Parallax and Luminosity Measurements of an L Subdwarf

We present the first parallax and luminosity measurements for an L subdwarf, the sdL7 2MASS J05325346+8246465. Observations conducted over 3 years by the USNO infrared astrometry program yield an astrometric distance of 26.7 ± 1.2 pc and a proper motion of 2.6241″ ± 0.0018″ yr−1. Combined with broadband spectral and photometric measurements, we determine a luminosity of log Lbol/L☉ = − 4.24 ± 0.06 and Teff = 1730 ± 90 K (the latter assuming an age of 5-10 Gyr), comparable to mid-type L field dwarfs. Comparison of the luminosity of 2MASS J05325346+8246465 to theoretical evolutionary models indicates that its mass is just below the sustained hydrogen-burning limit, and is therefore a brown dwarf. Its kinematics indicate a ~110 Myr, retrograde Galactic orbit that is both eccentric (3 kpc ≲ R≲ 8.5 kpc) and extends well away from the plane (Δ Z = ± 2 kpc), consistent with membership in the inner halo population. The relatively bright J-band magnitude of 2MASS J05325346+8246465 implies significantly reduced opacity in the 1.2 μ m region, consistent with inhibited condensate formation as previously proposed. Its as yet unknown subsolar metallicity remains the primary limitation in constraining its mass; determination of both parameters would provide a powerful test of interior and evolutionary models for low-mass stars and brown dwarfs.