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 \pm 400 K. Balmer-line fits yield a surface gravity \log g = 3.10 \pm 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 ( FG ) 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 .