IRAS observations of classical novae. II. Modeling the detections

We report the analysis of IRAS data on 64 classical novae extracted since our initial study of 41 novae (Harrison & Gehrz 1988, Paper I), and conclude that 45 of the 105 novae we investigated were detected by IRAS. A model is presented that compares the IRAS detections of novae with predictions of the strength of the far-infrared forbidden line emission from the gaseous ejecta. The distances are known for 23 of the 45 detected novae so that this sample of 23 can be used to compare the fluxes observed at earth with fluxes predicted by the line emission model. The fluxes from 9 of these 23 novae can be explained by line emission given relatively conservative assumptions about the physical conditions in their shells. Accepting more extreme values for these parameters allows us to attribute three more detections to possible line emission. There are 11 novae whose IRAS detections remain difficult to explain, even when dust emission and free-free emission are considered as alternatives to line emission.