Building the Galactic halo from globular clusters: evidence from chemically unusual red giants^⋆

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, 69120 Heidelberg, Germany
e-mail: martell@ari.uni-heidelberg.de; grebel@ari.uni-heidelberg.de
² Department of Physics and Astronomy and JINA (Joint Institute for Nuclear Astrophysics), Michigan State University, East Lansing, MI 48824, USA
e-mail: smolin19@msu.edu, beers@pa.msu.edu
³ Department of Physics and Astronomy, State University of New York College at Oneonta, Oneonta, NY 13820, USA
e-mail: smolinjp@oneonta.edu

Received: 6 July 2011
Accepted: 31 August 2011

Abstract

We present a spectroscopic search for halo field stars that originally formed in globular clusters. Using moderate-resolution SDSS-III/SEGUE-2 spectra of 561 red giants with typical halo metallicities (−1.8 ≤  [Fe/H]  ≤  −1.0), we identify 16 stars, 3% of the sample, with CN and CH bandstrength behavior indicating depleted carbon and enhanced nitrogen abundances relative to the rest of the data set. Since globular clusters are the only environment known in which stars form with this pattern of atypical light-element abundances, we claim that these stars are second-generation globular cluster stars that have been lost to the halo field via normal cluster mass-loss processes. Extrapolating from theoretical models of two-generation globular cluster formation, this result suggests that globular clusters contributed significant numbers of stars to the construction of the Galactic halo: we calculate that a minimum of 17% of the present-day mass of the stellar halo was originally formed in globular clusters. The ratio of CN-strong to CN-normal stars drops with Galactocentric distance, suggesting that the inner-halo population may be the primary repository of these stars.