High-Resolution Optical and Infrared Observations of Molecules in Comets

Abstract

Molecules in cometary nuclei are considered to contain information on the constituents in the proto-solar nebula at the time of solar system formation. The composition of cometary nuclei has been indirectly derived from spectroscopic observations of molecules in comae. Ambitious space missions to cometary nuclei for landing and sample return are planned to be realized in 4–7 years. Until then, spectroscopic observations remain a powerful way to remotely investigate molecules and ions in cometary comae. Recent bright comets such as Hale-Bopp (C/1995 O1) and Hyakutake (C/1996 B2) have provided detections of new molecules in comae. Observational results from high-resolution optical and infrared spectroscopy of Hale-Bopp, Hyakutake and recent comets, are presented. In particular, symmetric molecules, which have zero electric dipole moment and are therefore not observable in the radio range, have been detected in several recent comets by infrared observations. The new data can also be used to derive accurate abundances of known molecular species. We present the time-dependent excitation processes of the bands of these molecules and discuss the influence of the fluorescence calculations on the derivations of molecular abundances in comets.