ELECTRON DENSITIES IN FILAMENTARY NEBULAE

Abstract

The Cygnus Loop, the brightest parts of which are NGC 6960, NGC6992, and NGC 6995, is a well-known filamentary nebula quite different in structure from typical diffuse or planetary nebulae. There are a very few other filamentary nebulae of the same general type as the Cygnus Loop; a total of five objects of this class, of which the Cygnus Loop is the brightest, have been discussed by Minkowski.1 The second brightest is IC 443, which is apparently considerably more distant than the Cygnus Loop. In the Cygnus Loop no hot star has been found that might be responsible for the excitation and ionization of the nebula ;2 there are some B-type stars in the vicinity of IC443,8 but since the nebula lies in the Milky Way at b = +5°, these may well be projected field stars. Both the Cygnus Loop and IC 443 are radio sources,4 and thus these two objects are different in several ways from the more common types of emission nebulae. Oort has suggested that the energy radiated by the Cygnus Loop is derived ultimately from its expansion energy, the heating mechanism being the collision of the expanding shell with interstellar matter.5 Minkowski has made a thorough observational study of the velocities and line intensities in the Cygnus Loop, and bas concluded that his results agree with Oort's interpretation.1 Pikelner has worked out a somewhat similar, but not identical, theory, in which the energy is derived from the dissipation of shock waves propagating out from the center through a nebula with a fluctuating density distribution.6 Although the main idea of conversion of kinetic energy to heat seems fairly well established, the details of the Cygnus Loop and IC 443 are by no means completely understood, and further observations and interpretations of them and of similar objects are of value. A limited program of observations of the [On] λ 3729 /λ 3726 intensity ratio