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arXiv · 2606.18862

Revealing {\alpha}-Element's Past with Subaru/IRD: Oxygen Abundance of 35 Very Metal-Poor Stars from Near-IR OH lines

Bakuh Danang Setyo Budi·Wako Aoki·Nicholas Storm·Miho Ishigaki·Tadafumi Matsuno·Teruyuki Hirano·Masayuki Kuzuhara·Jun Nishikawa·Masashi Omiya·Takayuki Kotani·Tomoyuki Kudo·Sebastien Vievard·Motohide Tamura
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Abstract

Oxygen abundances in very and extremely metal-poor (V/EMP) stars provide critical constraints on early massive stars' nucleosynthesis. An Oxygen abundance analysis is presented for 35 V/EMP stars (-4.0<[Fe/H]< -1.5) using near-infrared H-band OH vibro-rotational lines from high-resolution Subaru/IRD spectra. To examine the reliability of these NIR OH lines, the results are compared with the abundances obtained from the 3D/NLTE-insensitive forbidden [OI] 6300{\AA} line using archival high-resolution optical spectra. After homogeneously rederiving stellar parameters and 1D/NLTE Fe abundances using Gaia photo-astrometry and literature optical Fe equivalent width data, oxygen abundance from OH and [OI] lines is determined through 1D/LTE spectral synthesis. A sensitivity analysis confirms that near-IR OH lines are highly sensitive to the adopted temperature compared to the forbidden line. A temperature-dependent discrepancy between the tracers is identified: in cool red giants (Teff <4600 K), OH-based abundances are systematically lower than [OI]-based abundance by 0.05 to 0.25 dex, while warmer red giants show higher OH-based abundances as expected from 3D effects. Despite this systematic offset, the numerous measurable NIR OH lines yield significantly smaller random abundance errors than that of the single, weak [OI] line. Leveraging this statistical precision, an empirical calibration as a function of Teff, log g, [Fe/H], and [C/Fe] is derived to align the 1D/LTE OH abundances onto the [OI] scale. Applying this correction substantially reduces the scatter and temperature dependence in the [O/Fe] versus [Fe/H] plane and flattens the trend, bringing the results into fairly good agreement with Galactic chemical evolution models.

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Bakuh Danang Setyo Budi, Wako Aoki, Nicholas Storm, Miho Ishigaki, Tadafumi Matsuno, Teruyuki Hirano, Masayuki Kuzuhara, Jun Nishikawa, Masashi Omiya, Takayuki Kotani, Tomoyuki Kudo, Sebastien Vievard, Motohide Tamura. 2026-06-17. Revealing {\alpha}-Element's Past with Subaru/IRD: Oxygen Abundance of 35 Very Metal-Poor Stars from Near-IR OH lines. https://doi.org/10.3847/1538-4357%2Fae8ba6

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