Prompt ultraviolet emission by OH in Comet Hyakutake

Abstract

Dissociation of H₂O by solar Ly-a produces a moderate amount of OH in highly rotationally excited levels within the A ²∑⁺ electronic state and within the vibrationally excited states of the X ²Π state. Bonchev et al. (2007) have recently reported detection of the prompt emission in the near infrared from the v=1 state of X ²Π. We report here the detection of the ultraviolet prompt emission from the A ²∑⁺ state during the very close approach to Earth of comet Hyakutake in 1996. The relative strengths of the lines are in excellent agreement with predictions based on laboratory studies going back several decades. Comparison of the column densities of H₂O (from the prompt emission using absolute g-factors) and of OH (from the fluorescent emission at nearby wavelengths using absolute g-factors) allows one to probe both the physics of dissociation and the dynamics of the water in comets.