Analysis of Anomalous Diffuse Interstellar Bands in the Spectrum of Herschel 36

Abstract

The extraordinary DIBs observed toward Herschel 36 (Dahlstrom et al. 2013) have been analyzed (Oka et al. 2013). The analysis led us to a new way to classify the carriers of DIBs depending on whether the molecules are polar or non-polar. The pronounced Extended Tails toward Red (ETR) observed for DIBs λ5780.5, λ5797.1, and λ6613.6 are explained as due to radiative excitation of high rotational levels of polar carrier molecules in an environment with high radiative temperature ~90 K. Other DIBs (e.g., λ5849.8, λ6196.0, and λ6379.3) which do not show ETR are likely due to non-polar molecules. Model calculations taking into account the interplay of radiative and collisional effects reproduce the observed ETR using realistic molecular parameters if the radiative temperature is sufficiently high (~90 K). The calculation suggests that the carriers of DIBs with ETR are likely medium size molecules with 3 - 6 heavy atoms unless the radiative temperature is much higher.

Dust evolution, a global view: II. Top-down branching, nanoparticle fragmentation and the mystery of the diffuse interstellar band carriers

“…Low-lying electronically excited states are also characteristic of defective graphene, C 60 anions, PAH cations and polyenes, probably making such species resistant to photo-dissociation in the harsh environment of the diffuse ISM. The above indicates that there may indeed be small species, with significantly less than 40 heavy atoms, that are resistant to photo-dissociation as has been inferred for the carriers of the DIB [32,69,70] and the extended red emission (ERE) [71].

Figure 9.A Jablonski diagram showing the possible transitions for species with low-lying electronically excited states.

…”

“…This object exhibits relatively weak DIBs, sits behind a dark lane with A V ∼4 mag. and N ( HI )=8.9×10 21 , shows (non-thermal) rotationally excited CH and CH + and vibrationally excited H 2 , due to strong IR radiation from an adjacent source but no CN and C 2 [32,69,85,104]. Along this line of sight, the DIBs are broad and many (approx.…”