  During an imaging survey of the Chamaeleon I star-forming region with the ACS
aboard HST, we have discovered a candidate substellar companion to the young
low-mass star CHXR 73 (~2 Myr, M=0.35 Msun). We measure a projected separation
of 1.3+/-0.03" for the companion, CHXR 73 B, which corresponds to 210 AU at the
distance of the cluster. A near-infrared spectrum of this source obtained with
CorMASS at the Magellan II telescope exhibits strong steam absorption that
confirms its late-type nature (>=M9.5). In addition, the gravity-sensitive
shapes of the H- and K-band continua demonstrate that CHXR 73 B is a young,
pre-main-sequence object rather than a field star. The probability that CHXR
73A and B are unrelated members of Chamaeleon I is ~0.001. We estimate the
masses of CHXR 73 B and other known substellar companions in young clusters
with a method that is consistent with the dynamical measurements of the
eclipsing binary brown dwarf 2M 0535-0546, which consists of a comparison of
the bolometric luminosities of the companions to the values predicted by the
evolutionary models of Chabrier & coworkers and Burrows & coworkers. We arrive
at mass estimates of 0.003-0.004, 0.024+/-0.012, 0.011+0.01/-0.003, and
0.012+0.008/-0.005 Msun for 2M 1207-3932 B, GQ Lup B, DH Tau B, and CHXR 73 B,
respectively. Thus, DH Tau B and CHXR 73 B appear to be the least massive
companions to stars outside the solar system that have been detected in direct
images, and may have masses that are within the range observed for extrasolar
planetary companions (M<=0.015 Msun). However, because these two objects (as
well as 2M 1207-3932 B) probably did not form within circumstellar disks around
their primaries, we suggest that they should be viewed as brown dwarf
companions rather than planets.
