Unveiling Oxygen Past with Subaru/IRD: Oxygen Abundance of Very Metal-Poor Stars Bakuh Danang Setyo Budi (1,2*), Wako Aoki (1,2), Nick Storm (3), Miho Ishigaki (2), Tadafumi Matsuno (4)
(1) Astronomical Science Program, The Graduate University for Advanced Studies, SOKENDAI, Mitaka, Tokyo, Japan
*danang.budi[at]grad.nao.ac.jp
(2) National Astronomical Observatory of Japan, Mitaka, Tokyo, Japan
(3) MPIA, Heidelberg, Germany
(4) Heidelberg University
Abstract
Oxygen, the third most abundant element in the universe, is mainly synthesized in massive stars by carbon-alpha capture reaction. Determining oxygen abundances in very metal-poor (VMP) stars ([Fe/H] < -3.0 ) is very challenging due to the weak and telluric-contaminated [OI] 630.03 nm forbidden line, the most reliable O indicator. It is widely known for O overabundance in VMP stars ([O/Fe] ~ 0.5), identifying stars with lower oxygen content requires alternative tracers. This study assesses the reliability of near-infrared OH vibration-rotation lines as oxygen indicators where the [OI] line is inaccessible. We analyse high-resolution Subaru/IRD spectra (R = 70,000), covering first-overtone OH lines in the H-band (1500-1700 nm), to derive oxygen abundances for 35 VMP stars using 1D/LTE model atmospheres. Comparing the O abundance from near-IR OH lines with optical [OI] line reveals OH abundances are very sensitive to the input stellar parameters, especially effective temperature and surface gravity. Significant scatter founds in the [O(OH)/Fe] versus [Fe/H] relation, notably in cooler or hotter red giants, where OH abundances are much lower or higher than [OI] abundances. In contrast, [O([OI])/Fe] values show less scatter, aligning with Galactic Chemical Evolution models. Future work will include 3D/LTE effects to enhance [OI] line abundance accuracy and address discrepancies between indicators. This study highlights limitations of OH lines for oxygen abundance determination with 1D/LTE methods. In the end of presentation, we will give a brief report progress on another project of Radial Velocity monitoring of VMP spectroscopic binaries with Seimei/GAOES-RV, with aims to determine chemical abundance pattern including lithium and neutron-capture element abundance for each component in the metal-poor binaries. This research may give some insight of potential studies with the new 3.8m telescope at Timau National Observatory.
Keywords: Stellar abundances - Nucleosynthesis - Galaxy chemical evolution - Population III stars
