Electron density and carriers of the diffuse interstellar bands

¹ Institute of Theoretical Physics and Astrophysics, University of Gdańsk, ul. Wita Stwosza 57, 80-952 Gdańsk, Poland e-mail: pg@iftia.univ.gda.pl
² Institute of Experimental Physics, University of Gdańsk, ul. Wita Stwosza 57, 80-952 Gdańsk, Poland e-mail: fizjks@iftia.univ.gda.pl
³ Korea Astronomy and Space Science Institute, 61-1 Whaam-dong, Yuseong-gu, Daejeon, 305-348, Republic of Korea e-mail: gala@boao.re.kr

Received: 10 January 2007
Accepted: 12 March 2007

Abstract

We have used the ionisation equilibrium equation to derive the electron density in interstellar clouds in the direction of 13 stars. A linear relation was found that allows determination of the electron density from the Mg I and Mg II column densities in diffuse clouds. The comparison of a normalised equivalent width of 12 DIBs with the electron density shows that the DIBs equivalent width do not change with electron density varying in the range  cm^-3. Therefore the DIBs carriers (1) can be observed only in one ionisation stage, or (2) the DIBs are arising in cloud regions (eg. cores or cloud coronas) for which we cannot determine the electron density.