Extinction and the Dimming of KIC 8462852

To test alternative hypotheses for the behavior of KIC 8462852, we obtained measurements of the star over a wide wavelength range from the UV to the mid-infrared from October 2015 through December 2016, using Swift, Spitzer and at AstroLAB IRIS. The star faded in a manner similar to the long-term fading seen in Kepler data about 1400 days previously. The dimming rate for the entire period reported is 22.1 +\- 9.7 milli-mag/yr in the Swift wavebands, with amounts of 21.0 +\- 4.5 mmag in the groundbased B measurements, 14.0 +\- 4.5 mmag in V, and 13.0 +\- 4.5 in R, and a rate of 5.0 +\- 1.2 mmag/yr averaged over the two warm Spitzer bands. Although the dimming is small, it is seen at >= 3 sigma by three different observatories operating from the UV to the IR. The presence of long-term secular dimming means that previous SED models of the star based on photometric measurements taken years apart may not be accurate. We find that stellar models with T_{eff} = 7000 - 7100 K and A_V ~ 0.73 best fit the Swift data from UV to optical. These models also show no excess in the near-simultaneous Spitzer photometry at 3.6 and 4.5 microns, although a longer wavelength excess from a substantial debris disk is still possible (e.g., as around Fomalhaut). The wavelength dependence of the fading favors a relatively neutral color (i.e., R_V >= 5, but not flat across all the bands) compared with the extinction law for the general ISM (R_V = 3.1), suggesting that the dimming arises from circumstellar material.