Diffuse Interstellar Bands and the Ultraviolet Extinction Curves: The Missing Link Revisited

Abstract

A large number of interstellar absorption features at ∼4000 Å-1.8 μm, known as the “diffuse interstellar bands” (DIBs), remains unidentified. Most recent works relate them to large polycyclic aromatic hydrocarbon (PAH) molecules or ultrasmall carbonaceous grains which are also thought to be responsible for the 2175 \mathringA extinction bump and/or the far-ultraviolet (UV) extinction rise at {λ }^-1> 5.9 μ {{{m}}}^-1. Therefore, one might expect some relation between UV extinction and DIBs. Such a relationship, if established, could put important constraints on the carrier of DIBs. Over the past four decades, whether DIBs are related to the shape of the UV extinction curves has been extensively investigated. However, the results are often inconsistent, partly due to the inconsistencies in characterizing UV extinction. Here we re-examine the connection between the UV extinction curve and DIBs. We compile the extinction curves and the equivalent widths of 40 DIBs along 97 sightlines. We decompose the extinction curve into three Drude-like functions composed of the visible/near-infrared component, the 2175 \mathringA bump, and the far-UV (FUV) extinction at {λ }^-1> 5.9 μ {{{m}}}^-1. We argue that the wavelength-integrated FUV extinction derived from this decomposition technique best measures the strength of the FUV extinction. No correlation is found between the FUV extinction and most (∼90%) of the DIBs. We have also shown that the color excess E(1300{--}1700), the extinction difference at 1300 and 1700 \mathringA often used to measure the strength of the FUV extinction, does not correlate with DIBs. Finally, we confirm the earlier findings of no correlation between the 2175 \mathringA bump and DIBs or between the 2175 \mathringA bump and the FUV extinction.

Dedicated to J. Mayo Greenberg (1922.1.14-2001.11.29) and Naomi Greenberg (1923.4.23-2015.8.19), who had always been a source of inspiration.