PI: Margaret Hanson, University of Cincinnati, firstname.lastname@example.org
Address: Department of Physics, PO Box 210011, Cincinnati, OH 45221-0011, USA
CoI: Philip Massey, Lowell Observatory
CoI: Nick Melena, University of Arizona
CoI: Laura Penny, College of Charleston
Title: Massive binaries in the Local Group's most massive young cluster
Abstract: The moderately reddened (A_V=11) Galactic cluster Westerlund 1 is thought by many to be the most massive (M ~ 10^5 M_\odot) young cluster in the Local Group. As such, it represents the largest collection of very massive stars in a nearby, small field and an exceptionally rich target for a search for massive binaries. In 2007 we undertook a 6 month photometric study involving nearly 100 independent queue observations of the field to derive light curves for 2000 stars. From this we have identified 14 candidate massive \it eclipsing binaries and are requesting follow up photometry for phase confirmation and then spectroscopic observations on the brightest 10 candidates. Spectroscopic classifications are available for 8 of the candidates and all were previously identified as very massive stars. All binary candidates selected for the spectroscopic program are reasonably bright and their light curves indicate periods of less than 5 days. We are requesting 100 I- and J-band queue images spread over 6 months (2.5 nights equivalent telescope time) in early 2010 to confirm these periods and to allow us to predict critical orbital phase points. We will follow up in June 2010 with 6 nights on SOAR to obtain H-band spectra for these 10 binaries to measure orbital velocities at orbital quadrature (maximum velocity separation) to derive full orbital solutions and stellar masses. This will help us better understand the evolutionary status of WR stars, constrain the mass-luminosity relationship for massive stars and provide an independent (isochrone- based) age of the high mass constituents of Westerlund 1.
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