Dust plays a central role in the unification theory of active galactic nuclei (AGNs). Whether the dust that forms the torus around an AGN is tenth micronsized like interstellar grains or much larger has a profound impact on correcting for the obscuration of the dust torus to recover the intrinsic spectrum and luminosity of the AGN. Here we show that the ratio of the optical extinction in the visual band (A V ) to the optical depth of the 9.7 µm silicate absorption feature (∆τ 9.7 ) could potentially be an effective probe of the dust size. The anomalously lower ratio of A V /∆τ 9.7 ≈ 5.5 of AGNs compared to that of the Galactic diffuse interstellar medium of A V /∆τ 9.7 ≈ 18 reveals that the dust in AGN torus could be substantially larger than the interstellar grains of the Milky Way and of the Small Magellanic Cloud, and therefore, one might expect a flat extinction curve for AGNs.

The 9.7 µm interstellar spectral feature, arising from the Si-O stretch of amorphous silicate dust, is the strongest extinction feature in the infrared (IR). In principle, the spectral profile of this feature could allow one to diagnose the mineralogical composition of interstellar silicate material. However, observationally, the 9.7 µm interstellar silicate extinction profile is not well determined. Here we utilize the Spitzer /IRS spectra of five early-type (one O-and four B-type) stars and compare them with that of unreddened stars of the same spectral type to probe the interstellar extinction of silicate dust around 9.7 µm. We find that, while the silicate extinction profiles all peak at ∼ 9.7 µm, two stars exhibit a narrow feature of FWHM ∼ 2.0 µm and three stars display a broad feature of FWHM ∼ 3.0 µm. We also find that the width of the 9.7 µm extinction feature does not show any environmental dependence. With a FWHM of ∼ 2.2 µm, the mean 9.7 µm extinction profile, obtained by averaging over our five stars, closely resembles that of the prototypical diffuse interstellar medium along the lines of sight toward Cyg OB2 No. 12 and WR 98a. Finally, an analytical formula is presented to parameterize the interstellar extinction in the IR at 0.9 µm λ 15 µm.