We present an analysis of high-resolution Keck/HIRES spectroscopic observations of J0815+4729 , an extremely carbon-enhanced , iron-poor dwarf star .
These high-quality data allow us to derive a metallicity of [ Fe/H ] = -5.49 { \pm } 0.14 from the three strongest \ion Fe1 lines and to measure a high [ Ca/Fe ] = 0.75 { \pm } 0.14 .
The large carbon abundance of A ( C ) = 7.43 { \pm } 0.17 ( or [ C/Fe ] \sim 4.49 { \pm } 0.11 ) places this star in the upper boundary of the low-carbon band in the A ( C ) - [ Fe/H ] diagram , suggesting no contamination from a binary AGB companion .
We detect the oxygen triplet at 777nm for the first time in an ultra-metal poor star , indicating a large oxygen-to-iron abundance ratio of [ O/Fe ] = 4.03 { \pm } 0.12 ( A ( O ) = 7.23 { \pm } 0.14 ) , significantly higher than the previously most metal-poor dwarf J2209-0028 with an oxygen triplet detection with [ O/Fe ] \sim 2.2 dex at [ Fe/H ] \sim - 3.9 .
Nitrogen is also dramatically enhanced with ( A ( N ) = 6.75 { \pm } 0.08 ) and an abundance ratio [ N/Fe ] \sim 4.41 { \pm } 0.08 .
We also detect Ca , Na and Mg , while provide upper limits for eight other elements .
The abundance pattern of J0815+4729 resembles that of HE 1327-2326 , indicating that both are second-generation stars contaminated by a \sim 21 - 27 M _ { \odot } single , zero-metallicity low-energy supernova with very little mixing and substantial fallback .
The absence of lithium implies an upper-limit abundance A ( Li ) < 1.3 dex , about 0.7 dex below the detected Li abundance in J0023+0307 which has a similar metallicity , exacerbating the cosmological lithium problem .