An Improbable Solution to the Underluminosity of 2M1207B: A Hot Protoplanet Collision Afterglow

Abstract

We introduce an alternative hypothesis to explain the very low luminosity of the cool (L-type) companion to the ∼25 , ∼8 Myr old brown dwarf 2M1207A. Recently, Mohanty et al. found that effective temperature estimates M Jup for 2M1207B (K) are grossly inconsistent with its lying on the same isochrone as the primary, being 1600 100 a factor of ∼10 underluminous at all bands between I (0.8 mm) and (3.6 mm). Mohanty et al. explain this L discrepancy by suggesting that 2M1207B is an 8 object surrounded by an edge-on disk comprised of large M Jup dust grains producing 2.5 mag of achromatic extinction. We offer an alternative explanation: the apparent flux reflects the actual source luminosity. Given the temperature, we infer a small radius (∼49,000 km), and for a range of plausible densities, we estimate a mass !. We suggest that 2M1207B is a hot protoplanet collision afterglow M Jup and show that the radiative timescale for such an object is 1% the age of the system. If our hypothesis is correct, the surface gravity of 2M1207B should be an order of magnitude lower than that predicted by Mohanty et al.