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

We have analyzed spectra from FUSE, COS, GHRS, and Keck HIRES 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 = 21,400 400 K. Balmer-line fits yield a surface gravity = 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 red giant branch (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 far-UV 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 = 0.45-0.55 and luminosity = 3.26-3.35. Comparison with stellar-evolution models suggests that Barnard 29 evolved from a zero-age horizontal branch star of mass M*/M between 0.50 and 0.55, near the boundary between the extreme and blue horizontal branches.