2008A&A...492..117M

Hydrous silicates occur in various cosmic environments, and are among the minerals with the most pronounced bands in the far infrared (FIR) spectral region. Given that Herschel and ALMA will open up new possibilities for astronomical FIR and sub-mm spectroscopy, data characterizing the dielectric properties of these materials at long wavelengths are desirable. We aimed at examining the FIR spectra of talc, picrolite, montmorillonite, and chamosite, which belong to four different groups of phyllosilicates. We tabulated positions and band widths of the FIR bands of these minerals depending on the dust temperature. By means of powder transmission spectroscopy, spectra of the examined materials were measured in the wavelength range 25-500µm at temperatures of 300, 200, 100, and 10K. Room-temperature measurements yield the following results. For talc, a previously unknown band, centered at 98.5µm, was found, in addition to bands at 56.5 and 59.5µm. For montmorillonite, several bands at wavelengths <110µm were detected, including a band at 105µm with an FWHM of about 10µm. Picrolite shows a sharp 77µm FIR band. Chamosite is characterized by bands in the 72-92µm range, and a prominent band at 277µm. At decreasing temperature, most of the bands shift to shorter wavelengths. Examining a potential counterpart of the 105µm band in the spectra of HD 142527 and HD 100546, we find that the broad band in the spectra of these young stars - extending from 85 to 125µm - cannot be due to montmorillonite or any of the hydrous silicates we studied, since these materials have sharper bands in the FIR wavelength range than previously assumed, especially at low temperatures.