The EDIBLES Survey IX: Simulations of the λ6614 DIB Profile Variations: A Surprising Connection with CH+

Abstract

The profiles of several diffuse interstellar bands (DIBs) show substructures that resemble unresolved rotational bands of the electronic transitions of large molecules. Their profiles show clear variations along the lines of sight, probing different physical conditions. Analysis of variations in such profiles can constrain the sizes and geometries of the DIB carriers and the physical conditions of the interstellar environments in which they reside. We investigate the properties of rotational band contours for perpendicular transitions in planar, oblate symmetric top molecules and compare such contours to the observed profile of the λ6614 DIB. We examine the shapes of the profiles as a function of the model parameters: the rotational constant B in the ground state, the relative change in the rotational constant of the excited state ΔB, the Coriolis coupling constant ζ, the rotational excitation temperature T_rot, and line width σ. We determine which parameters can reproduce the overall triple-peak profile of the λ6614 DIB and the variations across different lines of sight. We find that the substructures in the λ6614 DIB can be reproduced with an oblate top with rotational constant B = (2.2 ± 1.8) × 10^–3 cm^–1, ΔB = (−7.2 ± 0.4) × 10^–2%, and Coriolis coupling constant ζ = (2.9 ± 0.1) × 10^–1 cm^–1. Thus, if the λ6614 DIB carrier conforms to an oblate symmetric top geometry, it is most likely to be a ∼54C atom molecule. The profile variations correspond to changes in the rotational temperature from 81 to 92 K. We furthermore find that the intrinsic line width is a key parameter for each sightline and requires a range from 0.14 to 0.21 cm^–1 (or 2.8 to 4.2 km s^–1) across our sample to reproduce the observations. The intrinsic line width of the λ6614 DIB correlates with the width of the CH⁺ lines, suggesting an origin in the same environment. We conclude that the λ6614 DIB carrier resides in the same hot gas at low density that is probed by CH⁺.