The Orbit of the L dwarf + T dwarf Spectral Binary SDSS J080531.84+481233.0
Burgasser et al
We report four years of radial velocity monitoring observations of SDSS J080531.84+481233.0 that reveal significant and periodic variability, confirming the binary nature of the source. We infer an orbital period of 2.02±0.03 yr, a semi-major axis of 0.76+0.05−0.06 AU, and an eccentricity of 0.46±0.05, consistent with the amplitude of astrometric variability and prior attempts to resolve the system. Folding in constraints based on the spectral types of the components (L4±0.7 and T5.5±1.1), corresponding effective temperatures, and brown dwarf evolutionary models, we further constrain the orbital inclination of this system to be nearly edge-on (90o±19o), and deduce a large system mass ratio (M2/M1 = 0.86+0.10−0.12), substellar components (M1 = 0.057+0.016−0.014 M⊙, M2 = 0.048+0.008−0.010 M⊙), and a relatively old system age (minimum age = 4.0+1.9−1.2 Gyr). The measured projected rotational velocity of the primary (vsini = 34.1±0.7 km/s) implies that this inactive source is a rapid rotator (period ≲ 3 hr) and a viable system for testing spin-orbit alignment in very-low-mass multiples. The combination of well-determined component atmospheric properties and masses near and/or below the hydrogen minimum mass make SDSS J0805+4812AB an important system for future tests of brown dwarf evolutionary models.