A Review of Radio Interferometric Imaging of Comets

Abstract

ABSTRACT. We present an overview of cometary observations carried out with the VLA. These include OH imaging observations, studies of other molecules and searches for continuum emission. Primarily, we discuss the OH observations obtained of Comet Halley, a periodic comet, and Comet Wilson, a "new" comet. The emission from Comet Halley is confined to a region a few times 10 km. The emission from Comet Wilson shows sporadic blobs at large distances (~ 10 km from the center), which vary abruptly in both position and velocity. The generally accepted picture of excitation by UV pumping in a spherical outflow of cometary gas explains the coarse phenomenology, but other effects seem to cause significant modulations of the OH emission. We suggest that for Comet Halley, collisional effects near the edge of the quenching region, which is coincidently the approximate distance to the cometopause, may be important, and that for Comet Wilson, outgassing from cometesimals ejected from the nucleus may be significant. We further show that asymmetric line profiles are more common than previously thought. Lengthening integration times and lowering the velocity resolution results in a smooth, round shape for the comet's brightness distribution. In addition, spectral profiles become broader and more symmetric after long integration times and after integration over larger spatial regions. In addition to the OH observations we summarize the HoCO detection experiments for comets Halley and Machholz. I n t r o d u c t i o nIt is generally accepted t h a t water ice is the primary constituent of a cometary nucleus and sublimated H 2 0 flows radially away as the comet approaches the sun. It is very difficult to observe H 2 0 directly from the ground. T w o probable detections have been m a d e at 22 GHz (Jackson, Clark and Donn, 1976;Altenhoff et al, 1983). Snyder (1982) has summarized a *Alfred P. Sloan Fellow.