Metal-Poor Stars Observed with the Magellan Telescope. III. New Extremely and Ultra Metal-Poor Stars from SDSS/SEGUE and Insights on the Formation of Ultra Metal-Poor Stars

We report the discovery of one extremely metal-poor (EMP; [Fe/H]<-3) and one ultra metal-poor (UMP; [Fe/H]<-4) star selected from the SDSS/SEGUE survey. These stars were identified as EMP candidates based on their medium-resolution (R~2,000) spectra, and were followed-up with high-resolution (R~35,000) spectroscopy with the Magellan-Clay Telescope. Their derived chemical abundances exhibit good agreement with those of stars with similar metallicities. We also provide new insights on the formation of the UMP stars, based on comparison with a new set of theoretical models of supernovae nucleosynthesis. The models were matched with 20 UMP stars found in the literature, together with one of the program stars (SDSS J1204+1201), with [Fe/H]=-4.34. From fitting their abundances, we find that the supernovae progenitors, for stars where carbon and nitrogen are measured, had masses ranging from 20.5 M_sun to 28 M_sun and explosion energies from 0.3 to 0.9x10^51 erg. These results are highly sensitive to the carbon and nitrogen abundance determinations, which is one of the main drivers for future high-resolution follow-up of UMP candidates. In addition, we are able to reproduce the different CNO abundance patterns found in UMP stars with a single progenitor type, by varying its mass and explosion energy.

The Frequency of Field Blue-Straggler Stars in the Thick Disk and Halo System of the Galaxy

We present an analysis of a new, large sample of field blue-straggler stars (BSSs) in the thick disk and halo system of the Galaxy, based on stellar spectra obtained during the Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic Understanding and Exploration (SEGUE). Using estimates of stellar atmospheric parameters obtained from application of the SEGUE Stellar Parameter Pipeline, we obtain a sample of some 8000 BSSs, which are considered along with a previously selected sample of some 4800 blue horizontal-branch (BHB) stars. We derive the ratio of BSSs to BHB stars, F$_{rm BSS/BHB}$, as a function of Galactocentric distance and distance from the Galactic plane. The maximum value found for F$_{rm BSS/BHB}$ is $sim~$4.0 in the thick disk (at 3 kpc $<$ $|$Z$|$ $<$ 4 kpc), declining to on the order of $sim~1.5-2.0$ in the inner-halo region; this ratio continues to decline to $sim~$1.0 in the outer-halo region. We associate a minority of field BSSs with a likely extragalactic origin; at least 5$%$ of the BSS sample exhibit radial velocities, positions, and distances commensurate with membership in the Sagittarius Stream.