Follow-up observations of binary ultra-cool dwarfs

TL;DR

This study monitors 14 binary ultra-cool dwarfs over several years using high-resolution imaging to confirm multiplicity, derive orbital parameters, and estimate dynamical masses, aiming to calibrate the mass-luminosity relation for very low-mass stars and brown dwarfs.

Contribution

It provides new astrometric data and orbital parameters for 14 ultra-cool dwarf binaries, enabling future dynamical mass measurements and calibration of the mass-luminosity relation.

Findings

All 14 systems are confirmed as common proper motion pairs.

Six systems have short enough periods for future dynamical mass measurements.

Only about 8% of ultracool dwarfs are binaries suitable for orbital analysis within a reasonable timeframe.

Abstract

Astrometric observations of resolved binaries provide estimates of orbital periods and will eventually lead to measurement of dynamical masses. Only a few very low mass star and brown dwarf masses have been measured to date, and the mass-luminosity relation still needs to be calibrated. We have monitored 14 very low mass multiple systems for several years to confirm their multiplicity and, for those with a short period, derive accurate orbital parameters and dynamical mass estimates. We have used high spatial resolution images obtained at the Paranal, Lick and HST observatories to obtain astrometric and photometric measurements of the multiple systems at several epochs. The targets have periods ranging from 5 to 200 years, and spectral types in the range M7.5 - T5.5. All of our 14 multiple systems are confirmed as common proper motion pairs. One system (2MASSW J0920122+351742) is not resolved in our new images, probably because the discovery images were taken near maximum elongation. Six systems have periods short enough to allow dynamical mass measurements within the next 15 to 20years. We estimate that only 8% of the ultracool dwarfs in the solar neighborhood are binaries with separations large enough to be resolved, and yet periods short enough to derive astrometric orbital fits over a reasonable time frame with current instrumentation. A survey that doubles the number of ultracool dwarfs observed with high angular resolution is called for to discover enough binaries for a first attempt to derive the mass-luminosity relationship for very low-mass stars and brown dwarfs.