Discovery of a Young Substellar Companion in Chamaeleon

During an imaging survey of the Chamaeleon I star-forming region with the ACSaboard HST, we have discovered a candidate substellar companion to the younglow-mass star CHXR 73 (~2 Myr, M=0.35 Msun). We measure a projected separationof 1.3+/-0.03" for the companion, CHXR 73 B, which corresponds to 210 AU at thedistance of the cluster. A near-infrared spectrum of this source obtained withCorMASS at the Magellan II telescope exhibits strong steam absorption thatconfirms its late-type nature (>=M9.5). In addition, the gravity-sensitiveshapes 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 CHXR73A and B are unrelated members of Chamaeleon I is ~0.001. We estimate themasses of CHXR 73 B and other known substellar companions in young clusterswith a method that is consistent with the dynamical measurements of theeclipsing binary brown dwarf 2M 0535-0546, which consists of a comparison ofthe bolometric luminosities of the companions to the values predicted by theevolutionary models of Chabrier & coworkers and Burrows & coworkers. We arriveat mass estimates of 0.003-0.004, 0.024+/-0.012, 0.011+0.01/-0.003, and0.012+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 massivecompanions to stars outside the solar system that have been detected in directimages, and may have masses that are within the range observed for extrasolarplanetary companions (M<=0.015 Msun). However, because these two objects (aswell as 2M 1207-3932 B) probably did not form within circumstellar disks aroundtheir primaries, we suggest that they should be viewed as brown dwarfcompanions rather than planets.Comment: Astrophysical Journal, in pres