2001ApJ...561..272W

We have looked for rotational structure in the sharp λ6196 and λ6379 diffuse interstellar bands (DIBs) at a resolution ∼120,000 in seven stars where the interstellar λ7699 K I line is unresolved. The λ6196 DIB is bell-shaped with a flat core and differs slightly in width from star to star. It is accompanied by a weak DIB at λ6194.7, with which it does not maintain a constant depth ratio. The λ6379 DIB is asymmetric with a sharp double core, but the profile hardly varies between the stars apart from being undetectable for HD 37061. Weak features connect it to the weaker λ6376 DIB, with which it varies in unison. Simple rotational models do not fit the observed profiles of λ6196 and λ6376 at all well because of more prominent branch structure in the models. We achieve an acceptable fit by arbitrarily convolving the modeled profiles with Gaussians (0.2 to 0.3 cm^–1). The Gaussians correspond to an unexpected or anomalous broadening process that cannot be explained by the interstellar velocity distribution or the instrumental point-spread function. The fits give upper limits to the ratio of the rotational temperature to the moment of inertia of the molecular carriers. If the former lies in the range 10-100 K, then the molecules must be large, with moments of inertia comparable with that of the fullerene C₆₀. We present evidence that the anomalous broadening of the rotational profiles is intramolecular in origin, but it is not easily explained by broadening processes previously invoked in connection with the DIBs. We suggest that some other process such as a zero-point vibrational isotope shift may be involved that could be characteristic of many of the narrow bands.