The N2K consortium. II. A transiting hot Saturn around HD 149026 with a large dense core

Doppler measurements from Subaru and Keck have revealed radial velocity variations in the V 8.15, G0 IV star HD 149026 consistent with a Saturn-mass planet in a 2.8766 day orbit. Photometric observations at Fairborn Observatory have detected three complete transit events with depths of 0.003 mag at the predicted times of conjunction. HD 149026 is now the second-brightest star with a transiting extrasolar planet. The mass of the star, based on interpolation of stellar evolutionary models, is 1.3 +/- 0.1 M-circle dot; together with the Doppler amplitude K-1 = 43.3 m s(-1), we derive a planet mass M sin i = 0.36M(J) and orbital radius 0.042 AU. HD 149026 is chromospherically inactive and metal-rich with spectroscopically derived [Fe/H] = +0.36, T-eff 6147 K, log g 4.26, and v sin i 6.0 km s(-1). Based on Teff and the stellar luminosity of 2.72 L-circle dot, we derive a stellar radius of 1.45 R-circle dot. Modeling of the three photometric transits provides an orbital inclination of 85 degrees.3 +/- 1 degrees.0 and ( including the uncertainty in the stellar radius) a planet radius of (0.725 +/- 0.05) R-J. Models for this planet mass and radius suggest the presence of a similar to 67 M-circle dot core composed of elements heavier than hydrogen and helium. This substantial planet core would be difficult to construct by gravitational instability.