THE LOW-MASS ASTROMETRIC BINARY LSR 1610–0040

Abstract

ABSTRACT Even though it was discovered more than a decade ago, LSR 1610−0040 remains an enigma. This object has a peculiar spectrum that exhibits some features typically found in L subdwarfs, and others common in the spectra of more massive M dwarf stars. It is also a binary system with a known astrometric orbital solution. Given the available data, it remains a challenge to reconcile the observed properties of the combined light of LSR 1610−0040AB with current theoretical models of low-mass stars and brown dwarfs. We present the results of a joint fit to both astrometric and radial velocity measurements of this unresolved, low-mass binary. We find that the photocentric orbit has a period P = 633.0 ± 1.7 ?> days, somewhat longer than previous results, eccentricity of e = 0.42 ± 0.03 ?> , and we estimate that the semimajor axis of the orbit of the primary is a 1 ≈ 0.32 AU ?> , consistent with previous results. While a complete characterization of the system is limited by our small number of radial velocity measurements, we establish a likely primary mass range of 0.09–0.10 M ⊙ ?> from photometric and color–magnitude data. For a primary mass in this range, the secondary is constrained to be 0.06–0.075 M ⊙ ?> , making a negligible contribution to the total I-band luminosity. This effectively rules out the possibility of the secondary being a compact object such as an old, low-mass white dwarf. Based on our analysis, we predict a likely angular separation at apoapsis comparable to the resolution limits of current high-resolution imaging systems. Measuring the angular separation of the A and B components would finally enable a full, unambiguous solution for the masses of the components of this system.