Relations between mid-IR dust emission and UV extinction

We analyze low resolution Spitzer infrared (IR) 5-14 micron spectra of the diffuse emission toward a carefully selected sample of stars. The sample is composed of sight lines toward stars that have well determined ultraviolet (UV) extinction curves and which are shown to lie beyond effectively all of the extinguishing and emitting dust along their lines of sight. Our sample includes sight lines whose UV curve extinction curves exhibit a wide range of curve morphology and which sample a variety of interstellar environments. As a result, this unique sample enabled us to study the connection between the extinction and emission properties of the same grains, and to examine their response to different physical environments. We quantify the emission features in terms of the PAH model given by Draine & Li (2007) and a set of additional features, not known to be related to PAH emission. We compare the intensities of the different features in the Spitzer mid-IR spectra with the Fitzpatrick & Massa (2007) parameters which describe the shapes of UV to near-IR extinction curves. Our primary result is that there is a strong correlation between the area of the 2175 A UV bump in the extinction curves of the program stars and the strengths of the major PAH emission features in the mid-IR spectra for the same lines of sight.