Ultraviolet Color-Magnitude Diagram Studies of Intermediate Age Large Magellanic Cloud Star Clusters.I.NGC 1783

We have imaged the billion-year-old Large Magellanic Cloud star cluster NGC 1783 with the Wide Field/Planetary Camera 2 (WFPC2) in order to obtain broadband photometry at 1600 and 3300 Angstroms. We have identified for the first time the individual stars responsible for the unexpectedly strong ultraviolet light of intermediate-age stellar systems. The behavior of the hot component of aging stellar systems is not well known, and has important implications for the interpretation of light from high-redshift galaxies. Contrary to our expectations, there are far more hot stars present in this cool cluster than can be accounted for by the classical post-asymptotic giant branch. Of particular interest is a group of stars that is strongly concentrated in the core of the cluster. By constructing a relation between the WFPC2 UV colors and stellar effective temperatures, we have found that these stars do not lie in an area of the color-magnitude diagram that is crossed by any theoretical single-star stellar evolution tracks of the cluster's age. These data are insufficient to unambiguously identify the astrophysical nature of the anomalous UV-bright stars; we discuss possibilities for their origin.