ORFEUS-SPAS-II OBSERVATIONS OF THE ULTRAVIOLET-BRIGHT STAR BARNARD-29IN M13

The UV-bright star Barnard 29 in the globular cluster M13 was observed with the Berkeley spectrometer on the ORFEUS-SPAS II mission in 1996 November-December. The resulting spectrum extends from the interstella r cutoff at 912 Angstrom to similar to 1200 Angstrom at a resolution o f similar to 0.33 Angstrom. It shows numerous absorption features, bot h photospheric and interstellar, but no significant emission other tha n diffuse emission of local origin. The Kurucz synthetic stellar spect rum that best fits the data has T-eff = 21,000 K, log g = 3.0, and [M/ H] = -2.0. This effective temperature and surface gravity are consiste nt with previous results, but the derived metallicity is lower than th at of other M13 giants, for which [Fe/H] = -1.60. Using high-resolutio n synthetic spectra, we determine the photospheric abundances of C, S, and Fe, which are unobservable in the optical. We find log epsilon(C) = 6.15 +/- 0.10, log epsilon(S) = 5.34 +/- 0.50, and log epsilon(Fe) = 5.30(-0.26)(+0.22). Again, the Fe abundance is lower than expected. This anomaly may reflect selective condensation of metals onto dust gr ains at the end of the AGB phase, as has been suggested for some coole r post-AGB stars with peculiar Fe abundances.