The electron temperature distributions and internal kinematics of seven diffuse nebulae

Abstract

A Fabry-Pérot spectrophotometer is used to derive values of the intensity ratio Hα/[Nii] at 98 points in the seven bright diffuse nebulae M8, M20, M16, M17, NGC7000, M42, IC434. The fraction of nitrogen in the singly ionized state is estimated in the different objects, and is found to be sufficiently constant within any one nebula so that the above intensity ratio may be used to derive accurate electron temperature distributions. The position of the peak of the nebular line, its excess non-thermal width, its shape and relative intensity are used to derive kinematical models of these objects.

It is found that values of Hα/[Nii]≃1 are representative of the bright central cores of these nebulae. Temperatures between 7000K and 12000K were derived in the different objects. Although some of this apparent variation is due to the different conditions of excitation in the various nebulae, it is shown that a convincing progression of temperature in M8, M16, M17 is supported by radio recombination line results. The temperature variation within any one object was generally significantly less than 1500K.

No evidence was found for velocities of mass motion at more than twice the speed of sound. Relative radial velocities of generally less than 15 km sec⁻¹ characterized the velocity fields of M8, M20, M16, M42. The velocities in M17 were measured as about 20 km sec⁻¹. Motions in NGC 7000 and IC 434 were much lower (∼5 km sec⁻¹) although here the number of points taken was too small to construct meaningful kinematical models.

It is concluded that the internal motions of radiatively ionizedHii regions of Pop. I will not significantly affect the results of existing surveys for determining the rotation of the galaxy with radial velocities deduced from nebular emission lines.