Abstract

Stellar Occultations by Kuiper Belt Comets:
The Taiwan-America Occultation Survey (TAOS)

C. Lemme, W.P. Chen, and TAOS team
presented by C. Lemme

Der Vortrag fasst unser derzeitiges Wissen über den Kuipergürtel zusammen und stellt das Problem von Sternbedeckungen durch Kuipergürtelobjekte dar. Der Taiwan-America Occultation Survey (TAOS) nutzt diese Methode, um eine Zählung dieser Objekte durchzuführen, und damit das Wissen über den äußeren Teil unseres Sonnensystems deutlich über die bisherigen Grenzen hinaus zu erweitern.

The relatively short life-time of comets on one side and their large number on the other side have led to the idea, that they originate from a reservoir in the outer region of the solar system, where they are stored in an inactive state. The trajectories of the long period comets (those that take more than 200 years to circle the sun) can be well explained by the existence of the Oort cloud, a sphere of cometary material with a diameter of about 100000 AU. However, the orbits of short-period comets are smaller and tilted only slightly from the orbital plane of Earth, suggesting a different origin.

Edgeworth (1949, MNRAS 109:600) and Kuiper (1951, in Astrophysics, ed. Hynek) predicted the existence of a flattened ring of material beyond the orbit of Neptune which is now known as the Edgeworth-Kuiper Belt. But it was only in 1992 that the first Edgeworth-Kuiper Belt Object (EKO or KBO) in this region was found (Jewitt and Luu 1993, Nature 362:730). Presently there are more than 130 KBOs known.

For several reasons it is interesting to study the Kuiper belt. Being the origin of the short-period comets it is also relevant for the number of Earth-crossing bodies and thus for the frequency of biological extinction events. Containing the most pristine material, this region provides diagnostics on the early phases of Solar System formation. One of the most interesting problems is how far the Kuiper belt extends and to get a reliable estimation on the whole number of objects. Since the objects are quite small and the brightness in reflected sunlight declines r^-4 (r = distance to the sun), the prospects of direct observations are limited.

Because of their faintness, all the KBOs detected by direct imaging with large ground-based telescopes are limited to those with relatively large sizes (larger than about 100 km).

At least 70000 of objects with these sizes are expected to be located within 30 - 50 AU. Until now no objects beyond 50 AU have been observed, but a large number has been inferred.

The Taiwan-America Occultation Survey (TAOS) project will be able to measure the number of these objects down to a typical diameter of cometary nuclei (a few km). The basic idea is to infer the existence of a KBO by its chance occultation of a background star. The virtue of the occultation technique is that the detection is rendered by "borrowing" the light of the bright background star. The occultation rate will provide an albedo independent census of KBOs. The general problem of occultation of stars by KBOs and the basic principle of the TAOS project will be presented. Estimations of the expected detection rate will be given.

Dr. Claudia Lemme, Graduate Institute of Astronomy, National Central University, Taiwan
Address: 32054 Chung-Li, Taoyuan, Taiwan E-Mail: lemme@asiaa.sinica.edu.tw Internet: http://aswww.phy.ncu.edu.tw
Telephone: +886 3 422 7151 and +886 3 422 5968 Fax: +886 3 4262304

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