The Extreme CNO-enhanced Composition of the Primitive Iron-poor Dwarf Star J0815+4729

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 ± 0.14 from the three strongest Fe i lines and to measure a high [Ca/Fe] = 0.75 ± 0.14. The large carbon abundance of A(C) = 7.43 ± 0.17 (or [C/Fe] ∼ 4.49 ± 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 777 nm for the first time in an ultra-metal-poor star, indicating a large oxygen-to-iron abundance ratio of [O/Fe] = 4.03 ± 0.12 (A(O) = 7.23 ± 0.14), significantly higher than the previously most metal-poor dwarf J2209–0028 with an oxygen triplet detection with [O/Fe] ∼ 2.2 dex at [Fe/H] ∼ −3.9. Nitrogen is also dramatically enhanced with (A(N) = 6.75 ± 0.08) and an abundance ratio [N/Fe] ∼ 4.41 ± 0.08. We also detect Ca, Na, and Mg, while providing 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 ∼21–27 M⊙ 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.