Homogeneous studies of transiting extrasolar planets – I. Light-curve analyses

Abstract

Abstract I present a homogeneous analysis of the transit light curves of 14 well-observed transiting extrasolar planets. The light curves are modelled using jktebop, random errors are measured using Monte Carlo simulations and the effects of correlated noise are included using a residual-permutation algorithm. The importance of stellar limb darkening on the light-curve solutions and parameter uncertainties is investigated using five different limb darkening laws and including different numbers of coefficients as fitted parameters. The linear limb darkening law cannot adequately fit the Hubble Space Telescope (HST) photometry of HD 209458, but the other four laws give very similar results to each other for all transit light curves. In most cases fixing the limb darkening coefficients at theoretically predicted values does not bias the results, but does cause the error estimates to be too small. The available theoretical limb darkening coefficients clearly disagree with empirical values measured from the HST light curves of HD 209458; limb darkening must be included as fitted parameters when analysing high-quality light curves. In most cases the results of my analysis agree with the values found by other authors, but the uncertainties I find can be significantly larger (by factors of up to 3). Despite these greater uncertainty estimates, the analyses of sets of independent light curves for both HD 189733 and HD 209458 lead to results which do not agree with each other. This discrepancy is worst for the ratio of the radii (6.7σ for HD 189733 and 3.7σ for HD 209458), which depends primarily on the depth of the transit. It is therefore not due to the analysis method but is present in the light curves. These underlying systematic errors cannot be detected from the reduced data alone unless at least three independent light curves are available for an individual planetary system. The surface gravities of transiting extrasolar planets are known to be correlated with their orbital periods. New surface gravity values, calculated from the light-curve results and the stellar spectroscopic orbits, show that this correlation is still present. New high-precision light curves are needed for HD 149026, OGLE-TR-10, OGLE-TR-56, OGLE-TR-132 and GJ 436, and new radial velocity curves for the XO-1, WASP-1, WASP-2 and the OGLE (Optical Gravitational Lensing Experiment) planetary systems.