Wv. Dixon et M. Hurwitz, ORFEUS-SPAS-II OBSERVATIONS OF THE ULTRAVIOLET-BRIGHT STAR BARNARD-29IN M13, The Astrophysical journal, 500(1), 1998, pp. 29-32
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.