Transit Photometry of the Core‐dominated Planet HD 149026b

Abstract

We report g, V, and r photometric time series of HD 149026 spanning predicted times of transit of the Saturn-mass planetary companion, which was recently discovered by Sato and collaborators. We present a joint analysis of our observations and the previously reported photometry and radial velocities of the 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 RSun and a mass of 1.30 0.10 MSun, we estimate the planet radius to be 0.726 0.064 RJup, which implies a mean density of 1.07 +0.42-0.30 g/cm 3. This density is significantly greater than that predicted for models which include the effects of stellar insolation and for which the planet has only a small core of solid material. Thus we confirm that this planet likely contains a large core, and that the ratio of core mass to total planet mass is more akin to that of Uranus and Neptune than that of either Jupiter or Saturn.