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

Abstract

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.