# Observations of the Ultraviolet-Bright Star Barnard 29 in the Globular Cluster M13 (NGC 6205)

## TL;DR

This study analyzes UV and optical spectra of Barnard 29 in M13 to determine its physical properties and chemical composition, revealing it as a first-generation star with no significant chemical evolution since the RGB phase.

## Contribution

First detailed spectroscopic analysis of Barnard 29 combining multiple datasets to determine its physical parameters and chemical abundances, clarifying its evolutionary status.

## Key findings

- Barnard 29 has T_eff = 21,400 K and log g = 3.10.
- Chemical abundances match first-generation stars in M13.
- Star shows no evidence of third dredge-up or significant chemical evolution.

## Abstract

We have analyzed FUSE, COS, GHRS, and Keck HIRES spectra of the UV-bright star Barnard 29 in M13 (NGC 6205). By comparing the photospheric abundances derived from multiple ionization states of C, N, O, Si, and S, we infer an effective temperature T_eff = 21,400 +/- 400 K. Balmer-line fits yield a surface gravity log g = 3.10 +/- 0.03. We derive photospheric abundances of He, C, N, O, Mg, Al, Si, P, S, Cl, Ar, Ti, Cr, Fe, Ni, and Ge. Barnard 29 exhibits an abundance pattern typical of the first-generation stars in M13, enhanced in oxygen and depleted in aluminum. An underabundance of C and an overabundance of N suggest that the star experienced nonconvective mixing on the RGB. We see no evidence of significant chemical evolution since the star left the RGB; in particular, it did not undergo third dredge-up. Previous workers found that the star's FUV spectra yield an iron abundance about 0.5 dex lower than its optical spectrum, but the iron abundances derived from all of our spectra are consistent with the cluster value. We attribute this difference to our use of model atmospheres without microturbulence, which is ruled out by careful fits to optical absorption features. We derive a mass M_*/M_sun = 0.45 - 0.55 and luminosity log (L_*/L_sun) = 3.26 - 3.35. Comparison with stellar-evolution models suggests that Barnard 29 evolved from a ZAHB star of mass M_*/M_sun between 0.50 and 0.55, near the boundary between the extreme and blue horizontal branches.