Multiplicity and Optical Excess across the Substellar Boundary in Taurus

We present the results of a high-resolution imaging survey of 22 brown dwarfsand very low mass stars in the nearby (~145 pc) young (~1-2 Myr) low-densitystar-forming region Taurus-Auriga. We obtained images with the Advanced Camerafor Surveys/High Resolution Channel on HST through the F555W (V), F775W (i'),and F850LP (z') filters. This survey confirmed the binarity of MHO-Tau-8 anddiscovered a new candidate binary system, V410-Xray3, resulting in a binaryfraction of 9+/-5% at separations >4 AU. Both binary systems are tight (<10 AU)and they possess mass ratios of 0.75 and 0.46, respectively. The binaryfrequency and separations are consistent with low-mass binary properties in thefield, but the mass ratio of V410-Xray3 is among the lowest known. We find thatthe binary frequency is higher for very low mass stars and high-mass browndwarfs than for lower-mass brown dwarfs, implying either a decline in frequencyor a shift to smaller separations for the lowest mass binaries. Combining theseresults with multiplicity statistics for higher-mass Taurus members suggests agradual decline in binary frequency and separation toward low masses. Theimplication is that the distinct binary properties of very low-mass systems areset during formation and that the formation process is similar to the processwhich creates higher-mass stellar binaries, but occurs on a smaller scale. Weshow that there are no planets or very low-mass brown dwarfs with mass >3 M_Jat projected separation >40 AU orbiting any of the Taurus members in oursample. We identify several BDs with significant (>1 mag) V-band excesses. Theexcesses appear to be correlated with signatures of accretion, and ifattributed to accretion luminosity, may imply mass accretion rates severalorders of magnitude above those inferred from line-profile analyses. (abridged)Comment: Accepted for publication in ApJ; 15 pages, 8 figures in emulateapj forma