A UBVR CCD survey of the Magellanic Clouds

We present photometry and a preliminary interpretation of a UBVR survey of the Large and Small Magellanic Clouds, which covers 14.5 deg(2) and 7.2 deg(2), respectively. This study is aimed at obtaining well-calibrated data on the brighter, massive stars, complementing recent, deeper CCD surveys. Our catalog contains 179,655 LMC and 84,995 SMC stars brighter than V similar to 18.0, and is photometrically complete to U similar to B similar to V similar to 15.7 and R similar to 15.2, although stars in crowded regions are selectively missed. We compare our photometry to that of others, and describe the need for gravity-dependent corrections to our CCD U-band photometry. We provide a preliminary cross-reference between our catalog stars and the stars with existing spectroscopy. We discuss the population of stars seen toward the two Clouds, identifying the features in the color-magnitude diagram, and using existing spectroscopy to help construct H-R diagrams. We derive improved values for the blue-to-red star ratios in the two Clouds, emphasizing the uncertainties involved in this before additional spectroscopy. We compare the relative number of red supergiants and Wolf-Rayet stars in the LMC and SMC with that of other galaxies in the Local Group, demonstrating a very strong, tight trend with metallicity, with the ratio changing by a factor of 160 from the SMC to M31. We also reinvestigate the initial mass function of the massive stars found outside of the OB associations. With the newer data, we find that the initial mass function (IMF) slope of this field population is very steep, with Gamma similar to -4 +/- 0.5, in agreement with our earlier work. This is in sharp contrast to the IMF slope found for the massive stars with OB associations (Gammasimilar to -1.3). Although much more spectroscopy is needed to make this result firm, incompleteness can no longer be invoked as an explanation.