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

TL;DR

This paper proposes that the low luminosity of 2M1207B is due to it being a hot protoplanet collision afterglow, challenging previous explanations involving disk obscuration, and provides a new interpretation based on its luminosity and temperature.

Contribution

It introduces the novel hypothesis that 2M1207B is a collision afterglow, offering an alternative to disk-based explanations for its underluminosity and estimating its physical properties accordingly.

Findings

2M1207B's luminosity suggests a small radius and low mass.

The collision afterglow hypothesis explains the observed flux.

Surface gravity should be lower than previous estimates.

Abstract

We introduce an alternative hypothesis to explain the very low luminosity of the cool (L-type) companion to the ~25 M_Jup ~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 (0.8 um) and L' (3.6 um). Mohanty et al. explain this discrepency by suggesting that 2M1207B is an 8 M_Jup object surrounded by an edge-on disk comprised of large dust grains producing 2.5^m 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 < M_Jup. 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).