ORFEUS-SPAS II Observations of the Ultraviolet-bright Star Barnard 29 in M13

Abstract

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 interstellar cutoff at 912 A to ~1200 A at a resolution of ~0.33 A. It shows numerous absorption features, both photospheric and interstellar, but no significant emission other than diffuse emission of local origin. The Kurucz synthetic stellar spectrum 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 consistent with previous results, but the derived metallicity is lower than that of other M13 giants, for which [Fe/H] = -1.60. Using high-resolution 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^{+0.22}_{-0.26} . Again, the Fe abundance is lower than expected. This anomaly may reflect selective condensation of metals onto dust grains at the end of the AGB phase, as has been suggested for some cooler post-AGB stars with peculiar Fe abundances.