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

Judith Cohen, Norbert Christlieb, Ian Thompson, Andrew McWilliam, Stephen Shectman, Dieter Reimers, Lutz Wisotzki, Evan Kirby

Published 2013-10-06, updated 2013-10-12Version 3

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 (> 5 sigma) outliers in one, or more, elements between Mg and Ni; this rises to ~19% if very strong (>10 sigma) 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.