The First VLBI Detection of an Ultracool Dwarf: Implications for the Detectability of Sub-stellar Companions

TL;DR

This study uses high-resolution VLBI observations to detect radio emissions from ultracool dwarfs, aiming to identify sub-stellar companions through direct imaging or reflex motion, and sets limits on their presence and properties.

Contribution

First VLBI detection of an ultracool dwarf's radio emission, revealing potential binary nature and constraining the detectability of planetary companions via astrometry.

Findings

Detected unresolved radio emission from TVLM513-46546.

Suggested possible binary nature with ~1 mas separation.

No significant astrometric shift detected, constraining companion masses.

Abstract

We present milliarcsecond-resolution radio very long baseline interferometry (VLBI) observations of the ultracool dwarfs TVLM513-46546 (M8.5) and 2MASS J00361617+1821104 (L3.5) in an attempt to detect sub-stellar companions via direct imaging or reflex motion. Both objects are known radio emitters with strong evidence for periodic emission on timescales of about 2 and 3 hours, respectively. Using the inner seven VLBA antennas, we detect unresolved emission from TVLM513-46546 on a scale of 2.5 mas (~50 stellar radii), leading to a direct limit on the radio emission brightness temperature of T_B > 4x10^5 K. However, with the higher spatial resolution afforded by the full VLBA we find that the source appears to be marginally and asymmetrically resolved at a low S/N ratio, possibly indicating that TVLM513-46546 is a binary with a projected separation of ~1 mas (~20 stellar radii). Using the 7-hour baseline of our observation we find no astrometric shift in the position of TVLM513-46546, with a 3-sigma limit of about 0.6 mas. This is about 3 times larger than expected for an equal mass companion with a few-hour orbital period. Future monitoring of its position on a range of timescales will provide the required astrometric sensitivity to detect a planetary companion with a mass of ~10 M_J in a >15 d (>0.06 AU) orbit, or with a mass of ~2 M_J in an orbit of >0.5 yr (>0.3 AU).