Ionization and excitation of H2O+ in comet Kohoutek (1973f)

Intensities have been measured for the H2O+ A2A1→X2B1 bands in tail spectra of comet Kohoutek within a region of approx. 104 km of the nucleus. Accurate H2O photodecomposition rates have been calculated and H2O ionization rates for solar-wind interactions (electron impact and charge exchange) and gas-phase reactions estimated. The computed lifetime against photodecomposition for cometary H2O molecules at 1 AU is 25 hours. It was found that solar Lα and the solar ultraviolet flux in the region 1450 to 1860 A contribute equally and essentially control the photodissociation of H2O molecules; while the solar Lyman continuum determines the photoionization rate. Excitation rates and relative band intensities for H2O+ ions were computed for the processes H2O(X1A1)+hν→H2O+(A2A1)+e and H2O+(X2B1)+hν→H2O+(A2A1). The dominant excitation mechanism was shown to be resonance fluorescence. A dominant ionization process could not be determined. Column densities for H2O+ ions in the tail at projected nuclear distances, rho approx. =104 km, were found to be approx.1010/sup + - 1/ to approx.1011/sup + - 1/ cm-2 for heliocentric distances in the range 0.5 to 0.9 AU; while and upper limit for the H2O+ column density on the sunward side of the coma, > or approx. = 5 x 103 km, was estimated to be > or approx. 109/sup + - 1/ cm-2