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We present results from a Hubble Space Telescope imaging search for low-massbinary and planetary companions to 33 nearby brown dwarfs with spectral typesof T8-Y1. Our survey provides new photometric information for these faintsystems, from which we obtained model-derived luminosities, masses andtemperatures. Despite achieving a deep sensitivity to faint companions beyond0.2-0.5'', down to mass ratios of 0.4-0.7 outside ~5 au, we find no companionsto our substellar primaries. From our derived survey completeness, we place anupper limit of f < 4.9% at the 1-sigma level (< 13.0% at the 2-sigma level) onthe binary frequency of these objects over the separation range 1-1000 au andfor mass ratios above q = 0.4. Our results confirm that companions areextremely rare around the lowest-mass and coldest isolated brown dwarfs,continuing the marginal trend of decreasing binary fraction with primary massobserved throughout the stellar and substellar regimes. These findings supportthe idea that if a significant population of binaries exist around suchlow-mass objects, it should lie primarily below 2-3 au separations, with a truepeak possibly located at even tighter orbital separations for Y dwarfs.

An HST survey of 33 T8 to Y1 brown dwarfs: NIR photometry and multiplicity of the coldest isolated objects