The infrared synthetic spectrum of comet Halley

In order to prepare infrared sounding of comet Halley from the flyby VEGA probes, we have computed the synthetic spectrum between 2.5 and 15 μ of a typical comet at a heliocentric distance of ～ 0.8 AU. The present paper is particularly devoted to the contribution from the cometary gases. For a selection of 20 possible parent molecules, the most efficient excitation process is resonant fluorescence by the solar radiation field. The H₂O, CO, CO₂, CH₄, NH₃ and H₂CO molecules are the best candidates for detection by the IKS infrared spectrometers aboard the VEGA probes. For the water molecule, collisions are too rare to ensure thermal equilibrium in the whole coma ; therefore a limited number of fluorescence lines are expected to be present in the H₂O vibrational bands.