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