Multiplicity and Optical Excess across the Substellar Boundary in Taurus

Abstract

We present the results of a high-resolution imaging survey of 22 brown dwarfs and very low mass stars in the nearby (~145 pc) young (~1-2 Myr) low-density star-forming region Taurus-Auriga. We obtained images with the Advanced Camera for Surveys High Resolution Channel on HST through the F555W (V), F775W (i'), and F850LP (z') filters. This survey confirmed the binarity of [BHS98] MHO 8 (hereafter MHO-Tau-8) and discovered a new candidate binary system, [SS94] V410 X-ray 3 (hereafter V410-Xray3), resulting in a binary fraction of 9^(+10)_(-3)% at separations >4 AU. Both binary systems are tight (<10 AU), and they possess mass ratios of 0.75 and 0.46, respectively. The binary frequency and separations are consistent with low-mass binary properties in the field, but the mass ratio of V410-Xray3 is among the lowest known. We find that the binary frequency is higher for very low mass stars and high-mass brown dwarfs than for lower mass brown dwarfs, implying either a decline in frequency or a shift to smaller separations for the lowest mass binaries. Combining these results with multiplicity statistics for higher mass Taurus members suggests a gradual decline in binary frequency and separation toward low masses. The implication is that the distinct binary properties of very low-mass systems are set during formation and that the formation process is similar to the process that creates higher mass stellar binaries, but occurs on a smaller scale. We combine the survey detection limits with models for planetary-mass objects to show that there are no planets or very low-mass brown dwarfs with mass >3M_J at projected separation >40 AU orbiting any of the Taurus members in our sample, implying that planetary-mass companions at wide separations are rare. Finally, based on fits to the optical and near-infrared spectral energy distributions, we identify several BDs with significant (≳1 mag) V-band excesses. The excesses appear to be correlated with signatures of accretion and if attributed to accretion luminosity may imply mass accretion rates several orders of magnitude above those inferred from line profile analyses.

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