Fluorescence Efficiencies of Monodeuterio-Methane in Comets: Toward the Determination of the Deuterium/Hydrogen Ratio in Methane

We investigate the infrared (IR) emission from cometary monodeuterio-methane (CH3D), which is a key molecule in determining the deuterium-to-hydrogen ratio (D/H) in methane. It is shown that the IR emission is caused mainly by solar IR fluorescence and that the vibrational ground state of CH3D can be assumed to be in the LTE condition based on our discussion of the excitation conditions in the cometary inner coma. The line ratio between the strongest emission line [RR3(3)] in the CH3D v4 band and the R0 line in the CH4 v3 band is estimated to be about 0.06 for a rotational temperature of 100 K in the vibrational ground state when the D/H ratio in methane is 0.1. An attempt to detect CH3D in the IR region is strongly recommended, using a large telescope with a high-dispersion spectrograph in order to determine the D/H ratio in cometary methane.