The Separation Distribution of Ultrawide Binaries across Galactic Populations

Abstract

We present an extensive and pure sample of ultrawide binary stars with separations of 0.01 ≲ s/pc ≲ 1 in the solar neighborhood. Using data from Gaia DR2, we define kinematic subpopulations via the systems' tangential velocities, i.e., disk-like (v_(⊥,tot) ≤ 40 km s⁻¹), intermediate (v_(⊥,tot) = 40–85 km s⁻¹), and halo-like (v_(⊥,tot) ≥ 85 km s⁻¹) binaries, presuming that these velocity cuts represent a rough ordering in the binaries' age and metallicity. Through stringent cuts on astrometric precision, we can obtain pure binary samples at such wide separations with thousands of binaries in each sample. Fitting a smoothly broken power law for the separation distribution, we find that its slope at s = 10^(2.5–4) au is the same for all subpopulations, p(s) ∝ s^γ with γ ≈ −1.54. However, the logarithmic slope of p(s) steepens at s ≳ 10⁴ au. We find some evidences that the degree of steepening increases with the binaries' age, with a slope change of only Δγ ≈ 0.5 for disk-like stars, but Δγ ≳ 1 for halo-like stars. This trend is contrary to what might be expected if steepening at wide separations were due to gravitational perturbations by molecular clouds or stars, which would preferentially disrupt disk binaries. If we were to interpret steepening at s ≳ 10 au as a consequence of disruption by MAssive Compact Halo Objects (MACHOs), we would have to invoke a MACHO population inconsistent with other constraints. As a more plausible alternative, we propose a simple model to predict the separation distribution of wide binaries formed in dissolving star clusters. This model generically predicts γ ≃ −1.5 as observed, with steepening at larger separations due to the finite size of binaries' birth clusters.

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