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We report the discovery of one extremely metal-poor (EMP; [Fe/H]<-3) and oneultra 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 chemicalabundances exhibit good agreement with those of stars with similarmetallicities. We also provide new insights on the formation of the UMP stars,based on comparison with a new set of theoretical models of supernovaenucleosynthesis. The models were matched with 20 UMP stars found in theliterature, together with one of the program stars (SDSS J1204+1201), with[Fe/H]=-4.34. From fitting their abundances, we find that the supernovaeprogenitors, for stars where carbon and nitrogen are measured, had massesranging from 20.5 M_sun to 28 M_sun and explosion energies from 0.3 to0.9x10^51 erg. These results are highly sensitive to the carbon and nitrogenabundance determinations, which is one of the main drivers for futurehigh-resolution follow-up of UMP candidates. In addition, we are able toreproduce the different CNO abundance patterns found in UMP stars with a singleprogenitor type, by varying its mass and explosion energy.

astrophysical journal/the astrophysical journal

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