An observational program will be undertaken to determine the masses of the most massive stars. The work is motivated by significant (factors of two to three) differences in the masses of high mass stars derived from evolutionary models compared with those calculated from atmospheric parameters (i.e, surface gravities from Balmer profiles combined with distance, reddening, and bolometric magnitudes). Thus, either problems remain with theories of high mass star evolution (important/unknown physical complications may have not be included), or with the spectroscopic approach (incorrect surface gravities, luminosities, distances, etc. may be being used). To resolve the question and ascertain the source of the discrepancy, the masses of binary O stars will be directly measured from an analysis of eclipsing double-lined spectroscopic binaries (special attention will be paid to those with periods > 3-5 days which have therefore not interacted). In the first year, young associations and clusters in the Milky Way and Magellanic Clouds will be photometrically surveyed to find eclipsing binaries. Radial velocities for candidates will be obtained in the second year. The third year will be used to find the orbit solutions, and to calculate spectroscopic and evolutionary masses for comparison with the results. This project includes a significant outreach and undergraduate research component. The photometric data (its acquisition for the northern fields and reduction for both northern and southern targets) will be part of a key project of the National Undergraduate Research Observatory. Undergraduate students in Northern Arizona University Research Experiences for Undergraduates program and MIT field camp will help with the spectroscopic observations and orbit solutions.
