A Comprehensive Study of Infrared OH Prompt Emission in Two Comets. I. Observations and Effectiveg‐Factors

We present high-dispersion infrared spectra of hydroxyl (OH) in comets C/2000 WM 1 (LINEAR) and C/2004 Q2 (Machholz), acquired with the Near Infrared Echelle Spectrograph at the Keck Observatory atop Mauna Kea, Hawaii. Most of these rovibrational transitions result from photodissociative excitation of H 2 O giving rise to OH "prompt" emission. We present calibrated emission efficiencies (equivalent g -factors, measured in OH photons s -1 [H 2 O molecule] -1 ) for more than 20 OH lines sampled in these two comets. The OH transitions analyzed cover a broad range of rotational excitation. This infrared database for OH can be used in two principal ways: (1) as an indirect tool for obtaining water production in comets simultaneously with the production of other parent volatiles, even when direct detections of H 2 O are not available; and (2) as an observational constraint to models predicting the rotational distribution of rovibrationally excited OH produced by water photolysis.