Detection of a Temperature Inversion in the Broadband Infrared Emission Spectrum of TrES-4

TL;DR

This study measures the thermal emission spectrum of exoplanet TrES-4 using Spitzer data, revealing a thermal inversion layer likely caused by high stellar irradiation, and rules out tidal heating as the cause of its inflated radius.

Contribution

First detection of a temperature inversion in TrES-4's atmosphere through broadband infrared emission measurements, linking atmospheric structure to stellar irradiation levels.

Findings

TrES-4 exhibits a thermal inversion layer in its atmosphere.

The planet's emission spectrum is similar to HD 209458b.

Tidal heating is unlikely to explain TrES-4's inflated radius.

Abstract

We estimate the strength of the bandpass-integrated thermal emission from the extrasolar planet TrES-4 at 3.6, 4.5, 5.8, and 8.0 micron using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We find relative eclipse depths of 0.137 +/- 0.011%, 0.148 +/- 0.016%, 0.261 +/- 0.059%, and 0.318 +/- 0.044% in these four bandpasses, respectively. We also place a 2 sigma upper limit of 0.37% on the depth of the secondary eclipse in the 16 micron IRS peak-up array. These eclipse depths reveal that TrES-4 has an emission spectrum similar to that of HD 209458b, which requires the presence of water emission bands created by an thermal inversion layer high in the atmosphere in order to explain the observed features. TrES-4 receives more radiation from its star than HD 209458b and has a correspondingly higher effective temperature, therefore the presence of a temperature inversion in this planet's atmosphere lends support to the idea that inversions might be correlated with the irradiance received by the planet. We find no evidence for any offset in the timing of the secondary eclipse, and place a 3 sigma upper limit of |ecos(omega)|<0.0058 where e is the planet's orbital eccentricity and omega is the argument of pericenter. From this we conclude that tidal heating from ongoing orbital circulatization is unlikely to be the explanation for TrES-4's inflated radius.