Abundances of UV bright stars in globular clusters. I. ROA 5701 in omega Centauri and Barnard 29 in M 13

TL;DR

This study analyzes UV bright stars in globular clusters using high-resolution spectra to measure their elemental abundances, revealing significant iron deficiencies likely caused by dust-related processes, and highlighting their post-AGB evolutionary status.

Contribution

It provides the first detailed UV and optical spectral analysis of these stars, demonstrating iron depletion patterns consistent with gas-dust separation in their atmospheres.

Findings

Both stars are post-AGB stars with significant iron deficiency.

Barnard 29's abundance pattern matches M 13 giants except for stronger iron depletion.

ROA 5701 shows the lowest iron abundance among omega Cen stars analyzed.

Abstract

Two UV brights stars in globular clusters, ROA 5701 (omega Cen) and Barnard 29 (M 13) are analysed from high-resolution UV and optical spectra. The main aim is the measurement of iron abundances from UV spectra obtained with the HST-GHRS. In addition atmospheric parameters and abundances for He, C, N, O, and Si are derived from optical spectra (ESO CASPEC) for ROA 5701 or taken from literature for Barnard 29. Both stars are found to be post-asymptotic giant branch stars. Surprisingly, their iron abundances lie significantly below the cluster abundance in both cases. Barnard 29 lies 0.5 dex below the iron abundance derived for giant stars in M 13 and the iron abundance of ROA 5701 is the lowest of any star in omega Cen analysed so far. Barnard 29 shows the same abundance pattern as the red giant stars in M 13, except for its stronger iron deficiency. The iron depletion could be explained by gas-dust separation in the AGB progenitor's atmosphere, if iron condensed into dust grains which were then removed from the atmosphere by a radiatively driven wind. The interpretation of the abundance pattern for ROA 5701 is hampered by the star-to-star abundance variations seen in omega Cen, but its abundance pattern appears to be consistent with the gas-dust separation scenario.