Physical properties of cometary nuclei.

摘要

I present results on the physical and thermal properties of six cometary nuclei. This is a significant increase in the number of nuclei for which physical information is available. I have used imaging of the thermal continuum at mid-infrared and radio wavelengths and of the scattered solar continuum at optical wavelengths to study the effective radius, reflectivity, rotation state, and temperature of these objects. Traditionally the nucleus has been difficult to observe owing to an obscuring coma or extreme faintness. I have taken advantage of new mid-infrared array detectors to observe more comets than were possible before; I have also co-developed a technique to separate the coma and nucleus from a comet image. I developed a simple model of the thermal behavior of a cometary nucleus to help interpret the thermal flux measurements; the model is an extension to the Standard Thermal Model for asteroids. We have enough nuclei now to see the first demarcations of the "cometary" region on an albedo-diameter plot; I make a comparison of the cometary nuclei with outer Solar System small bodies and near-Earth asteroids. All of the cometary nuclei studied in this thesis are dark, with geometric albedos below 8%, and have effective diameters of around 3 to 8 km, except for comet Hale-Bopp C/1995 O1, which is in the next order of magnitude higher. I give an extensive discussion of the nuclear characteristics of comets Hale-Bopp and 2P/Encke, the two comets for which I have large datasets.