Ultraviolet and Optical Observations of Ob Associations and Field Stars in the Southwest Region of the Large Magellanic Cloud

Using ultraviolet photometry from the Ultraviolet Imaging Telescope (UIT ) combined with photometry and spectroscopy from three ground-based optical datasets we have analyzed the stellar content of OB associations and field areas in and around the regions N 79, N 81, N 83, and N 94 in the Large Magellanic 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 depend on different photometric reductions and calibrations. Although the datasets exhibit median photometric differences of up to 30%, the resulting uncorrected IMFs are reasonably similar, typically Γ ∼ −1.6 in the 5–60 M⊙ mass range. However, when we correct for the background contribution of field stars, the calculated IMF flattens to Γ = −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 universal IMF, the variability of the density of background stars could be the dominant factor creating the differences between calculated IMFs for OB associations. We have also combined the UIT data with the most extensive of these ground-based optical datasets — 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 found in field regions, far from any obvious OB associations [in accord with the suggestions of Garmany, Conti, & Chiosi (1982) for O-type stars in the solar neighborhood]. These stars are greater than 2 arcmin (30 pc) from the boundaries of existing OB associations in the region, which is a distance greater than most O-type stars with typical dispersion velocities will travel in their lifetimes. The origin of these massive field stars (either as runaways, members of low-density star-forming regions, or examples of isolated massive star formation) will have to be determined by further observations and analysis. Subject headings: Magellanic Clouds — open clusters and associations — stars: early type — stars: mass function — ultraviolet: stars — catalogs