2025PASJ...77..502A

The abundances of five elements, Na, Mg, Al, Si, and Sr, are investigated for 44 very metal-poor stars ($-4.0 < [{\rm Fe/H}] < -1.5$) in the Galactic halo system based on a local thermodynamic equilibrium (LTE) analysis of high-resolution near-infrared spectra obtained with the Infrared Doppler instrument (IRD) on the Subaru Telescope. Mg and Si abundances are determined for all 44 stars. The Si abundances are determined from up to 29 lines, which provide reliable abundance ratios compared to previous results from a few optical lines. The Mg and Si levels of these stars are overabundant relative to iron and are well explained by chemical-evolution models. No significant scatter is found in the abundance ratios of both elements with respect to iron, except for a few outliers. The small scatter of the abundance ratios of these elements provides constraints on the variations of stellar and supernova yields at very low metallicity. Al abundances are determined for 27 stars from near-infrared lines (e.g., 1312 nm), which are expected to be less affected by non-LTE (NLTE) effects than optical resonance lines. The average of the $[{\rm Al/Fe}]$ ratios is close to the solar value, and no dependence on metallicity is found over $-3.0 < [{\rm Fe/H}] < -2.0$. Na abundances are determined for 12 stars; they exhibit solar abundance ratios and no dependence on metallicity. The Sr abundances determined from the Sr II triplet are significantly higher than those from the optical resonance lines obtained by previous studies for our sample. This discrepancy shows a clear dependence on temperature and surface gravity, supporting models that predict large NLTE effects on the near-infrared lines for metal-poor red giants.