The HCN rotational spectral line in Hale-Bopp from observations at the JCMT
Cometary spectroscopy is unusually simple in that most gas lines are optically thin (transparent) most of the time. (A notable exception was provided by Hale-Bopp in which the amount of gas was so huge that some of the lines became optically thick (opaque) near perihelion. But in general, optically thin conditions prevail). This means that every atom and molecule is exposed to the full intensity of sunlight and is visible to the earth-bound observer. The mechanism of excitation depends on the wavelength of the photon.
The spectrum line profile results from the Doppler effect and the motions of gas atoms and molecules in the comet. In a spherical, isotropic coma, the line profile would be symmetric, since equal numbers of atoms approach towards and recede from the observer. In comets we more usually see asymmetric line profiles, which indicate asymmetric outgassing. The outgassing is directed towards the sun because the nucleus day-side is hotter than the night side.
The thermal and expansion motions of gas in the cometary coma are both of order 1 km/sec. High spectral resolution is needed to study spectral line profiles, and this is best attained by submillimeter spectroscopy using telescopes like the (sometimes) fabulous JCMT.
Last updated December 1997 by David Jewitt