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

We introduce an alternative hypothesis to explain the very low luminosity of the cool (L-type) companion to the ∼25 MJup ∼8 Myr-old brown dwarf 2M1207A. Recently, Mohanty et al. (2007) found that effective temperature estimates for 2M1207B (1600± 100 K) are grossly inconsistent with its lying on the same isochrone as the primary, being a factor of ∼10 underluminous at all bands between I (0.8μm) and L (3.6μm). Mohanty et al. explain this discrepency by suggesting that 2M1207B is an 8 MJup object surrounded by an edge-on disk comprised of large dust grains producing 2.5 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 < MJup. We suggest that 2M1207B is a hot protoplanet collision afterglow 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 predicted by Mohanty et al. (2007). Subject headings: circumstellar matter — planetary systems : formation— planetary systems : protoplanetary disks — stars: individual (2MASSW J1207334393254) — stars: low-mass, brown dwarfs — stars: pre-main-sequence