Allyson Bieryla

THE CANADA-FRANCE ECLIPTIC PLANE SURVEY—FULL DATA RELEASE: THE ORBITAL STRUCTURE OF THE KUIPER BELT*

We report the orbital distribution of the trans-Neptunian objects (TNOs) discovered during the Canada–France Ecliptic Plane Survey (CFEPS), whose discovery phase ran from early 2003 until early 2007. The follow-up observations started just after the first discoveries and extended until late 2009. We obtained characterized observations of 321 deg 2 of sky to depths in the range g ∼ 23.5–24.4 AB mag. We provide a database of 169 TNOs with high-precision dynamical classification and known discovery efficiency. Using this database, we find that the classical belt is a complex region with sub-structures that go beyond the usual splitting of inner (interior to 3:2 mean-motion resonance [MMR]), main (between 3:2 and 2:1 MMR), and outer (exterior to 2:1 MMR). The main classical belt (a = 40–47 AU) needs to be modeled with at least three components: the “hot” component with a wide inclination distribution and two “cold” components (stirred and kernel) with much narrower inclination distributions. The hot component must have a significantly shallower absolute magnitude (Hg) distribution than the other two components. With 95% confidence, there are 8000 +18001600 objects in the main belt with Hg 8.0, of which 50% are from the hot component, 40% from the stirred component, and 10% from the kernel; the hot component’s fraction drops rapidly with increasing Hg. Because of this, the apparent population fractions depend on the depth and ecliptic latitude of a trans-Neptunian survey. The stirred and kernel components are limited to only a portion of the main belt, while we find that the hot component is consistent with a smooth extension throughout the inner, main, and outer regions of the classical belt; in fact, the inner and outer belts are consistent with containing only hot-component objects. The Hg 8.0 TNO population estimates are 400 for the inner belt and 10,000 for the outer belt to within a factor of two (95% confidence). We show how the CFEPS Survey Simulator can be used to compare a cosmogonic model for the orbital element distribution to the real Kuiper Belt.

SYSTEMATIC BIASES IN THE OBSERVED DISTRIBUTION OF KUIPER BELT OBJECT ORBITS

The orbital distribution of Kuiper Belt objects (KBOs) provides important tests of solar system evolution models. However, our understanding of this orbital distribution can be affected by many observational biases. An important but difficult to quantify bias results from tracking selection effects; KBOs are recovered or lost depending on assumptions made about their orbital elements when fitting the initial (short) observational arc. Quantitatively studying the effects and significance of this bias is generally difficult, because only the objects where the assumptions were correct are recovered and thus available to study “the problem,” and because different observers use different assumptions and methods. We have used a sample of 38 KBOs that were discovered and tracked, bias-free, as part of the Canada–France Ecliptic Plane Survey to evaluate the potential for losing objects based on the two most common orbit and ephemeris prediction sources: the Minor Planet Center (MPC) and the Bernstein and Khushalani (BK) orbit fitting code. In both cases, we use early discovery and recovery astrometric measurements of the objects to generate ephemeris predictions that we then compare to later positional measurements; objects that have large differences between the predicted and actual positions would be unlikely to be recovered and are thus considered “lost.” We find systematic differences in the orbit distributions which would result from using the two orbit-fitting procedures. In our sample, the MPC-derived orbit solutions lost slightly fewer objects (five out of 38) due to large ephemeris errors at one year recovery, but the objects which were lost belonged to more “unusual” orbits such as scattering disk objects or objects with semimajor axes interior to the 3:2 resonance. Using the BK code, more objects (seven out of 38) would have been lost due to ephemeris errors, but the lost objects came from a range of orbital regions, primarily the classical belt region. We also compare the accuracy of orbits calculated from one year arcs against orbits calculated from multiple years of observations and find that two-opposition orbits without additional observations acquired at least two months from opposition are unreliable for dynamical modeling.