2009ApJ...706L.205F

We present milliarcsecond-resolution radio very long baseline interferometry (VLBI) observations of the ultracool dwarfs TVLM 513-46546 (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 hr and 3 hr, respectively. Using the inner seven VLBA antennas, we detect unresolved emission from TVLM 513-46546 on a scale of 2.5 mas (∼50 stellar radii), leading to a direct limit on the radio emission brightness temperature of T_B≳ 4x10⁵ K. However, with the higher spatial resolution afforded by the full VLBA we find that the source appears to be marginally resolved at a low signal-to-noise ratio, possibly indicating that TVLM 513-46546 is a binary with a projected separation of ∼1 mas (∼20 stellar radii). Using the 7 hr baseline of our observation, we find no astrometric shift in the position of TVLM 513-46546, with a 3σ limit of about 0.6 mas. This is about three 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 day (≳0.06 AU) orbit, or with a mass of ∼2 M_J in an orbit of ≳0.5 yr (≳0.3 AU).