DUST REVERBERATION - A MODEL FOR THE INFRARED VARIATIONS OF FAIRALL-9

Over a period of 10 yr starting in 1978, the ultraviolet flux from the quasar Fairall 9 was observed by Clavel, Wamsteker, & Glass to have varied by over a factor of 30. The infrared varied as well, with light curves very similar in shape but evincing a delay of about 1 yr relative to the UV. Clavel et al. suggested that the data were consistent with infrared reradiation of the UV light by dust. In this paper a detailed model is described in which dust heated by the central optical/UV source is responsible for the infrared emission from Fairall 9. Fits to the light curves at the four measured infrared wavelengths (the J, H, K, and L bands) are presented, which match both the spectral and temporal behavior of the infrared light. Good fits are obtained if the emitting grains are located in clouds where the grains are able to either survive episodes of high UV flux, or reform on a rapid time scale (approximately months) following such episodes. A detailed treatment is given of the emission from clouds of significant UV and infrared optical depth, illuminated on one face by a central UV continuum source. Using typical grain emissivity laws, inferred sublimation temperatures for the grains lie in the range 1300-2000 K, depending on the assumed opacity of the clouds. This range is consistent with either graphite or silicate grains, or some combination of the two. Because of the nature of the variations in Fairall 9, the model is able to distinguish whether the dust lies in a spherical or disklike geometry. The radial distribution can be constrained, however, and it appears that clouds directly exposed to the UV exist at distances ranging from less than or similar to 0.3 lt-yr to about 1.3 lt-yr. The variable part of the optical/UV continuum appears to have a flat spectrum extending into the near-infrared, suggesting an optically thin emission mechanism rather than emission from an optically thick accretion disk.