The Study of Comets at Radio Wavelengths

Abstract

Radio observations of comets complement studies at other wavelengths as well as providing certain kinds of unique information. Studies of continuum emission may probe the thermal emission of the nucleus and large-size dust particles. Spectroscopic observations of gas in the coma allow searches for parent molecular species that have no signatures in the optical spectral range. The 18-cm wavelength spectral lines of the OH radical now are relatively easy to detect, and observations of them permit long-term and short-term monitoring of the cometary gaseous output. Moreover, with the unique spectral resolution of radio techniques, aspects of the kinematics of the coma may be also studied, such as the gas expansion velocity and the anisotropy of gas production from the nucleus. In this review, we present recent results of cometary radio observations, and discuss what may be learned from such studies in the future.

A Review of Radio Interferometric Imaging of Comets

“…A similar conclusion can be reached when comparing the detections of Comet Halley at mm wavelengths by Altenhoff et al (1986) with the VLA non-detection at 2 cm (Hoban and Baum, 1987). Other possibilities to explain the continuum emission are reviewed by Crovisier and Schloerb (1990).…”

“…Although OH has been monitored regularly with single element radio telescopes (Crovisier and Schloerb, 1990), high resolution images can only be obtained with help of interferometers. As shown in Table 1, several comets have been imaged over the past few years; the results will be discussed in Sections 4 and 5.…”

Molecular Abundances in Comets

“…Whether or not the observed HCN abundance can explain all the CN radicals observed in the visible is still a debated question. From first evaluations this is not the case (Crovisier and Schloerb, 1991).…”

“…First claimed to be detected in comet Kohoutek 1973 XII (Huebner et al 1974), hydrogen cyanide was definitely identified at millimetre wavelengths in P/Halley (Crovisier and Schloerb 1991 and references therein) and several subsequent comets (Bockelée-Morvan et al 1990, 1994cSchloerb and Ge, 1992). Several rotational transitions (J 1-0, 3-2 and 4-3) have now been observed.…”

“…Other potential CN-parents (HC3N, CH3CN...) were unsuccessfully searched for at radio wavelengths (Bockelée-Morvan et al 1987;Crovisier and Schloerb, 1991;Crovisier et al 1993), and an early radio detection of CH3CN (Ulich and Conklin, 1974) could never be confirmed. The possibility that CN could be released from organic grains has been invoked (see the discussion in Greenberg and Shalabiea, 1994).…”