Warm Spitzer Occultation Photometry of WASP-26b at 3.6{\mu}m and 4.5{\mu}m

TL;DR

This study reports the first detection of WASP-26b's occultation at 3.6 and 4.5 micrometers using warm Spitzer, analyzing its atmospheric properties and orbit, and discusses implications for thermal inversions.

Contribution

First detection of WASP-26b occultation at infrared wavelengths and analysis of its atmospheric temperature and orbital eccentricity.

Findings

Occultation depths at 3.6μm and 4.5μm are 0.00126 and 0.00149.

Brightness temperatures are approximately 1825K and 1725K.

Orbit is consistent with being circular, with e < 0.04.

Abstract

We present new warm Spitzer occultation photometry of WASP-26 at 3.6{\mu}m 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{\mu}m and 4.5{\mu}m of 0.00126 +/- 0.00013 and 0.00149 +/- 0.00016 corresponding to brightness temperatures of 1825+/-80K and 1725+/-89K, respectively. We find that the eccentricity of the orbit is consistent with a circular orbit at the 1{\sigma} level with a 3{\sigma} upper limit of e < 0.04. According to the activity-inversion relation of Knutson et al. (2010), WASP-26b is predicted to host a thermal inversion. The brightness temperatures deduced from the eclipse depths are consistent with an isothermal atmosphere, although it is within the uncertainties that the planet may host a weak thermal inversion. The data are equally well fit 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. (2010) 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.