Infrared spectroscopy and imaging of the Crab Nebula

Abstract

Infrared spectrophotometry and a forbidden Fe II 1.644 micron emission-line image of the Crab Nebula are presented. Bright Fe II 1.644 micron emission has been detected in three filaments, with values similar to the emission from the supernova remnants IC 443 and the Cygnus Loop. However, unlike the forbidden Fe II emission from the latter objects, which is shock-excited, the emission-line filaments in the Crab are photoionized by the power-law synchrotron emission, which produces an extended, warm photodissociation region where the forbidden Fe II line is collisionally excited. The forbidden Fe II line image shows structure very similar to H-alpha and other emission lines, confirming that the infrared forbidden Fe II line has its origin in the filamentary gas where the optically emitting species are excited.