High Resolution Imaging of Very Low Mass Spectral Binaries: Three Resolved Systems and Detection of Orbital Motion in an L/T Transition Binary

TL;DR

This study uses high-resolution imaging to resolve three very low mass spectral binary systems, reports orbital motion detection, and suggests many such binaries have small separations that are difficult to resolve directly.

Contribution

First high-resolution imaging resolution of three very low mass spectral binaries, with orbital motion detection and implications for binary separation distributions.

Findings

Resolved three binaries with separations <8 AU.

Detected orbital motion in one binary over 6 years.

Estimated a minimum binary fraction of 47% in the sample.

Abstract

We present high resolution Laser Guide Star Adaptive Optics imaging of 43 late-M, L and T dwarf systems with Keck/NIRC2. These include 17 spectral binary candidates, systems whose spectra suggest the presence of a T dwarf secondary. We resolve three systems: 2MASS J1341$-$3052, SDSS J1511+0607 and SDSS J2052$-$1609; the first two are resolved for the first time. All three have projected separations $<8$ AU and estimated periods of $14-80$ years. We also report a preliminary orbit determination for SDSS J2052$-$1609 based on six epochs of resolved astrometry between 2005$-$2010. Among the 14 unresolved spectral binaries, 5 systems were confirmed binaries but remained unresolved, implying a minimum binary fraction of $47^{+12}_{-11}\%$ for this sample. Our inability to resolve most of the spectral binaries, including the confirmed binaries, supports the hypothesis that a large fraction of very low mass systems have relatively small separations and are missed with direct imaging.