Properties of Diffuse Interstellar Bands at Different Physical Conditions of the ISM

Abstract

Diffuse interstellar bands (DIBs) can trace different conditions of the ISM along the sightline toward the observed stars. A small survey was made in optical wavelengths, producing high resolution and high signal to noise spectra. We present measurements of 19 DIBs' properties in 50 sightlines towards hot stars, distributed at a variety of galactic coordinates and interstellar reddening. Equivalent widths were obtained by fitting asymmetric Gaussian and variable continuum to DIBs. Conditions of the ISM were calculated from 8 atomic and molecular interstellar lines. Two distinctively different types of DIBs were identified, by carefully comparing correlation coefficients between DIBs and reddening and by different behaviour in UV shielded ($\zeta$) and non-shielded ($\sigma$) sightlines. A ratio of DIBs at 5780 Å and~5797 Å proved to be reliable enough to distinguish between two different sightline types. Based on linear relations between DIB equivalent width and reddening for $\sigma$ and $\zeta$ sightlines, we divide DIBs into type {\sc i} (where both linear relations are similar) and type {\sc ii} (where they are significantly different). Linear relation for $\zeta$ type sightlines always show a higher slope and larger x-intercept parameter than the relation for $\sigma$ sightlines. Scatter around the linear relation is reduced after the separation, but it does not vanish completely. This means that UV shielding is the dominant factor of the DIB equivalent width vs.\ reddening relation shape for $\zeta$ sightlines, but in $\sigma$ sightlines other physical parameters play a major role. No similar dependency on gas density, electron density or turbulence was observed. A catalog of all observed interstellar lines is made public.

19 pages, 13 figures, accepted to ApJ

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