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

TL;DR

This study used Hubble Space Telescope imaging to search for binary companions around 33 very cold brown dwarfs of spectral types T8 to Y1, finding no companions and setting upper limits on binary frequency.

Contribution

First comprehensive HST survey targeting the multiplicity of the coldest isolated brown dwarfs, providing new photometric data and constraining binary occurrence rates.

Findings

No companions detected within 1-1000 au separation range.

Upper limit of 4.9% binary frequency at 1-sigma confidence.

Binary fraction likely decreases with primary mass and may peak at very tight separations.

Abstract

We present results from a Hubble Space Telescope imaging search for low-mass binary and planetary companions to 33 nearby brown dwarfs with spectral types of T8-Y1. Our survey provides new photometric information for these faint systems, from which we obtained model-derived luminosities, masses and temperatures. Despite achieving a deep sensitivity to faint companions beyond 0.2-0.5'', down to mass ratios of 0.4-0.7 outside ~5 au, we find no companions to our substellar primaries. From our derived survey completeness, we place an upper limit of f < 4.9% at the 1-sigma level (< 13.0% at the 2-sigma level) on the binary frequency of these objects over the separation range 1-1000 au and for mass ratios above q = 0.4. Our results confirm that companions are extremely rare around the lowest-mass and coldest isolated brown dwarfs, continuing the marginal trend of decreasing binary fraction with primary mass observed throughout the stellar and substellar regimes. These findings support the idea that if a significant population of binaries exist around such low-mass objects, it should lie primarily below 2-3 au separations, with a true peak possibly located at even tighter orbital separations for Y dwarfs.