Chandra X-ray Measurement of Heavy Element Abundances of Wolf-Rayet Stars in the Galactic Center

TL;DR

This study uses deep Chandra X-ray observations to measure heavy element abundances in Wolf-Rayet stars in the Galactic Center, providing insights into star formation history and stellar nucleosynthesis.

Contribution

First X-ray based measurements of multiple heavy element abundances in Galactic Center Wolf-Rayet stars, revealing their metallicity and dust depletion effects.

Findings

Arches and Quintuplet WR stars have similar Si, S, Ar abundances.

Ca and Fe show different levels, likely due to dust depletion.

Metallicity of WR winds is near-solar or sub-solar, consistent with nucleosynthesis models.

Abstract

Elemental abundances hold important information about the star formation history in the Galactic Center. The thermal X-ray spectra of certain stars can provide a robust probe of elemental abundances, mainly through the presence of K-shell emission lines. In this work, based on deep archival {\it Chandra} observations, we obtain X-ray measurements of five heavy elements (Si, S, Ar, Ca and Fe) for three sources in the Arches cluster, one source in the Quintuplet cluster, as well as a field source known as Edd 1, which are all probable WR stars exhibiting a high quality X-ray spectrum. A two-temperature, non-equilibrium ionization plasma model is employed for the spectral fit, taking into account light element compositions characteristic of WR star winds, which is substantially depleted in hydrogen but enriched in nitrogen and/or carbon. It is found that the Arches and Quintuplet WR stars share similar abundances of Si, S, and Ar, while exhibiting distinct Ca and Fe abundances, which may be understood as due to dust depletion of the latter two elements in Quintuplet. The observed near-solar or sub-solar metallicity of the WR star winds can be naturally understood as the result of nucleosynthesis and internal mixing of the parent star, which have a supersolar initial metallicity as expected for the Galactic center in general. Implications of our findings on the origin of the young star clusters and isolated massive stars in the Galactic center, as well as the elemental composition of the accretion flow onto Sgr A*, are addressed.