Trigonometric Parallaxes for Two Late‐Type Subdwarfs: LSR 1425+71 (sdM8.0) and the Binary LSR 1610−00 (sd?M6pec)

Abstract

Trigonometric parallax astrometry and BVI photometry are presented for two late-type subdwarf candidates, LSR 1425+71 (sdM8.0) and LSR 1610?00 (sd?M6pec). For the former we measure an absolute parallax of 13.37 ? 0.51 mas yielding -->MV = 15.25 ? 0.09. The astrometry for LSR 1610?00 shows that this object is an astrometric binary with a period of 1.66 ? 0.01 yr. The photocentric orbit is derived from the data; it has a moderate eccentricity ( -->e ? 0.44 ? 0.02) and a semimajor axis of 0.29 ? 0.01 AU based on our measured absolute parallax of 31.02 ? 0.26 mas. Our radial velocity measure of ?108.1 ? 1.6 km s -->?1 for LSR 1610?00 at epoch 2006.179, when coupled with the observation of ?95 ? 1 km s -->?1 at epoch 2005.167 by Reiners & Basri, indicates a systemic radial velocity of ?101 ? 1 km s -->?1 for the LSR 1610?00AB pair. The Galactic velocity components for LSR 1425+71 and LSR 1610?00AB are (U, V, W) = (84 ? 6, ?202 ? 13, 66 ? 14) km s?1 and (U, V, W) = (36 ? 2, ?232 ? 2, ?61 ? 2) km s?1, respectively. For both stars, the velocities are characteristic of halo population kinematics. However, modeling shows that both stars have orbits around the Galaxy with high eccentricity that pass remarkably close to the Galactic center. LSR 1425+71 has a luminosity and colors consistent with its metal-poor subdwarf spectral classification, while LSR 1610?00 has a luminosity and most colors indicative of being only mildly metal-poor, plus a uniquely red -->B ? V color. The companion to LSR 1610?00 must be a low-mass, substellar brown dwarf. We speculate on the paradoxical nature of LSR 1610?00 and possible sources of its peculiarities.