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 Sloan Digital Sky Survey/Sloan Extension for Galactic Understanding and Exploration survey. These stars were identified as EMP candidates based on their medium-resolution (R ~ 2000)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 comparisons with a new set of theoretical models of supernovae (SNe) 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 SNe progenitors, for stars where carbon and nitrogen are measured, had masses ranging from 20.5 M⊙ to 28 M⊙ and explosion energies from 0.3 to 0.9 x 10⁵¹ erg. These results are highly sensitive to the carbon and nitrogen abundance determinations, which is one of the main drivers for a 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.