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 defocussed so the radius of each point spread function was 17 arcsec (44 pixels). This approach minimises 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 M_b = 1.289 {+0.090 -0.090} {+0.039 -0.000} MJup and R_b = 1.371 {+0.032 -0.035} {+0.021 -0.000} RJup, respectively (statistical and systematic uncertainties). These quantities give a surface gravity and density of g_b = 17.03 +0.97 -0.54 m/s2 and rho_b = 0.500 {+0.032 -0.021} {+0.000 -0.008} rhoJup, 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.

💡 Research Summary

This paper presents a high‑precision photometric study of the transiting exoplanet system WASP‑4, employing a deliberately defocused telescope to achieve unprecedented photometric stability. By spreading the stellar point‑spread function (PSF) to a radius of 17 arcseconds (44 pixels), the authors mitigate both random photon noise and systematic errors such as intra‑pixel sensitivity variations, flat‑field imperfections, and atmospheric scintillation. Four complete transits were observed with a 1.54‑m telescope, each yielding light curves with root‑mean‑square (RMS) scatter between 0.60 and 0.88 mmag per data point—significantly better than conventional focused observations on similar equipment.

The raw frames were bias‑, dark‑, and flat‑field corrected, after which custom circular apertures were applied to account for the enlarged PSF and to minimize contamination from nearby stars. The resulting high‑signal‑to‑noise light curves were modeled using the JKTEBOP code, fitting for the planet‑to‑star radius ratio (k), orbital inclination (i), mid‑transit time (T0), and limb‑darkening coefficients. Statistical uncertainties were derived via a combination of Markov Chain Monte Carlo (MCMC) simulations and bootstrap resampling, while systematic uncertainties were assessed by repeating the analysis with several stellar evolution model grids (Yonsei‑Yale, Padova, Teramo, etc.). The stellar effective temperature (T_eff = 5500 K) and metallicity (