High-precision photometry by telescope defocussing - II. The transiting planetary system WASP-4

We present and analyse light curves of four transits of the Southern hemisphere extrasolar planetary system WASP-4, obtained with a telescope defocused so the radius of each point spread function was 17 arcsec (44 pixels). This approach minimizes both random and systematic errors, allowing us to achieve scatters of between 0.60 and 0.88 mmag per observation over complete transit events. The light curves are augmented by published observations and analysed using the JKTEBOP code. The results of this process are combined with theoretical stellar model predictions to derive the physical properties of the WASP-4 system. We find that the mass and radius of the planet are Mb = 1.289+0.090-0.090+0.039-0.000MJup and Rb = 1.371+0.032-0.035+0.021-0.000RJup, respectively (statistical and systematic uncertainties). These quantities give a surface gravity and density of gb = 17.03+0.97-0.54ms-2 and ρb = 0.500+0.032-0.021+0.000-0.008 ρJup, and fit the trends for short-period extrasolar planets to have relatively high masses and surface gravities. WASP-4 is now one of the best-quantified transiting extrasolar planetary systems, and significant further progress requires improvements to our understanding of the physical properties of low-mass stars.