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

## TL;DR

This study uses telescope defocussing to obtain high-precision light curves of the WASP-4 transiting exoplanet system, enabling accurate determination of planetary and stellar properties with minimized observational errors.

## Contribution

It demonstrates the effectiveness of telescope defocussing in achieving high-precision photometry for transiting exoplanets and refines the physical parameters of the WASP-4 system.

## Key findings

- Achieved millimagnitude precision in transit light curves.
- Derived precise mass and radius of WASP-4b.
- Confirmed high mass and surface gravity consistent with short-period exoplanets.

## Abstract

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.