PRECISE ESTIMATES OF THE PHYSICAL PARAMETERS FOR THE EXOPLANET SYSTEM HD 17156 ENABLED BY HUBBLE SPACE TELESCOPE FINE GUIDANCE SENSOR TRANSIT AND ASTEROSEISMIC OBSERVATIONS

We present observations of three distinct transits of HD 17156b obtained with the Fine Guidance Sensors on board the Hubble Space Telescope. We analyzed both the transit photometry and previously published radial velocities to find the planet-star radius ratio Rp /R* = 0.07454 ± 0.00035, inclination i = 86.49+0.24-0.20 deg, and scaled semimajor axis a/R* = 23.19+0.32-0.27. This last value translates directly to a mean stellar density determination ρ* = 0.522+0.021-0.018 g cm-3. Analysis of asteroseismology observations by the companion paper of Gilliland et al. provides a consistent but significantly refined measurement of ρ* = 0.5308 ± 0.0040. We compare stellar isochrones to this density estimate and find M* = 1.275 ± 0.018 Msun and a stellar age of 3.37+0.20-0.47 Gyr. Using this estimate of M* and incorporating the density constraint from asteroseismology, we model both the photometry and published radial velocities to estimate the planet radius Rp = 1.0870 ± 0.0066 RJ and the stellar radius R* = 1.5007 ± 0.0076 Rsun. The planet radius is larger than that found in previous studies and consistent with theoretical models of a solar-composition gas giant of the same mass and equilibrium temperature. For the three transits, we determine the times of mid-transit to a precision of 6.2 s, 7.6 s, and 6.9 s, and the transit times for HD 17156 do not show any significant departures from a constant period. The joint analysis of transit photometry and asteroseismology presages similar studies that will be enabled by the NASA Kepler Mission.