Normal and Outlying Populations of the Milky Way Stellar Halo at [Fe/H] < –2

Creators: Cohen, Judith G.; Christlieb, Norbert; Thompson, Ian; McWilliam, Andrew; Shectman, Stephen; Reimers, Dieter; Wisotzki, Lutz; Kirby, Evan

Abstract

From detailed abundance analysis of >100 Hamburg/ESO candidate extremely metal-poor (EMP) stars we find 45 with [Fe/H] < –3.0 dex. We identify a heretofore unidentified group: Ca-deficient stars with sub-solar [Ca/Fe] ratios and the lowest neutron-capture abundances; the Ca-deficient group comprises ~10% of the sample, excluding Carbon stars. Our radial velocity distribution shows that the carbon-enhanced stars with no s-process enhancements, CEMP-no, and which do not show C2 bands are not preferentially binary systems. Ignoring Carbon stars, approximately 15% of our sample are strong (≥5σ) outliers in one or more elements between Mg and Ni; this rises to ~19% if very strong (≥10σ) outliers for Sr and Ba are included. Examples include: HE0305–0554 with the lowest [Ba/H] known; HE1012–1540 and HE2323–0256, two (non-velocity variable) C-rich stars with very strong [Mg,Al/Fe] enhancements; and HE1226–1149, an extremely r-process rich star.

Additional Information

© 2013 American Astronomical Society. Received 2013 June 8; accepted 2013 September 23; published 2013 November 1. Based in part on observations obtained in part at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. We are very grateful to the Palomar, Las Campanas, and Keck time allocation committees for their long-term support of this campaign during the initial phase of moderate resolution spectroscopy which began in 2000 and ended in 2006 as well as the subsequent high resolution spectroscopy. J. Cohen acknowledges partial support from NSF grants AST-0507219 and AST-0908139. I. Thompson acknowledges partial support from NSF AST-0507325. E.N.K. acknowledges support from the Southern California Center for Galaxy Evolution, a multicampus research program funded by the University of California Office of Research, and partial support from NSF grant AST-1009973. This work was partially supported by Sonderforschungsbereich SFB 881 "The Milky Way System" (subproject A4) of the German Research Foundation (DFG). We are grateful to the many people who have worked to make the Keck Telescopes and their instruments, and the Magellan Telescopes and their instruments, a reality and to operate and maintain these observatories. 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.

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Submitted - 1310.1527v3.pdf

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