Detection of Planetary Emission from the Exoplanet Tres-2 Using Spitzer/IRAC
We present here the results of our observations of TrES-2 using the Infrared Array Camera on Spitzer. We monitored this transiting system during two secondary eclipses, when the planetary emission is blocked by the star. The resulting decrease in flux is 0.127% ± 0.021%, 0.230% ± 0.024%, 0.199% ± 0.054%, and 0.359% ± 0.060% at 3.6 μm, 4.5 μm, 5.8 μm, and 8.0 μm, respectively. We show that three of these flux contrasts are well fit by a blackbody spectrum with T_(eff) = 1500 K, as well as by a more detailed model spectrum of a planetary atmosphere. The observed planet-to-star flux ratios in all four IRAC channels can be explained by models with and without a thermal inversion in the atmosphere of TrES-2, although with different atmospheric chemistry. Based on the assumption of thermochemical equilibrium, the chemical composition of the inversion model seems more plausible, making it a more favorable scenario. TrES-2 also falls in the category of highly irradiated planets which have been theoretically predicted to exhibit thermal inversions. However, more observations at infrared and visible wavelengths would be needed to confirm a thermal inversion in this system. Furthermore, we find that the times of the secondary eclipses are consistent with previously published times of transit and the expectation from a circular orbit. This implies that TrES-2 most likely has a circular orbit, and thus does not obtain additional thermal energy from tidal dissipation of a non-zero orbital eccentricity, a proposed explanation for the large radius of this planet.
Additional Information© 2010 American Astronomical Society. Print publication: Issue 2 (2010 February 20); received 2009 September 16; accepted for publication 2010 January 2; published 2010 February 1. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with the National Aeronautics and Space Administration (NASA). This research was supported in part by NASA under grant NNG05GJ29G (issued through the Origins of Solar Systems Program) and also by an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center (administered by Oak Ridge Associated Universities through a contract with NASA). Facilities: Spitzer (IRAC)
Published - ODonovan2010p7149Astrophys_J.pdf
Accepted Version - 0909.3073.pdf