Constraining the multiplicity statistics of the coolest brown dwarfs: Binary fraction continues to decrease with spectral type

Clémence Fontanive*, Beth Biller, Mariangela Bonavita, Katelyn Allers

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

Binary statistics of the latest-type T and Y brown dwarfs are sparse and it is unclear whether the trends seen in the multiplicity properties of their more massive counterparts hold for the very coolest brown dwarfs. We present results from a search for substellar and planetary-mass companions to a sample of 12 ultracool T8-Y0 field brown dwarfs with the Hubble Space Telescope/Wide Field Camera 3. We find no evidence for resolved binary companions among our sample down to separations of 0.7-2.5 au. Combining our survey with prior searches, we place some of the first statistically robust constraints to date on the multiplicity properties of the coolest, lowest mass brown dwarfs in the field. Accounting for observational biases and incompleteness, we derive a binary frequency of f = 5.5+-3.3 5.2 per cent for T5-Y0 brown dwarfs at separations of 1.5-1000 au, for an overall binary fraction of ftot = 8 ± 6 per cent. Modelling the projected separation as a lognormal distribution, we find a peak in separation at ρ0 = 2.9-1.4 +0.8 au with a logarithmic width of σ = 0.21+-0.08 0.14. We infer a mass ratio distribution peaking strongly towards unity, with a power-law index of γ = 6.1-2.7 +4.0, reinforcing the significance of the detection of a tighter and higher mass ratio companion population around lower mass primaries. These results are consistent with prior studies and support the idea of a decreasing binary frequency with spectral type in the Galactic field.

Original languageEnglish
Pages (from-to)2702-2727
Number of pages26
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date26 Jun 2018
Publication statusPublished - 11 Sept 2018

Keywords / Materials (for Non-textual outputs)

  • Binaries: visual
  • Brown dwarfs
  • Stars: fundamental parameters
  • Stars: statistics


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