25.11

HST Imaging of Distant Comet Nuclei

K. J. Meech, O. R. Hainaut (IfA)

The Wide Field Camera (WFC) on the Hubble Space Telescope has been used to image 4 distant comet nuclei, as part of a long-term program to search for physical differences between short-period comets and Oort cloud comets. WFC data were obtained on comets C/1987 H1 (Shoemaker) and C/1994 K1 (Shoemaker) during cycle 5 (1995 December) and on C/1988 B1 (Shoemaker) and C/1987 F1 (Torres) during cycle 6 (1997 April and May). Observations of C/1983 (Cernis) are planned. Each 3.6 hour integration through the F675W filter was split into 28 dithered exposures, and guided at comet rates. The most difficult part of the analysis is the removal of the cosmic rays because the untrailed comet images are undersampled, and the comet moves with respect to the background objects, making the use of pre-existing routines impossible. We have developed a technique to shift and median combine the images to remove cosmic rays. Because this technique also removes all the stars, artificial comets of known brightness are introduced into the data to robustly determine detection limits. We will discuss these techniques and present the final limiting magnitudes for the comet fields. For the 1995 data, the comet nuclei are fainter than m > 27, resulting in very small limits on their radii, assuming = 0.04. The nucleus size limits for the 1997 data assume a magnitude of 28. These limits are comparable to the average size of the known short-period comet nuclei ( 4.9 km). The implications concerning the activity of these comets will be discussed.

This work is supported in part by a grant from STScI No. GO-06447.01-95A and from NASA, NAG5-4495.