X-RAY-IRRADIATION OF INTERSTELLAR GRAINS IN ACTIVE GALAXIES - EVAPORATION AND INFRARED-SPECTRA

X-ray-irradiated dust grains are likely to be present in Seyfert galaxies, quasars, starburst galaxies, and perhaps near X-ray sources in our own Galaxy. Infrared observations have shown that small grains (< 10 angstrom) subjected to ambient interstellar UV radiation will heat sporadically to more than 1000 K and emit in the near-infrared. X-rays will heat even larger grains in this way. A population of X-ray-illuminated grains varying in size will radiate an infrared spectrum with a long-wavelength blackbody peak, at the equilibrium temperature of the large grains, plus some excess nonequilibrium continuum emission extending into the 1-3-mu-m range. X-rays will efficiently evaporate dust grains smaller than 10 angstrom, so X-ray illuminated dust should have no polycyclic aromatic hydrocarbon (PAH) features. The turnover shortward of 3-mu-m is robust and represents the onset of evaporative cooling in grains. The characteristics of the IR spectrum from X-ray-heated grains are not particularly sensitive to the shape of the incident X-ray spectrum; however, UV emission stimulated by X-ray photoelectrons originating in the gas phase can substantially increase the strength of the near-IR excess.