Origin of diffuse interstellar lines

Abstract

SINCE their discovery over 40 years ago, the origin of the diffuse interstellar absorption lines has been a mystery although there have been many suggestions put forward. Developments in spectroscopy and astronomy have intensified rather than reduced the mystery. This letter takes into account some of these recent developments and suggests a rather definite (but unconfirmed) identification of the lines. Excellent surveys of the diffuse interstellar lines have been given by Herbig¹ and by Wu², so the many studies of these lines will not be mentioned again. For our purposes, it is sufficient to note that the strongest line which lies at 4,428 Å has a half width of 20 Å, and that 38 weaker lines of varying strengths and widths lie at longer wavelengths. The strength of the lines is strongly correlated with regions in space containing interstellar grains (particularly small grains) and there is strong evidence that all the lines arise from a single species or from closely related species. The only feature in the spectrum lying to the violet of 4,428 Å which seems to be related to the diffuse lines is a continuum in the 2,200 Å region but this must certainly involve much higher electronic states of the absorbing species.