2024ApJ...963..120S

The 9.7 and 18 μm interstellar spectral features, arising from the Si-O stretching and O-Si-O bending modes of amorphous silicate dust, are the strongest extinction features in the infrared. Here we use the “pair method” to determine the silicate extinction profile by comparing the Spitzer/IRS spectra of 49 target stars with obvious extinction with those of unreddened stars of the same spectral type. The 9.7 μm extinction profile is determined from all the 49 stars and the 18 μm profile is determined from six stars. It is found that the profile has a peak wavelength at ∼9.2-9.8 and ∼18-22 μm, respectively. The peak wavelength of the 9.7 μm feature seems to become shorter for stars of late spectral types; meanwhile the FWHM seems irrelevant to the spectral type, which may be related to circumstellar silicate emission. The silicate optical depth at 9.7 μm, Δτ_9.7, mostly increases with the color excess in J - K_S (${E}_{{{JK}}_{S}}$). The mean ratio of the visual extinction to the 9.7 μm silicate absorption optical depth is A_V/Δτ_9.7 ≈ 17.8, in close agreement with that of the solar-neighborhood diffuse interstellar medium. When ${E}_{{{JK}}_{S}}\gt 4$, this proportionality changes. The correlation coefficient between the peak wavelength and the FWHM of the 9.7 μm feature is 0.4, which indicates a positive correlation considering the uncertainties of the parameters. The method is compared with replacing the reference star by an atmospheric model spectral energy distribution and no significant difference is observed.