Anomalous Diffuse Interstellar Bands in the Spectrum of Herschel 36. II. Analysis of Radiatively Excited CH+, CH, and DIBs

TL;DR

This study analyzes radiatively excited molecules and diffuse interstellar bands toward Herschel 36, revealing how high radiative temperatures influence DIB profiles and suggesting small polar molecules as DIB carriers.

Contribution

It introduces a model for the effect of radiative excitation on DIBs, linking observed spectral features to molecular properties and environmental conditions.

Findings

High radiative temperatures cause extended tails in DIBs.

Small polar molecules with high beta values explain pronounced ETRs.

Non-polar or larger molecules likely cause DIBs without ETRs.

Abstract

Absorption spectra toward Herschel 36 for the A^1Pi <-- X^1Sigma transitions of CH+ in the J=1 excited rotational level and the A^2Delta <-- X^2Pi transition of CH in the J=3/2 excited fine structure level have been analyzed. These excited levels are above their ground levels by 40.1 K and ~25.7 K and indicate high radiative temperatures of the environment, 14.6 K and 6.7 K, respectively. The effect of the high radiative temperature is more spectacular in some diffuse interstellar bands (DIBs) observed toward Her 36; remarkable extended tails toward red (ETR) were observed. We interpret these ETRs as due to a small decrease of rotational constants upon excitation of excited electronic states. Along with radiative pumping of a great many high-J rotational levels, this causes the ETRs. In order to study this effect quantitatively, we have developed a model calculation in which the effects of collision and radiation are treated simultaneously. The simplest case of linear molecules is considered. It has been found that the ETR is reproduced if the fraction of the variation of the rotational constant, beta = (B'-B)/B, is sufficiently high (3-5%) and the radiative temperature is high (T_r > 50 K). Although modeling for general molecules is beyond the scope of this paper, the results indicate that the prototypical DIBs at 5780.5, 5797.1, and 6613.6 A which show the pronounced ETRs are due to polar molecules sensitive to the radiative excitation. The requirement of high beta favors relatively small molecules with 3-6 heavy atoms. DIBs at 5849.8, 6196.0, and 6379.3 A which do not show the pronounced ETRs are likely due to non-polar molecules or large polar molecules with small beta.