A. M. S. Smith; D. R. Anderson; N. Madhusudhan; J. Southworth; A. C. Cameron; J. Blecic; J. Harrington; C. Hellier; P. F. L. Maxted; D. Pollacco; D. Queloz; B. Smalley; Ahmj Triaud;P. J. Wheatley


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Abbreviated Journal Title

Astron. Astrophys.


planetary systems; planets and satellites: atmospheres; stars:; individual: WASP-24; planets and satellites: individual: WASP-24b; infrared: planetary systems; COLLISION-INDUCED ABSORPTION; STELLAR ATMOSPHERE MODELS; SPITZER-SPACE-TELESCOPE; EXOPLANET HD 189733B; LIMB-DARKENING LAW; EXTRASOLAR PLANET; SURFACE GRAVITIES; SECONDARY ECLIPSE; GIANT PLANETS; HOT JUPITERS; Astronomy & Astrophysics


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 Stromgren u and y bands, with the CAHA 2.2-m telescope. Results. We measure occultation depths of 0.159 +/- 0.013 per cent at 3.6 mu m and 0.202 +/- 0.018 per cent at 4.5 mu m. The corresponding planetary brightness temperatures are 1974 +/- 71 K and 1944 +/- 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 +/- 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.

Journal Title

Astronomy & Astrophysics



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