High-Precision Radio and Infrared Astrometry of LSPM J1314+1320AB - I: Parallax, Proper Motions, and Limits on Planets

TL;DR

This study uses multi-epoch radio and infrared astrometry to precisely measure the distance, motion, and orbital parameters of the ultracool dwarf binary LSPM J1314+1320AB, and places limits on potential planetary companions.

Contribution

It provides the first combined radio and infrared astrometric measurements for this binary, improving distance and orbital estimates, and constrains the presence of planets in the system.

Findings

Distance measured at 17.249 pc with high precision.

Secondary star's radio emission is significantly stronger than the primary.

Excludes planets of 0.7 to 10 Jupiter masses with periods of 600 to 10 days.

Abstract

We present multi-epoch astrometric radio observations with the Very Long Baseline Array (VLBA) of the young ultracool-dwarf binary LSPM J1314+1320AB . The radio emission comes from the secondary star. Combining the VLBA data with Keck near-infrared adaptive-optics observations of both components, a full astrometric fit of parallax ($\pi_{\rm abs}=57.975\pm0.045$ mas, corresponding to a distance of $d=17.249\pm0.013$ pc), proper motion ($\mu_{\rm \alpha cos \delta}=-247.99\pm0.10$ mas yr$^{-1}$, $\mu_{\delta}=-183.58\pm0.22$ mas yr$^{-1}$), and orbital motion is obtained. Despite the fact that the two components have nearly identical masses to within $\pm2$%, the secondary's radio emission exceeds that of the primary by a factor of $\gtrsim$30, suggesting a difference in stellar rotation history, which could result in different magnetic field configurations. Alternatively, the emission could be anisotropic and beamed toward us for the secondary but not for the primary. Using only reflex motion, we exclude planets of mass 0.7 to 10 $M_{\rm jup}$ with orbital periods of 600 to 10 days, respectively. Additionally, we use the full orbital solution of the binary to derive an upper limit for the semi-major axis of 0.23 AU for stable planetary orbits within this system. These limits cover a parameter space that is inaccessible with, and complementary to, near-infrared radial velocity surveys of ultracool dwarfs. Our absolute astrometry will constitute an important test for the astrometric calibration of Gaia.