H] ~ -3.7, [Ni/Fe] = 0.08 ± 0.06. This result appears to be inconsistent with the supernova models of Nakamura and colleagues that seek to understand the observed behavior of the Fe peak elements by varying the position of the model mass cut relative to the Si-burning regions. (5) The heavy neutron capture elements Sr and Ba exhibit a large scatter, with the effect being larger for Sr than Ba. The disparate behavior of these two elements has been attributed to the existence of (at least) two different mechanisms for their production. (6) For the remarkable object CS 22949-037, we confirm the result of McWilliam and colleagues that [C/Fe], [Mg/Fe], and [Si/Fe] are supersolar by ~1.0 dex. Further, we find [N/Fe] = 2.7 ± 0.4. None of these results are understandable within the framework of standard models. We discuss them in terms of partial ejection of supernova mantles and massive (200-500 M☉) zero heavy-element hypernovae. The latter model actually predicted overproduction of N and underproduction of Fe peak elements. (7) We use robust techniques to determine abundance trends as a function of [Fe/H]. In most cases one sees an apparent upturn in the dispersion of relative abundance [X/Fe] for [Fe/H] < -2.5. It remains unclear whether this is a real effect or one driven by observational error. The question needs to be resolved with a much larger and homogeneous data set, both to improve the quality of the data and to understand the role of unusual stars such as CS 22949-037.

Subfields: Astronomy and Astrophysics

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The Y2 Isochrones for α-Element Enhanced Mixtures (2002, background): “Note that, as in VandenBerg et al. (2000), we have assumed that the abundance changes in Mn and Al are anticorrelated with those of the -elements (VandenBerg et al. 2000; Norris et al. 2001).”
The Hamburg/ESO R-process enhanced star survey (HERES). II. Spectroscopic analysis of the survey sample (2005, methodology): “Such trends have been noted in previous works on metal-poor giants and dwarfs, for example by Norris et al. (2001) and Cohen et al. (2004).”
THE MOST METAL-POOR STARS. II. CHEMICAL ABUNDANCES OF 190 METAL-POOR STARS INCLUDING 10 NEW STARS WITH [Fe/H] ⩽ −3.5,, (2012, methodology): “…Research, School of Physics, Astronomy & Mathematics, University of Hertfordshire, College Lane,Hatfield, Hertfordshire, AL10 9AB, UK; s.g.ryan@herts.ac.uk 1996; Norris et al. 2001; Johnson 2002; Cayrel et al. 2004; Beers & Christlieb 2005; Cohen et al. 2008; Lai et al. 2008; Frebel & Norris 2011).”
The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models (2011, result): “Similar characteristics have been detected in the giant CS 22949–037, previously detected by Norris et al. (2001); Depagne et al. (2002).”
Spectroscopic Studies of Very Metal-poor Stars with the Subaru High Dispersion Spectrograph. III. Light Neutron-Capture Elements (2005, methodology): “The error of equivalent width measurements is estimated using the relation W (kn1=2pix )/ R S/Nð Þ½ , where R is the resolving power, S/N is the signal-to-noise ratio per pixel, and n1 =2 pix is the number of pix- els over which the integration is performed (Norris et al. 2001).”
First stars VIII. Enrichment of the neutron-capture elements in the early Galaxy (2007, unknown): “…[Zr/Fe] as functions of [Fe/H], as determined here and by Honda et al. (2004), together with data selected from earlier papers (Ryan et al. 1991; Norris et al. 2001; Gratton et al. 1987; Gilroy et al. 1988; Gratton et al. 1988; Gratton & Sneden 1991; Edvardsson et al. 1993; Gratton & Sneden…”
Nucleosynthesis in the Early Galaxy (2007, background): “McWilliam (1998), Burris et al. (2000), Norris et al. (2001), Johnson & Bolte (2002), Lambert & Allende Prieto (2002), Truran et al. (2002), Honda et al. (2004) and Barklem et al. (2005) reported the observation of a large scatter in [Sr/Ba] in low-metallicity stars.”
HIGH-RESOLUTION SPECTROSCOPY OF EXTREMELY METAL-POOR STARS IN THE LEAST EVOLVED GALAXIES: URSA MAJOR II and COMA BERENICES (2009, methodology): “To characterize the quality of the data, we use the ”figure of merit” introduced by Norris et al. (2001) to compare these observations to others that have been obtained for dwarf galaxies.”
The evolution of the Milky Way from its earliest phases: Constraints on stellar nucleosynthesis (2004, background): “Norris et al. (2001) introduced a factor F (defined as R x (S/N) / λ, where R is the resolution of the spectrum and S/N the signal to noise ratio), representative of the quality of the observational data.”
First stars. I. The extreme r-element rich, iron-poor halo giant CS 31082-001 - Implications for the r-process site(s) and radioactive cosmochronology (2002, background): “Previous studies of these extreme halo stars (Bessell & Norris 1984; Norris et al. 2001; see other references in Table 2 of Cayrel 1996) have revealed abundance patterns that are very unlike those found in the disk and in halo stars with metallicities above [Fe/H] ∼ −2.5 (e.g., Pagel &…”
Carbon-enhanced Metal-poor Stars. I. Chemical Compositions of 26 Stars (2006, methodology): “This object was reinvestigated by Norris et al. (2001, 2002), Depagne et al. (2002), and Cayrel et al. (2004).”