2MASS J09393548-2448279: The coldest and least luminous brown dwarf binary known?

Determinations of the luminosity and atmospheric properties of the T8 brown dwarf 2MASS J09393548-2448279 are presented, based on Spitzer IRAC and IRS observations and ground-based astrometry. We find log₁₀ (L _bol/L_⊙)=-5.69 ± 0.03 for this source, comparable to the current low-luminosity record holder 2MASS J04151954-0935066. However, modeling of near-and mid-infrared spectral data indicates an effective temperature of 600 ± 35 K., roughly 100 K cooler than 2M0415. These parameters require a highly inflated radius for 2M0939 (R ≈ 0.13 R _⊙) which cannot be reconciled with brown dwarf structure models. However, if this source is an unresolved, equal-mass binary, then the reduced luminosity of each component (L_bol ≈ 10^-6 L _⊙) can be brought into agreement with the inferred atmospheric parameters for an age of 0.4-12 Gyr and component masses of 0.01-0.05 M _⊙. This hypothesis can be tested through future high-resolution imaging and/or spectroscopic observations.