Thermal emission from WASP-24b at 3.6 and 4.5 $m

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 $m using the Spitzer Space Telescope. It has been suggested that there is a correlation between stellar activity and the presence of inversions, so we analysed existing HARPS spectra in order to calculate log R'$_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 Strömgren u and y bands, with the CAHA 2.2-m telescope.
Results: We measure occultation depths of 0.159 plusmn 0.013 per cent at 3.6 $m and 0.202 plusmn 0.018 per cent at 4.5 $m. The corresponding planetary brightness temperatures are 1974 plusmn 71 K and 1944 plusmn 85 K respectively. Atmosphere models with and without a thermal inversion fit the data equally well; we are unable to constrain the presence of an inversion without additional occultation measurements in the near-IR. We find log R'$_HK$ = -4.98 plusmn 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.