New extremely metal-poor stars in the Galactic halo
We present a detailed abundance analysis based on high-resolution and high signal-to-noise spectra of eight extremely metal-poor (EMP) stars with [ Fe/H ] ≾ − 3.5 dex, four of which are new. Only stars with 4900 K < T_(eff) < 5650 K are included. Two stars of the eight are outliers in each of several abundance ratios. The most metal-poor star in this sample, HE 1424–0241, has [ Fe/H ] ~ − 4 dex and is thus among the most metal-poor stars known in the Galaxy. It has highly anomalous abundance ratios unlike those of any other known EMP giant, with very low Si, Ca, and Ti relative to Fe, and enhanced Mn and Co, again relative to Fe. Only (low) upper limits for C and N can be derived from the nondetection of the CH and NH molecular bands. HE 0132-2429, another sample star, has excesses of N and Sc with respect to Fe. The strong outliers in abundance ratios among the Fe-peak elements in these C-normal stars, not found at somewhat higher metallicities ([ Fe/H ] ~ − 3 dex), are definitely real. They suggest that at such low metallicities we are beginning to see the anticipated and long sought stochastic effects of individual supernova events contributing to the Fe-peak material within a single star. With spectra reaching well into the near-UV we are able to probe the behavior of copper abundances in such extreme EMP stars. A detailed comparison of the results of the analysis procedures adopted by our 0Z project compared to those of the First Stars VLT Large Project finds a systematic difference for [ Fe/H ] of ~0.3 dex, our values always being higher.
Additional Information©2008 American Astronomical Society. Received 2007 July 6, accepted for publication 2007 August 30. We are grateful to the many people who have worked to make the Keck Telescope and HIRES a reality and to operate and maintain the Keck Observatory. The authors wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented herein would have been possible. Some of the data presented herein were obtained at the Palomar Observatory. This publication makes use of data from the Two Micron All-Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration and the National Science Foundation. J. G. C. is grateful to NSF grant AST 05-07219 for partial support. N. C. is a Research Fellow of the Royal Swedish Academy of Sciences supported by a grant from the Knut and Alice Wallenberg Foundation. He also acknowledges financial support from Deutsche Forschungsgemeinschaft through grants Ch 214/3 and Re 353/44. We thank W. Huang for help accessing the SDSS database.
Published - COHapj08a.pdf
Accepted Version - 0709.0029.pdf
Supplemental Material - datafile3.txt