Unveiling Oxygen's Past with Subaru/IRD: Oxygen Abundance of Very Metal-Poor Stars

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 ($-4.0 < \text{[Fe/H]} < -1.5$) 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 $\text{[OI]}$ 6300 Å line using archival high-resolution optical spectra. After homogeneously rederiving stellar parameters and 1D/NLTE Fe abundances using \textit{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 ($\Delta\log\epsilon(\text{O})/\Delta T\sim\pm0.25\text{ dex}/\pm100\text{ K}$) compared to the forbidden line. A temperature-dependent discrepancy between the tracers is identified: in cool red giants ($T_\text{eff} \lesssim 4600$ K), OH-based abundances are systematically lower than $\text{[OI]}$-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 $\text{[OI]}$ line. Leveraging this statistical precision, an empirical calibration as a function of $T_\text{eff}$, $\log g$, $\text{[Fe/H]}$, and $\text{[C/Fe]}$ is derived to align the 1D/LTE OH abundances onto the $\text{[OI]}$ scale. Applying this correction substantially reduces the scatter and temperature dependence in the $\text{[O/Fe]}$ versus $\text{[Fe/H]}$ plane and flattens the trend, bringing the results into fairly good agreement with Galactic chemical evolution models.