CFHT Information Bulletin Number 37, Semester 97II

Probing the Centaurs and Edgeworth-Kuiper belt object populations, the source of short period comets

- A key project for the MEGACAM camera of the Canada-France-Hawaii 3.6m telescope.

Festou M. C (Observatoire Midi Pyrénées), Duncan M. (Queen University, Kingston, CA), Levison H., & Stern S.A. (Southwest Research Inst., Boulder, CO, USA)

The scientific context

There are two good reasons to expect the presence of comets beyond the orbit of Pluto in a disc-like structure (possibly in a spherical cloud) extending out to a few hundred a.u.:

1 - The Oort cloud is not stable over the age of the solar system, mainly because of the perturbing actions of molecular clouds passing in the sun's neighborhood, on average once every few 100 million years, and of the Galatic tide. As a result, a reservoir of comets is required to replenish the Oort cloud.

2 - The population of the existing short period comets cannot be explained by the injection of comets coming from the Oort cloud. The existence of a large population of comets residing closer to the sun than the Oort cloud comets, but farther away from the planets was thus postulated in the 80's. It received the name of Edgeworth-Kuiper belt (EKB), for the two astronomers who proposed its existence in 1948 and 1951, respectively, for different reason.

The discovery in 1992 by David Jewitt and Jane Luu of a new family of objects, most likely comets beyond the orbit of Pluto, the first members of the EKB, is one of the most important discoveries made in planetary science during the last 20 years. Objects in the belt should be pristine remnants of the primordial nebula from which the solar system formed and it is of paramount importance to gather as much information as we can about their true nature.

Jewitt and Luu's key observation, supported by the discovery of an additional three dozen new members, leaves little doubt about the reality of the belt. However, the properties of this belt, both photometric and astrometric, are far from well characterized and finding more objects to understand the true nature of this family of objects is of prime importance. So far, only bright (large and nearby) objects have been discovered. The dynamical structure of the belt reveals that one fith of the objects are Centaurs, namely, objects that orbit closer to the sun than Neptune and have fairly short dynamical lifetimes, and that the rest are transneptunian objects, that form the inner (r < 35 a.u.) and the outer part (r > 35 a.u.) of the EKB and that have much longer dynamical lifetimes. No object situated at more than 50 a.u. has been observed and this is the result of strong observational bias. Centaurs are objects injected in the inner solar system by the chaotic perturbations that control the EKB object orbits. Most objects in the inner belt are found at resonances with Neptune (like Pluto-Charon).

Scientific objectives

Three outstanding issues remain to be solved:

  1. what is the extent of the belt,
  2. what is the size distribution of the EKB objects and Centaurs, and
  3. what are the photometric properties of the new objects (are they comets or an entirely new family of objects?).
Besides the discovery of new objects (and the determination of their orbits) that will lead, hopefully, to the observation of smaller members, the most basic information one can hope to collect is the objects' size distribution. As the number of known objects will increase, one may anticipate characterizing the size distributions of members of subfamilies such as those located in specific resonances. At the present time, the orbital elements of the objects are not well known and the orbital parameter space has been only partly explored. Photometric properties are largely unknown and the object diameters are derived by assuming surface properties of comet-like objects, i.e. low albedo reflectors. In that respect, Centaurs might be considered to some extent as proxies for the EKB objects from which they derive, provided their orbital evolution has not altered their surface and core properties by exposing them to heat fluxes too large for their most volatile constituents to remain.

Some technical details

Today's technology and observing techniques (a key parameter for this program) correspond to limit magnitudes of about R = 25.3. To improve on Jewitt, Luu and other's recent results, one must improve performance in two areas:

  1. go much deeper than this limit to probe the outer parts of the belt (the larger the object family considered the greater the depth) and
  2. cover a much larger fraction of the orbital parameter space by scanning a much larger fraction of the sky (e.g. higher inclinations).
These two objectives are easier to reach in sites where the average seeing is good (the on-going search programs enjoy average seeing rarely below 1" while the overall CFHT performance, including the optical train, should be 0.8" or lower). The difficulties associated with handling the very large expected data sets and the special treament required to extract the results (the objects are moving at sidereal rates of order 0.5" per hour) should not be underestimated and it is essential to start developing reduction and analysis tools as early as possible. It is also to be remarked that the discovery of an object is based first on the observation of a correct order of magnitude for the object's proper motion (and an associated acceptable magnitude). It is absolutely required, at least for a significant fraction of the newly discovered objects, that a preliminary orbit be determined (i) to establish its true membership, and (ii) to allow its re-observation at the following opposition, which will permit the determination of a reliable orbit used in dynamical studies (and later, physical studies).

Many past programs have failed because of the limited "limiting'' magnitude that was reached (22-23). The UH program on 2-3m class telescopes has been very successful not only because of the quality of the Hawaii site but because the program was conducted during the best nights the observers had access to. Such a selection can be performed only if one has a large number of nights one's disposal.

CFHT Information Bulletin Number 37, Semester 97II
Copyright © 1997, Canada-France-Hawaii Telescope