2015ApJ...801....6B

We present an analysis of the diverse spectral profiles observed for the λ6614 diffuse interstellar band (DIB). This includes the anomalous Herschel 36 profile, exhibiting a prominent, broad red tail, and the typically observed narrow profiles, exhibiting much narrower, but noticeable red tails. This study was motivated by the inability of previous rotational contour modeling work to account for the narrow and broad red tails. We show that the full profiles, for all the observations, can consistently be modeled as a superposition of two overlapping DIBs, with peaks at 6613.6 and 6614.2 Å. Each DIB is plausibly fit using a prolate, parallel band, symmetric top spectral contour model. For λ6613.6, there are small differences in the rotational constants, less than 1%, between the upper and lower transition states; whereas, for λ6614.2, the differences are much larger, ∼ -5%. These results are consistent with λ6614.2 being the source of the narrow and broad red tails. The fit residuals are shown to be consistent with contributions from overlapping spectra, attributed to closely spaced vibrational sequences, originating from low frequency vibrations. We suggest that such sequences may be the source of the anomalous broadening needed to obtain good spectral fits to narrow DIB profiles. We discuss how λ6614.2 and the other Herschel 36 extended red tail DIBs help bridge the association gap between the narrow, absorption DIBs and the even broader and more redshifted emission features observed for the Red Rectangle. Finally, the broader implications of this study, in the context of identifying DIB molecular carriers, are discussed.