Ultraviolet and optical observations of OB associations and field stars inthe southwest region of the large Magellanic Cloud

Using ultraviolet photometry from the Ultraviolet Imaging Telescope (UIT) c ombined with photometry and spectroscopy from three ground-based optical da ta sets we have analyzed the stellar content of OB associations and field a reas in and around the regions N79, N81, N83, and N94 in the Large Magellan ic Cloud. In particular, we compare data for the OB association Lucke-Hodge 2 (LH 2) to determine how strongly the initial mass function (IMF) may dep end on different photometric reductions and calibrations. Although the data sets exhibit median photometric differences of up to 30%, the resulting un corrected IMFs are reasonably similar, typically Gamma similar to -1.6 in t he 5-60 M. mass range. M _ However, when we correct for the background cont ribution of field stars, the calculated IMF flattens to Gamma = 1.3 +/- 0.2 (similar to the Salpeter IMF slope). This change underlines the importance of correcting for field star contamination in determinations of the IMF of star formation regions. It is possible that even in the case of an univers al IMF, the variability of the density of background stars could be the dom inant factor creating the differences between calculated IMFs for OB associ ations. We have also combined the UIT data with the most extensive of these ground-based optical data sets-the Magellanic Cloud Photometric Survey-to study the distribution of the candidate O-type stars in the field. We find a significant fraction, roughly half, of the candidate O-type stars are fou nd in field regions, far from any obvious OB associations (in accord with t he 1982 suggestions of Garmany, Conti, & Chiosi for O-type stars in the sol ar neighborhood). These stars are greater than 2' (30 pc) from the boundari es of existing OB associations in the region, which is a distance greater t han most O-type stars with typical dispersion velocities will travel in the ir lifetimes. The origin of these massive field stars (either as runaways, members of low-density star-forming regions, or examples of isolated massiv e star formation) will have to be determined by further observations and an alysis.