Program: GN-2010B-Q-92

Massive stars via mass loss and supernova explosions are the primary driving mechanism for the chemical enrichment and evolution of galaxies. Eclipsing binary (EB) systems are powerful tools to test stellar structure and evolution models of massive stars. We propose follow-up GMOS spectroscopy of the most massive EB system ever reported with a measured apsidal motion: M31V_J00442326+4127082. This important 19th-mag eccentric 5.75-day EB (placed in M31) provides a unique opportunity (not known in the Galaxy or the Magellanic Clouds) to probe the internal structure of massive stars and test the predictions of theoretical interior models. The requested observations will be combined with previous photometry and GMOS spectroscopy to secure precise stellar physical properties (masses, radii, temperatures and luminosities), and with apsidal motion, determine the internal structure of stars with masses ~45 Msun. As an added bonus, the proposed field of view of the target contains three additional O/B EB systems with components >20 Msun that provide reliable fundamental properties of massive stars as well as yielding a more reliable M31 distance. The Gemini-North GMOS observations are vital to obtain the required radial velocity curves that will yield direct, precise measures of the fundamental properties of massive O-type stars.