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NOAO Newsletter - NOAO Highlights - June 2000 - Number 62

A Deep Ecliptic Survey for Kuiper Belt Objects

Based on a Solicited Contribution from Marc W. Buie

Marc Buie, Robert Millis, and Larry Wasserman (Lowell Observatory); Jim Elliot (MIT); and Mark Wagner (OSU) are using the KPNO Mosaic camera with the Mayall 4-m telescope to identify Kuiper Belt Objects (KBOs, also known as Trans-Neptunian Objects). The goals of their survey are to answer several fundamental scientific questions:

The large area surveyed, combined with the sensitivity of the Mosaic camera, enables the discovery of 15 to 20 new KBOs on each clear night at the 4-m, the raw data necessary to address these questions by acquiring a statistically large sample of KBOs.

The discovery of the Kuiper Belt is an exciting development in planetary astronomy. A vast region beyond Neptune, once thought to be essentially empty except for Pluto and its satellite, Charon, is now known to be populated by ~105 bodies. Most KBOs are asteroidal in size, but a few could be comparable to, or even larger than, Pluto. The 250+ KBOs discovered to date display very interesting dynamical properties; their extremely broad range of observed colors promises remarkable physical properties, as well.

Caption: Positions of all Kuiper Belt Objects known to date are plotted at their discovery epochs with respect to the orbits of the outer planets. The numbers indicate ecliptic longitude. Large-scale angular inhomogeneities reflect the locations of the search fields.

It is now realized that the Kuiper Belt is likely to be analogous to the planet-forming circumstellar dust disks seen around other stars. This tie-in comes from noting that circumstellar dust in some cases must be continuously supplied, presumably from a population of colliding bodies. Our own Kuiper Belt may allow study of the source regions for that dust, while observations around other stars show the end product of collisional grinding. Tying these end-member observations together requires understanding the full dynamical state of the outer solar system through the discovery and follow-up of many more KBOs. In particular, most of the properties of KBOs, and the belt itself, may ultimately be tied to the importance of collisional excitations within the belt.

Caption: Like that for the main belt asteroids, the dynamics of the Kuiper Belt Objects will only be revealed through orbital parameters of an extremely large number of objects. The left-hand panel, which shows parameters for the first 200 asteroids discovered, is comparable to our knowledge of KBOs today. The center panel is the result for the first 2,000 asteroids known, i.e., our view of the main belt in the early 1980s. The right-hand panel shows that different families of asteroids are readily revealed in the main belt after some 50,000 main belt asteroids are studied.

The observations by the Lowell group, combined with those from other observers, have begun to outline the Kuiper Belt population. Analogy with the main belt asteroids illustrates the critical importance of sampling a large number of objects. Twenty years ago it was tempting to think that the dynamical distribution of the main belt asteroids was fully understood. However, the full complexity of the gravitational sculpting in the main belt is only now being revealed through orbits of nearly 50,000 asteroids. An analogous plot with 50,000 KBOs will be just as revealing.

A unique aspect of the Buie et al. survey is the use of a powerful new technique of searching for moving objects. Though they make extensive use of automatic computer source detection, they also use direct visual examination of all data; the human brain is a powerful tool for identifying image motion. In the past, the most commonly used technique for finding moving objects was to blink two or more registered images. In fact, it was this precise technique that was used to discover Pluto. Buie et al. code the images with color. They load the first epoch image into the red plane of an image display. The second epoch image is loaded into the blue and green planes of the display. Once the images are registered, all stationary objects will be displayed in shades of gray. Any object that moves creates a red/cyan image pair that is readily distinguishable from all the gray objects.

The beauty of this technique is that it takes advantage of the built-in image processing abilities of the human brain, rather than relying so much on memory, as is required for traditional blinking. The color-blink method is insensitive to CCD artifacts and blemishes as well as cosmic ray strikes. Most of the time it is not even necessary to perform the normal bias subtraction and flat-fielding steps.

Another distinct advantage is that it only needs two frames; most computer-based search algorithms need three or four frames to function reliably. The reduced number of frames allows much more sky to be searched with the Mosaic camera.

Finding the objects with Mosaic and color-blinking is not enough. To fully understand the discovered objects, their orbits need to be determined. Most of the discovery verification images are taken with WIYN, benefiting greatly from the WIYNQ experiment. In addition, the Lowell group is being helped as well by other investigators using their own telescopes. The process of follow-up and orbit refinement is now the limiting step in the study of the distribution of KBOs.

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