Comets

While it has long been recognized that the primary outgassing product which drives most of the observable phenomena of comets is HO, direct observation of HO to date has been possible in only one comet because of interference by terrestrial water vapor. The primary photodisassociation product, OH, can be detected at both UV and radio wavelengths. Only the radio lines can be observed with velocity resolution finer than the outflow velocity. The only cometary images which are resolved both spatially and in velocity have been obtained with the VLA. However, the scalelength of OH is fairly large so that in the D configuration typically only 1/2 to 1/3 of the total flux is detected. Because the emission is variable from day to day, the missing spacings must be filled in simultaneously with the VLA observations. The use of the E configuration to increase the surface brightness sensitivity and a total power system to measure the spectra with individual antennas would allow images which contain all of the OH flux. This would unambiguously resolve long-standing uncertainties about the degree of symmetry in outgassing of comets.

So far, only the 1667 MHz transition of OH has been imaged because it is not possible to image multiple transitions with adequate frequency resolution and sensitivity. All four 18cm OH lines need to be imaged simultaneously in both sense of polarization to obtain full data about the cometary emission. Because the velocity range to be studied is not large, 8 sets of 64 1.5kHz channels would be adequate. Interference excision would be needed, especially at 1612 MHz and 1720 MHz.