A Significant Overluminosity in the Transiting Brown Dwarf CWW 89Ab

We observed eclipses of the transiting brown dwarf CWW 89Ab at 3.6 and 4.5 μm using Spitzer/IRAC. The CWW 89 binary system is a member of the 3.0 ±0.25 Gyr old open cluster Ruprecht 147 and is composed of a Sun-like primary and an early M-dwarf secondary separated by a projected distance of 25 au. CWW 89Ab has a radius of 0.937 ±0.042 and a mass of 36.5 ±0.1 , and is on a 5.3 day orbit about CWW 89A with a non-zero eccentricity of e = 0.19. We strongly detect the eclipses of CWW 89Ab in both Spitzer channels as δ _3.6 = 1147 ±213 ppm and δ _4.5 = 1097 ±225 ppm after correcting for the dilution from CWW 89B. After accounting for the irradiation that CWW 89Ab receives from its host star, these measurements imply that the brown dwarf has an internal luminosity of . This is 16 times, or 9.3σ, higher than model predictions given the known mass, radius, and age of CWW 89Ab. As we discuss, this overluminosity is explainable neither by an inaccurate age determination, nor additional stellar heating, nor tidal heating. Instead, we suggest that the anomalous luminosity of CWW 89Ab is caused by a dayside temperature inversion - though a significant error in the evolutionary models is also a possibility. Importantly, a temperature inversion would require a superstellar C/O ratio in CWW 89Ab's atmosphere. If this is indeed the case, it implies that CWW 89Ab is a 36.5 object that formed via core accretion processes. Finally, we use our measurement of CWW 89Ab's orbital eccentricity, improved via these observations, to constrain the tidal quality factors of the brown dwarf and the host star CWW 89A to be and , respectively.