THE ORBIT OF THE L DWARF + T DWARF SPECTRAL BINARY SDSS J080531.84+481233.0*

Abstract

ABSTRACT SDSS J080531.84+481233.0 is a closely separated, very-low-mass (VLM) binary identified through combined-light spectroscopy and confirmed as an astrometric variable. Here we report four years of radial velocity monitoring observations of the system that reveal significant and periodic variability, confirming the binary nature of the source. We infer an orbital period of 2.02 ± 0.03 years, a semimajor axis of 0.76 au, and an eccenticity 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 (90° ± 19°), and deduce a large system mass ratio (M 2/M 1 = ), substellar components (M 1 = M ☉, M 2 = M ☉), and a relatively old system age (minimum age = Gyr). The measured projected rotational velocity of the primary ( = 34.1 ± 0.7 km s−1) implies that this inactive source is a rapid rotator (period ≲ 3 hr) and a viable system for testing spin–orbit alignment in VLM multiples. Robust model-independent constraints on the component masses may be possible through measurement of the reflex motion of the secondary at wavelengths in which it contributes a greater proportion of the combined luminence, while the system may also be resolvable through sparse-aperature mask interferometry with adaptive optics. 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.