We report the discovery of one extremely metal-poor ( EMP ; [ \mathrm { Fe } / \mathrm { H } ] < -3 ) and one ultra metal-poor ( UMP ; [ \mathrm { Fe } / \mathrm { H } ] < -4 ) star selected from the SDSS/SEGUE survey .
These stars were identified as EMP candidates based on their medium-resolution ( R~ { } \sim 2 , 000 ) spectra , and were followed-up with high-resolution ( R~ { } \sim 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 ( ( catalog SDSS~J1204 $ + $ 1201 ) ) , with [ \mathrm { Fe } / \mathrm { 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 \mathrm { M } _ { \odot } to 28 \mathrm { M } _ { \odot } , and explosion energies from 0.3 to 0.9 \times 10 ^ { 51 } \mathrm { 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 .