Absorption of crystalline water ice in the far infrared at different temperatures

Abstract

The optical properties of ice in the far infrared are important for models of protoplanetary and debris disks. In this report, we derive a new set of data for the absorption (represented by the imaginary part of the refractive index κ) of crystalline water ice in this spectral range. The study includes a detailed inspection of the temperature dependence, which has not been conducted in such detail before. We measured the transmission of three ice layers with different thicknesses at temperatures ϑ = 10 . . . 250 K and present data at wavelengths λ = 80 . . . 625 μm. We found a change in the spectral dependence of κ at a wavelength of 175 ± 6 μm. At shorter wavelengths, κ exhibits a constant flat slope and no significant temperature dependence. Long-ward of that wavelength, the slope gets steeper and has a clear, approximately linear temperature dependence. This change in behaviour is probably caused by a characteristic absorption band of water ice. The measured data were fitted by a power-law model that analytically describes the absorption behaviour at an arbitrary temperature. This model can readily be applied to any object of interest, for instance a protoplanetary or debris disk. To illustrate how the model works, we simulated the spectral energy distribution (SED) of the resolved, large debris disk around the nearby solar-type star HD 207129. Replacing our ice model by another, commonly used data set for water ice results in a different SED slope at longer wavelengths. This leads to changes in the characteristic model parameters of the disk, such as the inferred particle size distribution, and affects the interpretation of the underlying collisional physics of the disk.

Constraining the detectability of water ice in debris disks

“…We note that the direct observation of a spatially resolved inner radius of the debris disk system is limited by the fixed occulting spot size, for instance, 0.6" in HST observation. Alternatively, the analysis of debris disks SEDs allows constraining the inner radius as well, but is limited [0.1 µm, 1000 µm] with n(a) ∝ a −3.5 (Dohnanyi 1969) Dust composition and Amorphous ice (Potapov et al 2018b, Curtis et al 2005, and Li & Greenberg 1998) References of corresponding optical data Crystalline ice (Reinert et al 2015, Häßner et al 2018, Mishima et al 1983, Potapov et al 2018b, Warren 1984, Curtis et al 2005, and Li & Greenberg 1998 Astrosil (Draine 2003) Fractional ratio of ice F ice in icy dust mixtures 0 (pure Astrosil), 0.25, 0.5, 0.75, and 1 (pure ice) Porosity of grains P 0 (compact), 0.25, 0.5, 0.75 Sublimation temperature 100 K: Pure amorphous ice (Brown et al 2006;Kobayashi et al 2010) 105 K: Pure crystalline ice 100 K: Amorphous ice in dust aggregates 105 K: Crystalline ice in dust aggregates Table 1. Model parameters for the simulation of our reference debris disk model.…”

Relating grain size distributions in circumstellar discs to the spectral index at millimetre wavelengths

“…Mishima et al 1983) but only recent observations at higher resolution and sensitivity have brought the issue back into focus. Consequently, temperature-dependent data for more and more materials are becoming available (Reinert et al 2015;Häßner et al 2018;Mutschke & Mohr 2019).…”

“…A set of five different materials is assumed: compact astronomical silicate (Draine 2006) with assumed bulk density ρ = 3.3 g/cm 3 , porous astronomical silicate (mixing rule: Bruggeman 1935) with a filling factor f = 10 % and ρ = 0.33 g/cm 3 , amorphous carbon (Zubko et al 1996) with ρ = 2 g/cm 3 , pyroxene (H. Mutschke and P. Mohr, private comm., manuscript in preparation) with ρ = 3.3 g/cm 3 , and crystalline water ice (Reinert et al 2015) with ρ = 1.0 g/cm 3 . Only the data sets for the latter two materials include a dependence on grain temperature.…”

Solution to the debris disc mass problem: planetesimals are born small?

“…Additional uncertainties come from (unknown) porosity of grains (Henning & Stognienko 1996), as well as from the temperature dependence of the opacity. For all of the materials listed above, opacity tends to decrease toward lower temperatures (Reinert et al 2015;Häßner et al 2018;Potapov et al 2018;Mutschke & Mohr 2019, H. Mutschke & P. Mohr, in prep. ).…”

“…Since we do not know what the composition of debris dust looks like in reality, we can invoke results of laboratory measurements with THz techniques performed for some materials of potential astrophysical relevance. As an example, we consider crystalline water ice (Reinert et al 2015), forsterite (Mutschke & Mohr 2019), pyroxene (H. Mutschke & P. Mohr, in prep. ), and amorphous carbon (Zubko et al 1996).…”