A Significant Overluminosity in the Transiting Brown Dwarf CWW 89Ab

Abstract

We observed eclipses of the transiting brown dwarf CWW 89Ab at 3.6 and 4.5μm usingSpitzer/IRAC. TheCWW 89 binary system is a member of the 3.0±0.25 Gyr old open cluster Ruprecht 147 and is composed of aSun-like primary and an early M-dwarf secondary separated by a projected distance of 25 au. CWW 89Ab has aradius of 0.937±0.042RJand a mass of 36.5±0.1MJ, and is on a 5.3 day orbit about CWW 89A with a non-zero eccentricity ofe=0.19. We strongly detect the eclipses of CWW 89Ab in bothSpitzerchannels asδ3.6=1147±213 ppm andδ4.5=1097±225 ppm after correcting for the dilution from CWW 89B. Afteraccounting for the irradiation that CWW 89Ab receives from its host star, these measurements imply that thebrown dwarf has an internal luminosity of=- ()LLlog4.19 0.14bol. This is 16 times, or 9.3σ, higher thanmodel predictions given the known mass, radius, and age of CWW 89Ab. As we discuss, this overluminosity isexplainable neither by an inaccurate age determination, nor additional stellar heating, nor tidal heating. Instead, wesuggest that the anomalous luminosity of CWW 89Ab is caused by a dayside temperature inversion—though asignificant error in the evolutionary models is also a possibility. Importantly, a temperature inversion would requirea superstellar C/O ratio in CWW 89Ab’s atmosphere. If this is indeed the case, it implies that CWW 89Ab is a36.5MJobject that formed via core accretion processes. Finally, we use our measurement of CWW 89Ab’s orbitaleccentricity, improved via these observations, to constrain the tidal quality factors of the brown dwarf and the hoststar CWW 89A to be>Q10BD4.15and*>Q109, respectively.