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Abstract:

We report on over 3 weeks of continuous ultra-high precision photometry of the bright, nearby, detached (P= 1.02 day; A8m + dK7) eclipsing binary delta Cap. The observations were carried out with the Canadian Micro-satellite MOST during Aug/Sept. 2010. Extensive contemporaneous spectroscopy was secured with the 2-m TSU Automatic Spectroscopic Telescope (AST) and complementary BVR photometry was obtained with ground based telescopes. Delta Cap is an astrophysically important star because it is the nearest and brightest eclipsing binary with a bright pulsating component that can be used (with astereoseismic analyses) to test and calibrate stellar interior and pulsation models. When a pulsating star is a member of an eclipsing binary, the analyses of the it's light and radial velocity observations yield the precise determination of all fundamental orbital and physical properties for the component stars. Moreover the MOST observations during the primary eclipses are a powerful tool for mode identification as portions of the pulsating A-star are blocked from view. Also because delta Cap is nearby and has a reliable parallax (pi (Hipp) = 84.27+/- 0.19 mas), the component stars' luminosities and temperatures are also directly determinable. In addition to its well behaved 1.02-d periodic light variations arising from the eclipses and tidal effects, the MOST light curves clearly show small ( 0.01-0.02 mag) complex light variations. We present the results of the analysis the eclipsing binary light and radial velocity curves using PHOEBE. Also presented are the initial asteroseismic analyses of the A8m component based on the MOST photometry and contemporaneous radial velocity observations. Preliminary models indicate this star is a hybrid gamma Dor-delta Scuti pulsator.