High-resolution profiles of the diffuse interstellar feature at 5780 A.

Abstract

High-resolution profiles (..delta..lambdaapprox. =0.2 A) were obtained of the diffuse interstellar feature at 5780 A in 18 heavily reddened stars with the Wisconsin echelle spectrograph at the Cassegrain focus of the Mayall 4 m telescope. This feature is, in all cases, asymmetrical with its steep side being toward the blue. On attempting to match theoretical profiles to the observed lambda5780 profile in HD 183143 we find that theoretical profiles for such processes as autoionization, predissociation, or preionization do not provide acceptable fits to the observational data, while good fits can be obtained for the extinction profiles provided by small (rapprox. =750 A), cold grains containing impurities that produce narrow no-phonon absorption lines. If lambda5780 is in fact due to this latter process, then the asymmetry of the feature provides information on the sizes of interstellar grains, while the width provides information on the internal temperatures of grains. Significant differences in the feature asymmetry were noted for several stars, a result that can readily be explained as being due to small differences in the particle size in different galactic regions. Although changes in the width of the feature at 5780 A were noted, it is difficult to decide if the variationsmore » are due to cloud motions, observational errors, or changes in grain temperatures. However, it is possible that the broad weak feature superposed on lambda5780 is due to the same process that produces lambda5780 but in hot (Tapprox. =100-200 K) grains situated near the stars being observed. It is concluded that a careful study of the profiles of the narrow diffuse interstellar features may provide interesting information on the internal temperatures and geometrical characteristics of interstellar particles. (AIP)« less