Emergent Fluxes of Hot Jupiters: Evidence for an Abrupt Rise in Emergent Fluxes under Strong Irradiation

We are studying the emergent fluxes and spectral colors of transiting hot Jupiters, using secondary eclipses from Spitzer and ground-based facilities. To achieve a large and uniform sample, we have re-analyzed 457 secondary eclipses for 121 planets observed by Spitzer at 3.6 and 4.5 microns, including eclipses of 14 planets not previously published. We use eclipse depths and stellar model atmospheres to calculate the emergent flux at the top of the exoplanetary atmospheres. We find evidence for an abrupt rise in the emergent fluxes from the day sides of strongly irradiated hot Jupiters, which occurs in the population at an equilibrium temperature between 1725 and 1990 Kelvins (99-percent confidence limits). We discuss the physical mechanisms that could cause such an effect, including the dissociation of water vapor. We also discuss the spectral colors of hot Jupiters, as a prelude to eclipse spectroscopy by JWST. We focus on the relation between the 3.6 and 4.5 micron Spitzer fluxes, since these are the most numerous measurements. Previous work has found that the 4.5 micron flux becomes relatively more prominent compared to the 3.6 micron flux in the most strongly irradiated planets. This has been interpreted as due to spectral emission caused by atmospheric temperature inversions, and we test that hypothesis using our large sample.