Thermal emission from WASP-24b at 3.6 and 4.5 {\mu}m

TL;DR

This study measures the thermal emission of exoplanet WASP-24b at 3.6 and 4.5 micrometers using Spitzer, analyzes stellar activity, and assesses atmospheric models with or without thermal inversion, highlighting the need for additional data for definitive conclusions.

Contribution

First simultaneous measurements of WASP-24b's thermal emission at 3.6 and 4.5 micrometers, combined with stellar activity analysis and refined system parameters.

Findings

Occultation depths of 0.159% at 3.6 μm and 0.202% at 4.5 μm

Brightness temperatures around 1950 K

No conclusive evidence for atmospheric thermal inversion

Abstract

Aims. We observe occultations of WASP-24b to measure brightness temperatures and to determine whether or not its atmosphere exhibits a thermal inversion (stratosphere). Methods. We observed occultations of WASP-24b at 3.6 and 4.5 {\mu}m using the Spitzer Space Telescope. It has been suggested that there is a correlation between stellar activity and the presence of inversions, so we analysed existing HARPS spectra in order to calculate log R'HK for WASP-24 and thus determine whether or not the star is chromospherically active. We also observed a transit of WASP-24b in the Str\"{o}mgren u and y bands, with the CAHA 2.2-m telescope. Results. We measure occultation depths of 0.159 \pm 0.013 per cent at 3.6 {\mu}m and 0.202 \pm 0.018 per cent at 4.5 {\mu}m. The corresponding planetary brightness temperatures are 1974 \pm 71 K and 1944 \pm 85 K respectively. Atmosphere models with and without a thermal inversion fit the data equally well; we are unable to constrain the presence of an inversion without additional occultation measurements in the near-IR. We find log R'HK = -4.98 \pm 0.12, indicating that WASP-24 is not a chromospherically active star. Our global analysis of new and previously-published data has refined the system parameters, and we find no evidence that the orbit of WASP-24b is non-circular. Conclusions. These results emphasise the importance of complementing Spitzer measurements with observations at shorter wavelengths to gain a full understanding of hot Jupiter atmospheres.