Warm Spitzer Photometry of XO-4b, HAT-P-6b and HAT-P-8b

TL;DR

This study analyzes Warm Spitzer observations of three exoplanets during secondary eclipses to determine atmospheric properties and temperature inversions, providing insights into their thermal structures and orbital characteristics.

Contribution

First detailed infrared secondary eclipse measurements for XO-4b, HAT-P-6b, and HAT-P-8b, with updated orbital ephemerides and analysis of temperature inversion hypotheses.

Findings

XO-4b shows a temperature inversion signature.

HAT-P-8b's atmosphere lacks a temperature inversion.

All planets have orbits consistent with being circular.

Abstract

We have analyzed Warm Spitzer/IRAC observations of the secondary eclipses of three planets, XO-4b, HAT-P-6b and HAT-P-8b. We measure secondary eclipse amplitudes at 3.6{\mu}m and 4.5{\mu}m for each target. XO-4b exhibits a stronger eclipse depth at 4.5{\mu}m than at 3.6{\mu}m, which is consistent with the presence of a temperature inversion. HAT-P-8b shows a stronger eclipse amplitude at 3.6{\mu}m, and is best-described by models without a temperature inversion. The eclipse depths of HAT-P-6b can be fitted with models with a small or no temperature inversion. We consider our results in the context of a postulated relationship between stellar activity and temperature inversions and a relationship between irradiation level and planet dayside temperature, as discussed by Knutson et al. (2010) and Cowan & Agol (2011), respectively. Our results are consistent with these hypotheses, but do not significantly strengthen them. To measure accurate secondary eclipse central phases, we require accurate ephemerides. We obtain primary transit observations and supplement them with publicly available observations to update the orbital ephemerides of the three planets. Based on the secondary eclipse timing, we set upper boundaries for e cos(\omega) for HAT-P-6b, HAT-P-8b and XO-4b and find that the values are consistent with circular orbits.