High-precision photometry by telescope defocussing. III. The transiting planetary system WASP-2

TL;DR

This paper demonstrates high-precision ground-based photometry of the WASP-2 transiting exoplanet system using telescope defocussing, achieving record low scatter and deriving detailed physical properties of the system.

Contribution

It introduces a defocussed photometry technique for precise transit observations and accounts for nearby faint star contamination in system modeling.

Findings

Achieved 0.42-0.73 mmag photometric precision for transits.

Derived planetary mass and radius with combined statistical and systematic errors.

Confirmed low equilibrium temperature and absence of atmospheric temperature inversion.

Abstract

We present high-precision photometry of three transits of the extrasolar planetary system WASP-2, obtained by defocussing the telescope, and achieving point-to-point scatters of between 0.42 and 0.73 mmag. These data are modelled using the JKTEBOP code, and taking into account the light from the recently-discovered faint star close to the system. The physical properties of the WASP-2 system are derived using tabulated predictions from five different sets of stellar evolutionary models, allowing both statistical and systematic errorbars to be specified. We find the mass and radius of the planet to be M_b = 0.847 +/- 0.038 +/- 0.024 Mjup and R_b = 1.044 +/- 0.029 +/- 0.015 Rjup. It has a low equilibrium temperature of 1280 +/- 21 K, in agreement with a recent finding that it does not have an atmospheric temperature inversion. The first of our transit datasets has a scatter of only 0.42 mmag with respect to the best-fitting light curve model, which to our knowledge is a record for ground-based observations of a transiting extrasolar planet.