MODELING LINEAR MOLECULES AS CARRIERS OF THE λ5797 Å AND λ6613 Å DIFFUSE INTERSTELLAR BANDS

Electronic transitions of polar linear molecules have been modeled and compared to archival high resolution spectra of the diffuse interstellar bands (DIBs) at 5797 and 6613 Å. These two bands are notable for fine structure that has most commonly been attributed to the rotational structure of electronic transitions of gas-phase molecules.a Most strikingly, the 5797 DIB has a sharp, narrow center peak that is characteristic of the Q branch of parallel transitions with non-zero Λ. This work is also motivated by Oka et al.’s analysis of the anomalously extended redward tails seen in certain DIBs toward Herschel 36, which are reminiscent of electronic transitions of polar linear molecules at high radiative temperatures.b The determination of rotational distributions, which includes radiative and collisional effects, is based on the model presented in the earlier work. Thus far, the most promising models are a Π ← Π transition for the 5797 DIB and a ∆ ← Π transition for the 6613 DIB, with the effects of spin-orbit coupling examined in each case. The degree of consistency of these transitions with respect to the anomalous DIBs toward Herschel 36 is also discussed.