Carbon Abundances of Metal-Poor Star Determined from the C I 1.068-1.069 micron Lines

TL;DR

This study uses non-LTE analysis of C I lines at 1.068-1.069 microns to determine carbon abundances in 46 metal-poor stars, extending the method's applicability to very low metallicities and comparing results with previous techniques.

Contribution

It demonstrates the viability of using strong C I lines for carbon abundance measurements in extremely metal-poor stars, providing insights into non-LTE effects and their impact on abundance determinations.

Findings

C I lines are visible down to [Fe/H] ~ -3.7.

Results align with previous studies' trends of [C/Fe] vs. [Fe/H].

C I-based abundances tend to be higher than CH-based ones in very metal-poor giants.

Abstract

A non-LTE analysis of C I lines at 1.068-1.069 micron was carried out for selected 46 halo/disk stars covering a wide metallicity range (-3.7 <[Fe/H]< +0.3), based on the spectral data collected with IRCS+AO188 of the Subaru Telescope, in order to examine whether and how these strong neutral carbon lines of multiplet 1 can be exploited for establishing stellar carbon abundances, especially for very metal-poor stars where CH molecular lines have been commonly used. These C I lines were confirmed to be clearly visible for all stars down to [Fe/H] ~ -3.7, from which C abundances could be successfully determined. The resulting [C/Fe] vs. [Fe/H] diagram revealed almost the same trend established from previous studies. When the results for individual stars are compared with the published data collected from various literature, while a reasonable agreement is seen as a whole, a tendency is observed that our abundances are appreciably higher than those from CH lines especially for very metal-poor giants of low gravity. Since the abundances of these C I lines are subject to rather large non-LTE corrections (typically by several tenths dex) whose importance progressively grows as the metallicity is lowered, attention should be paid to how the collisional rates (especially due to neutral hydrogen) are treated in non-LTE calculations.