Warm Spitzer occultation photometry of WASP-26b at 3.6 and 4.5 mu m

Abstract

We present new warm Spitzer occultation photometry of WASP-26 at 3.6 and 4.5 mu m along with new transit photometry taken in the g, r and i bands. We report the first detection of the occultation of WASP-26b, with occultation depths at 3.6 and 4.5 mu m of 0.001 26 +/- 0.000 13 and 0.001 49 +/- 0.000 16 corresponding to brightness temperatures of 1825 +/- 80 and 1725 +/- 89 K, respectively. We find that the eccentricity of the orbit is consistent with a circular orbit at the 1 sigma level (e = 0.0028(-0.0022)(+0.0097), 3 sigma upper limit e < 0.04). According to the activity-inversion relation of Knutson et al., WASP-26b is predicted to host a thermal inversion. The brightness temperatures deduced from the eclipse depths are consistent with an isothermal atmosphere, although the planet may host a weak thermal inversion given the uncertainties on these values. The data are equally well fitted by atmospheric models with or without a thermal inversion. We find that variation in activity of solar-like stars does not change enough over the time-scales of months or years to change the interpretation of the Knutson et al. activity-inversion relation, provided that the measured activity level is averaged over several nights. Further data are required to fully constrain the thermal structure of the atmosphere because the planet lies very close to the boundary between atmospheres with and without a thermal inversion.