Transit Photometry of the Core‐dominated Planet HD 149026b

We report g, V, and r photometric time series of HD 149026 spanning predictedtimes of transit of the Saturn-mass planetary companion, which was recentlydiscovered by Sato and collaborators. We present a joint analysis of ourobservations and the previously reported photometry and radial velocities ofthe central star. We refine the estimate of the transit ephemeris to Tc [HJD] =2453527.87455^{+0.00085}_{-0.00091} + N * 2.87598^{+0.00012}_{-0.00017}.Assuming that the star has a radius of 1.45 +/- 0.10 R_Sun and a mass of 1.30+/- 0.10 M_Sun, we estimate the planet radius to be 0.726 +/- 0.064 R_Jup,which implies a mean density of 1.07^{+0.42}_{-0.30} g/cm^3. This density issignificantly greater than that predicted for models which include the effectsof stellar insolation and for which the planet has only a small core of solidmaterial. Thus we confirm that this planet likely contains a large core, andthat the ratio of core mass to total planet mass is more akin to that of Uranusand Neptune than that of either Jupiter or Saturn.Comment: 18 pages, 4 figures, submitted to Ap