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 massloss 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.

Spectroscopic and photometric data for likely member stars of five Galactic globular clusters (M3, M53, M71, M92, and NGC 5053) and three open clusters (M35, NGC 2158, and NGC 6791) are processed by the current version of the SEGUE Stellar Parameter Pipeline (SSPP), in order to determine estimates of metallicities and radial velocities for the clusters. These results are then compared to values from the literature. We find that the mean metallicity ( [Fe/H] ) and mean radial velocity ( RV ) estimates for each cluster are almost all within 2σ of the adopted literature values; most are within 1σ. We also demonstrate that the new version of the SSPP achieves small, but noteworthy, improvements in [Fe/H] estimates at the extrema of the cluster metallicity range, as compared to a previous version of the pipeline software. These results provide additional confidence in the application of the SSPP for studies of the abundances and kinematics of stellar populations in the Galaxy.Subject headings: methods: data analysis -stars: abundances, fundamental parameters -surveys -techniques: spectroscopic of M35, M71, M92, NGC 2158, and NGC 6791, including sky spectra and calibration objects. Some of these targets had low average signal-to-noise spectra; for consistency with previous papers in this series, only those spectra with S/N > 10/1 were considered for subsequent analysis. After processing by the SSPP some targets had no estimates for RV or [Fe/H]; these were excluded as well. After these cuts were made, there remained 487, 495, 579, 1094, 775, 495, 579, and 1087 stars considered for M3, M35, M53, M71, M92, NGC 2158, NGC 5053, and NGC 6791, respectively. Cluster Membership SelectionPaper I has shown that the stellar spectra processed through the SSPP have typical uncertainties of 141 K, 0.23 dex, and 0.23 dex for T eff , log g, and [Fe/H], respectively. Uncertainties in the radial velocity depends on the spectral type and apparent magnitude (and fall in the range 5-20 km s −1 ; for most of the cluster stars the error is usually much less than 10 km s −1 . In this section we discuss how the adopted true members for each cluster are selected, based in part on their estimated metallicities and radial velocities. Likely Member Star SelectionThe procedure for determining the likely members of each cluster is the same as described by Paper II, and will only be discussed briefly here. Two procedures were designed for selecting likely true member stars, one for globular clusters and one for open clusters. The difference is primarily due to the lower number density of stars on the CMD of an open cluster compared to that of a globular cluster. However, the techniques are sufficiently different that, due to the highly evolved nature of NGC 2158 and NGC 6791, the procedure for open clusters could not be applied to these particular clusters because it relies on a function fit to the main stellar locus which, in these cases, would be double-valued around the main-sequence turnoff. Hence, we have employed the procedure for globular c...