a SUBARU Archival Search for Faint Trans-Neptunian Objects

Abstract

We present the results of a survey for trans-Neptunian objects (TNOs) based on Subaru archival images, originally collected by Sheppard et al. in 2005 as part of a search for irregular satellites of Uranus. The survey region covers 2.8 deg², centered on Uranus and observed near opposition on two adjacent nights. Our survey reaches half its maximum detection efficiency at R = 25.69 ± 0.01. The objects detected correspond to 82 TNOs, five Centaurs, and five irregular satellites. We model the cumulative number of TNOs brighter than a given apparent magnitude with both a single power law (SPL) and a double power law (DPL). The best-fit SPL, with one object per square degree at magnitude R ₀ = 22.6^+0.3 _-0.4 and a slope of α = 0.51^+0.5 _-0.6, is inconsistent with the results of similar searches with shallower limiting magnitudes. The best-fit DPL, with a bright-end slope α₁ = 0.7^+0.2 _-0.1, a faint-end slope α₂ = 0.3^+0.2 _-0.2, a differential number density at R = 23σ₂₃ = 2.0^+0.5 _-0.5, and a magnitude break in the slope at R _eq = 24.3^+0.8 _-0.1, is more likely than the SPL by a Bayes factor of ~26. This is the first survey with sufficient depth and areal coverage to identify the magnitude at which the break occurs without relying on the results of other surveys.

We estimate barycentric distances for the 73 objects that have 24 h arcs; only two have heliocentric distances as large as ~50 AU. We combine the distribution of observed distances with the size distribution that corresponds to a DPL luminosity function to set a tight constraint on the existence of a distant TNO population. We can exclude such a population at 60 AU, with 95% confidence, assuming it has the same size distribution and albedo as the observed TNOs, if it exceeds 8% of mass of the observed TNOs.

Based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.