Astronomy and Astrophysics – Astrophysics
Scientific paper
Mar 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...423..305p&link_type=abstract
The Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 423, no. 1, p. 305-309
Astronomy and Astrophysics
Astrophysics
11
Asymptotic Giant Branch Stars, Globular Clusters, Horizontal Branch Stars, Stellar Evolution, Ultraviolet Emission, Color-Magnitude Diagram, Far Ultraviolet Radiation, Hertzsprung-Russell Diagram, Spacelab
Scientific paper
Two-color ultraviolet images of the globular cluster NGC 1851 were obtained with the Ultraviolet Imaging Telescope (UIT) during the 1990 December Astro-1 Spacelab mission. A total of 133 stars are detected at 2490 A and 74 stars at 1520 A. An ultraviolet color-magnitude diagram based on the 46 well-photometered stars that appear in both images is presented. Thirty-nine of the 45 horizontal branch (HB) stars fall below the zero-age horizontal branch (ZAHB) of Z = 0.001 and Y = 0.23 predicted by Sweigart by as much as 0.6 mag if the interstellar reddening to the cluster is E(B - V) = 0.02. Supporting ground-based V and B observations, however, show excellent correlation with the same model ZAHB. A newly detected hot subdwarf star with Teff is approximately 26,000 K appears to be an extreme horizontal branch star. The measured flux for this star also falls approximately 0.6 mag below the position on the color-magnitude diagrams (CMD) predicted by parameters derived from ground-based spectroscopy by Landsman. The far-ultraviolet image is dominated by the ultraviolet-bright member star UV 5, which contributes 30% of the total flux at 1520 A. The UIT photometry is consistent with the classification of UV 5 as a post-asymptotic giant branch (PAGB) star. To a limit of 16.5 mag at 1520 A we find no ultraviolet counterpart within 6 minutes of the position of the X-ray source MX 0513-40.
Bohlin Ralph Charles
Cheng Kwang-Ping
Greason Michael R.
Hintzen Paul M. N.
Landsman Wayne B.
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