Probing the 9.7\mu m Interstellar Silicate Extinction Profile through
  the Spitzer/IRS Spectroscopy of OB Stars
  The 9.7$\mu$m 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$\mu$m 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$\mu$m. We find that, while the silicate extinction
profiles all peak at ~9.7$\mu$m, two stars exhibit a narrow feature of FWHM
~2.0$\mu$m and three stars display a broad feature of FWHM ~3.0$\mu$m. We also
find that the width of the 9.7$\mu$m extinction feature does not show any
environmental dependence. With a FWHM of ~2.2$\mu$m, the mean 9.7\mu m
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 $0.9\mu {\rm
m} < \lambda < 15\mu {\rm m}$.
