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

2018/06/25 by Clémence Fontanive, C. Fontanive, Beth Biller +3
Physics and Astronomy · #Astronomy #Astronomy and Astrophysical Research #Astrophysics #Astrophysics and Star Formation Studies #Binary number #Brown dwarf #Geometry #Hubble space telescope #Mass ratio #Multiplicity (mathematics) #Physics #Stars #Stellar classification #Stellar, planetary, and galactic studies #Wide Field Camera 3 #astro-ph.SR

paper · pdf · doi:10.1093/mnras/sty1682

26 pages, 22 figures, accepted for publication in MNRAS

openalex publication_date 2018/06/25 · arxiv created 2018/07/07 · openalex created_date 2018/07/10 · arxiv updated 2018/07/25 · openalex updated_date 2026/08/05

Abstract

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^+5.2_-3.3% for T5-Y0 brown dwarfs at separations of 1.5-1000 AU, for an overall binary fraction of f_tot = 8±6%. Modelling the projected separation as a lognormal distribution, we find a peak in separation at ρ₀ = 2.9^+0.8_-1.4 AU with a logarithmic width of σ = 0.21^+0.14_-0.08. We infer a mass ratio distribution peaking strongly towards unity, with a power law index of γ = 6.1^+4.0_-2.7, 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.

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