Shocked forbidden O I 63 micron line emission from the supernova remnant IC 443

Abstract

Observations of the fine-structure emission from the forbidden O I 63 micron line in the SNR IC 443 are presented. It is shown that the emission correlates well with the distribution of line emission from shock-excited molecular hydrogen, which leads to the conclusion that the line is shock-excited. X-ray heating as well as UV-heating from a photodissociation region is ruled out as a possible excitation mechanism for the emission. It is shown that the forbidden O I 63 micron line is an important contributor to the total emission in the IRAS 60 micron band, estimated as approximately 40-75 percent of the total band flux. An attempt to shock model the line emission from IC 443 is made; however, to match the observational evidence, it has to be assumed that the shock is J-type, and that the oxygen chemistry is suppressed so that oxygen remains in atomic form and does not get converted into H2O. However, no theoretical rationale for these assumptions can be provided.