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R. L. Allen, G. M. Bernstein (U.Mich), R. Malhotra (LPI)
The population of the Kuiper Belt within 50~AU of the Sun has likely been severely depleted by gravitational perturbations from the giant planets, particularly Neptune. The density of Kuiper Belt objects might be expected to be higher just beyond 50 AU, where planetary perturbations are very weak. In 1998 and 1999, we surveyed for Kuiper Belt Objects (KBOs) in 6 fields of the ecliptic (total sky area 1.4 deg2) with limiting magnitudes between R=25.0 and R=25.8. This is deep enough to detect KBOs of diameter \gtrsim 160~km at a distance of 65~AU. We discovered 24 objects. None of these objects, however, is beyond 55~AU.
Our survey places a 95% CL upper limit of \Sigma < 5\,{\rm deg}-2 on the surface density of KBOs larger than ~60~km beyond 55~AU. This can be compared to the surface density of ~\,{\rm deg}-2 of \ge160~km KBOs at distances 30---50~AU determined from this survey and previous shallower surveys. A comparison of the volume densities indicates the region from 55---65~AU has a volume density at most 72 40---50~AU, at a 95 50~AU is excluded in this model-independent estimate, implying that some process or event in the history of the Solar System has truncated the distribution of 200-km planetesimals at ~0~AU. A dense primordial disk could be present beyond 50~AU if it contains only smaller objects, or is sufficiently thin and inclined to have escaped detection in our 6 survey fields.
This work is supported by NASA Planetary Astronomy grant #NAG5-7860.
The author(s) of this abstract have provided an email address for comments about the abstract: rhiannon@umich.edu