Carbon in J0815+4729: Casting a shadow of doubt on the interpretation of CH absorption in the most primitive stars
Abstract
Aims. We revisit the carbon abundance in the primitive iron-poor star J0815+4729 ([Fe/H] = -5.49). J0815+4729 is so carbon-enhanced and oxygen-rich that, in addition to CH and C2 molecular lines, weaker neutral atomic transitions are measurable in its spectrum, providing a unique opportunity to explore the line formation of C I and O I transitions in such an extremely iron-deficient star. Methods. Based on high-resolution spectroscopy and newly constructed atomic models for carbon and oxygen, we present the first investigation to derive carbon and oxygen abundances of this hyper iron-poor star from neutral atomic transitions under non-local thermodynamic equilibrium (NLTE) conditions. Results. The carbon abundance derived from C I lines is A(C)NLTE = 8.08+-0.07, consistent with that inferred from C2 lines (A(C)LTE =7.96 +- 0.08), but around 0.6 dex higher than the value from CH lines (A(C)LTE = 7.47 +- 0.17). Since CH lines are the most commonly-used carbon abundance indicator for extremely metal-poor stars, this result casts doubt on the quantitative interpretation of CH absorption in the most primitive stars. We confirm that J0815+4729 is one of the most carbon-enhanced stars with [Fe/H] <= -4.5 known to date. We have also determined A(O)NLTE as 7.25 +- 0.11, with NLTE corrections of around -0.11 dex. The carbon-to-oxygen ratio of the star is [C/O] = 1.06 +- 0.1.
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BibTeX
@misc{zhang2026carbon,
title = {{Carbon in J0815+4729: Casting a shadow of doubt on the interpretation of CH absorption in the most primitive stars}},
author = {Junbo Zhang and Carlos Allende Prieto and Yeisson Osorio and Jonay I. González Hernández and David S. Aguado and Ivan Hubeny and Shuai Liu and Jianrong Shi and Rafael Rebolo and Gang Zhao},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2608.26731},
eprint = {2608.26731},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.26731},
note = {CC BY-NC-ND 4.0},
}
RIS
TY - UNPB
TI - Carbon in J0815+4729: Casting a shadow of doubt on the interpretation of CH absorption in the most primitive stars
AU - Junbo Zhang
AU - Carlos Allende Prieto
AU - Yeisson Osorio
AU - Jonay I. González Hernández
AU - David S. Aguado
AU - Ivan Hubeny
AU - Shuai Liu
AU - Jianrong Shi
AU - Rafael Rebolo
AU - Gang Zhao
PY - 2026
DA - 2026/10/09
PB - arXiv
DO - 10.48550/arxiv.2608.26731
UR - https://arxiv.org/abs/2608.26731
AB - Aims. We revisit the carbon abundance in the primitive iron-poor star J0815+4729 ([Fe/H] = -5.49). J0815+4729 is so carbon-enhanced and oxygen-rich that, in addition to CH and C2 molecular lines, weaker neutral atomic transitions are measurable in its spectrum, providing a unique opportunity to explore the line formation of C I and O I transitions in such an extremely iron-deficient star. Methods. Based on high-resolution spectroscopy and newly constructed atomic models for carbon and oxygen, we present the first investigation to derive carbon and oxygen abundances of this hyper iron-poor star from neutral atomic transitions under non-local thermodynamic equilibrium (NLTE) conditions. Results. The carbon abundance derived from C I lines is A(C)NLTE = 8.08+-0.07, consistent with that inferred from C2 lines (A(C)LTE =7.96 +- 0.08), but around 0.6 dex higher than the value from CH lines (A(C)LTE = 7.47 +- 0.17). Since CH lines are the most commonly-used carbon abundance indicator for extremely metal-poor stars, this result casts doubt on the quantitative interpretation of CH absorption in the most primitive stars. We confirm that J0815+4729 is one of the most carbon-enhanced stars with [Fe/H] <= -4.5 known to date. We have also determined A(O)NLTE as 7.25 +- 0.11, with NLTE corrections of around -0.11 dex. The carbon-to-oxygen ratio of the star is [C/O] = 1.06 +- 0.1.
ER -