The Environment Dependence of Diffuse Interstellar Bands

Abstract

Diffuse interstellar bands (DIBs) show different behaviour according to their environment. For some DIBs a decrease in strength is observed in the direction of dense clouds and towards H II regions. We have studied these dependencies in Orion for two DIBs in particular, the narrow well defined DIB at 6196 Å and the strong DIB at 6284 Å. A key characteristic of interstellar environment is the physical state of hydrogen in the line of sight, which can be molecular (H₂), atomic (H I), or ionized (H II). Usually, there is a range of physical conditions along the line of sight. We find that there is a good correlation of DIB strength with atomic hydrogen column density. The correlation of DIB strength with reddening (which is proportional to all matter in line of sight) breaks down for E _(B-V) < 0.08 where molecular hydrogen is absent and some hydrogen is thought to be ionized. Aparently, DIBs (both λ6196 and λ6284) are absent or much weaker in the H II medium. In lines of sight towards M42, where hydrogen is mainly atomic, λ6196 is weak but λ6284 is as strong (or stronger) as elsewhere. Finally, DIBs are indeed weaker per unit reddening (or unit extinction) for lines of sight that penetrate a dense cloud, but λ6196 tends to weaken less quickly with increasing reddening than λ6284. We interpret the results in terms of the influence of UV photons, and conclude that λ6196 is both ionized and double ionized (destroyed) by less energetic photons than λ6284. This is consistent with a molecular origin for the DIBs.