Speaker
Description
Oxygen abundances in very and extremely metal-poor (V/EMP) stars provide critical constraints on early massive stars' nucleosynthesis. We present an oxygen abundance analysis for 35 V/EMP stars ($-4.0<\text{[Fe/H]}<-1.5$) using near-infrared $H$-band OH lines from high-resolution Subaru/IRD spectra. To evaluate these atmospheric diagnostics, we compare OH-based abundances against the 3D/NLTE-insensitive [OI] 6300Å line using archival optical spectra. Oxygen abundances are determined via 1D/LTE spectral synthesis.
We identify a strong temperature-dependent discrepancy between the tracers. In cooler red giants ($T_\text{eff}\lesssim4600$K), OH-based abundances are systematically lower than [OI] by 0.05-0.25 dex, while warmer RGs show a higher one, directly consistent with expected 3D atmospheric effects on molecular lines in general. Despite this systematic offset, thanks to numerous near-IR OH lines yields significantly smaller random errors than relying on a single, weak [OI] line.
Leveraging this statistical precision, we derive an empirical calibration to align our more precise 1D/LTE OH-based abundances onto the robust [OI]-based abundance. This correction is not equivalent to canonical 3D/NLTE OH-based abundances, but it substantially reduces scatter in the [O/Fe] trend, bringing our OH-based abundances into better agreement with Galactic chemical evolution models. Additionally, we are ready to extend this study to even lower metallicity RGs with future NIR high-resolution spectroscopic observations.
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