Far-infrared continuum absorption of olivine at low temperatures

Abstract

The far-infrared continuum opacity of cold dust is an important quantity for the study of debris disks in planetary systems and of protoplanetary disks. Olivine is considered the most abundant crystalline dust species in such environments. We present spectroscopic absorption measurements on olivine plates of the order of a millimeter thickness at wavelengths between 60 and 400 μm for temperatures down to 10 K. Our data reveal a strong temperature dependence of the continuum absorption coefficient, i.e. more than an order of magnitude decrease at 100 μm for 10 K compared to room temperature. The absolute values are generally much lower than those measured with olivine powders embedded into polyethylene pellets, even if the difference between plate and powder samples is taken into account by theoretical models. In contrast to this, the room temperature data are in relatively good agreement with simulations using optical constants determined from reflection measurements. At low temperatures, the absorption coefficient of olivine was measurable with sufficient accuracy only up to 90 μm for 10 K and up to 110 μm for 100 K. These data reveal a drastic change in the spectral slope (from β ∼ 2.0 to β > 5.0) for the continuum underlying the 69-μm band, which is not predicted by the low-temperature optical constants determined for forsterite.

Far-infrared continuum absorption of forsterite and enstatite at low temperatures

“…The disagreement among the spectra measured with the same polarization direction and the excess over the theoretical limit both indicate strong errors in the measured absolute transmission values. Similar errors were already noted by Mutschke et al (2013) and were mainly assigned to the displacement of the beam by refraction at the sample surfaces. For the very thick samples used here, and the complicated optical path with four partly bent cryostat windows, such beam displacements can influence the detected signal significantly.…”

“…The absolute values of the mass absorption coefficient at low temperature, however, differ significantly, i.e., they are higher by more than one order of magnitude (at λ = 100 µm) in the Mennella et al results. On the other hand, Mutschke et al (2013) measured an olivine powder (Mg 1.84 Fe 0.16 SiO 4 , sample originally prepared by Imai et al 2009) similar to that measured by Mennella et al (1998) at θ = 10 K and found values closer to those measured by Chihara et al (2001). Furthermore, Coupeaud et al (2011) measured samples of originally amorphous silicates, which have been crystallized at high temperature.…”

“…Their absorption values are comparable to those by Mennella et al (1998); however, their materials contain a large portion of other crystallization products such as periclase (MgO). Mutschke et al (2013) presented the first low-temperature transmission measurements on an olivine single crystal (Mg 1.78 Fe 0.22 SiO 4 ) in the 100 µm wavelength range and found a continuum absorption, which was again an order of magnitude lower (at θ = 10 K) than the lowest values measured on powders. The difference between the room temperature and the 10 K absorption amounted to more than one order of magnitude, but also at room temperature the values were a factor of 3.7 lower than those measured with the olivine powder.…”

“…In fact, the single-phonon transitions (IR bands) located at shorter wavelengths are supposed to cause a decrease in the absorption coefficient proportional to λ −2 , and the phonon difference processes, which are expected to dominate the absorption spectrum at higher temperatures, should also produce a λ −2 spectral decrease (e.g., Mishima et al 1983). Moreover, the spectral slope found with the absorption coefficients of forsterite and enstatite powders (Mennella et al 1998;Chihara et al 2001;Imai et al 2009;Mutschke et al 2013) is typically an inverse quadratic dependence on wavelength or steeper.…”

“…Similarly, a number of laboratory studies have been undertaken to determine the continuum absorption coefficient of amorphous and crystalline silicates in the visible (e.g., Zeidler et al 2011;Pitman et al 2013) and in the far-infrared/submillimeter spectral ranges (Mennella et al 1998;Chihara et al 2001;Boudet et al 2005;Coupeaud et al 2011;Mutschke et al 2013;Demyk et al 2017a,b). Studying the latter is especially demanding, since most of the physical absorption mechanisms relevant at these low photon energies show a strong temperature dependence.…”