Physical properties of the WASP-44 planetary system from simultaneous multi-colour photometry

TL;DR

This study uses simultaneous multi-colour photometry to refine the physical parameters of the WASP-44 exoplanet system, providing more accurate measurements of the star and planet radii and testing wavelength-dependent radius variations.

Contribution

It presents the first simultaneous multi-band optical and near-infrared photometry of WASP-44 transits, improving system parameter measurements and testing theoretical radius variation predictions.

Findings

Refined star and planet radii by factors of 3 and 4.

Ruled out extreme optical radius variations.

Data insufficient to confirm wavelength-dependent radius changes.

Abstract

We present ground-based broad-band photometry of two transits in the WASP-44 planetary system obtained simultaneously through four optical (Sloan g', r', i', z') and three near-infrared (NIR; J, H, K) filters. We achieved low scatters of 1-2 mmag per observation in the optical bands with a cadence of 48 s, but the NIR-band light curves present much greater scatter. We also observed another transit of WASP-44 b by using a Gunn-r filter and telescope defocussing, with a scatter of 0.37 mmag per point and an observing cadence around 135 s. We used these data to improve measurements of the time of mid-transit and the physical properties of the system. In particular, we improved the radius measurements of the star and planet by factors of 3 and 4, respectively. We find that the radius of WASP-44 b is 1.002 R_Jup, which is slightly smaller than previously thought and differs from that expected for a core-free planet. In addition, with the help of a synthetic spectrum, we investigated the theoretically-predicted variation of the planetary radius as a function of wavelength, covering the range 370-2440 nm. We can rule out extreme variations at optical wavelengths, but unfortunately our data are not precise enough (especially in the NIR bands) to differentiate between the theoretical spectrum and a radius which does not change with wavelength.