Extremely metal-poor stars. VIII. High-resolution, high signal-to-noise ratio analysis of five stars with [Fe/H] greater than or similar to-3.5

High-resolution, high signal-to-noise ratio (<S/N> = 85) spectra have been obtained for five stars, CD -24 degrees 17504, CD - 38 degrees 245, CS 2217 2-002, CS 22885-096, and CS 22949-037, having [Fe/H] less than or similar t o -3.5 according to previous lower S/N material. LTE model atmosphere techn iques 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 compa re our results with high-quality higher abundance literature data for other metal-poor stars and with the canonical Galactic chemical enrichment resul ts of Timmes and colleagues and obtain the following basic results. (1) Lar ge supersolar values of [C/Fe] and [N/Fe], not predicted by the canonical m odels, exist at lowest abundance. For C at least, the result is difficult t o attribute to internal mixing effects. (2) We confirm that there is no upw ard trend in [alpha /Fe] as a function of [Fe/H], in contradistinction to s ome reports of the behavior of [O/Fe]. (3) The abundances of aluminum, afte r correction for non-LTE effects, are in fair accord with theoretical predi ction. (4) We confirm earlier results concerning the Fe peak elements that [Cr/Fe] and [Mn/Fe] decrease at lowest abundance while [Co/Fe] increases, b ehaviors that had not been predicted. We find, however, that [Ni/Fe] does n ot vary with [Fe/H], and at [Fe/H] similar to -3.7, [Ni/Fe] = 0.08 +/- 0.06 . This result appears to be inconsistent with the supernova models of Nakam ura 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 exhib it a large scatter, with the effect being larger for Sr than Ba. The dispar ate behavior of these two elements has been attributed to the existence of (at least) two different mechanisms for their production. (6) For the remar kable object CS 22949-037, we confirm the result of McWilliam and colleague s that [C/Fe], [Mg/Fe], and [Si/Fe] are supersolar by similar to1.0 dex. Fu rther, we find [N/Fe] = 2.7 +/- 0.4. None of these results are understandab le within the framework of standard models. We discuss them in terms of par tial ejection of supernova mantles and massive (200-500 M-circle dot) zero heavy-element hypernovae. The latter model actually predicted overproductio n of N and underproduction of Fe peak elements. (7) We use robust technique s 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 drive n by observational error. The question needs to be resolved with a much lar ger and homogeneous data set, both to improve the quality of the data and t o understand the role of unusual stars such as CS 22949-037.