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OSSOS finds an Exponential Cutoff in the Size Distribution of the Cold Classical Kuiper belt

Presentation #105.07 in the session Physical Properties of Centaurs & KBOs.

Published onOct 20, 2022
OSSOS finds an Exponential Cutoff in the Size Distribution of the Cold Classical Kuiper belt

The cold main classical Kuiper Belt consists of those non-resonant small solar system bodies with low orbital inclinations and orbital semi-major axes between 42.4 au and 47.7 au. These objects likely formed in situ and the population has experienced minimal collisional modification since formation. Using the Outer Solar System Origins Survey (OSSOS) ensemble sample and characterization, combined with constraints from deeper surveys and supported by evidence from the Minor Planet Center catalog and the Deep Ecliptic Survey, we determine the absolute magnitude Hr distribution of the cold classical belt from Hr ≈ 5 to 12 (roughly diameters of 400 km to 20 km). We conclude that the cold population’s Hr distribution exhibits an exponential cutoff at large sizes. Exponential cutoffs at large sizes are not a natural outcome of pair-wise particle accretion but exponentially tapered power-law size distributions are a feature of numerical simulations of planetesimal formation via a streaming instability. Our observation of an exponential cutoff agrees with previous observational inferences that no large objects (D > 400 km) exist in the cold population. We note that the asymptotic slope of the Hr distribution is consistent with α ~ 0.4 and this asymptotic slope is also found in streaming instability modelling of planetesimal formation and is thus not necessarily associated with achieving collisional equilibrium. Studies of the transneptunian region are providing the parameters that will enable future streaming-instability studies to determine the initial conditions of planetesimal formation in the ≈ 45 au region of the Sun’s protoplanetary disk.

Figure 1

Hr distribution of the cold main Kuiper belt. The grey-dash line represents the distribution of raw detections in the OSSOS++ sample. Red-orange curve (shown as a dotted line for Hr > 8.3 where our sample debiasing is less secure) represents the debiased OSSOS++ sample with the shading indicating the Poisson 95% confidence range. The black lines represent two exponentially tapered functions matched to the debiased OSSOS++ data, with forced large-Hr (small object) asymptotic slopes (dotted: α = 0.5; solid: α = 0.4). For Hr < 9 the two model curves are nearly identical. The debiased OSSOS++ measurements are well matched by the exponential taper form. The boxes represent literature derived estimates. The cyan diamond with uncertainty represents a direct debiasing of detected cold classicals in Bernstein et al. 2004. The black open circles are located where the MPC database indicates a cumulative total of 3 (Hr ~ 5.13) and 11 (Hr ~ 5.51) main-belt cold objects.

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