Water ice: Temperature-dependent refractive indexes and their astrophysical implications

Abstract

Context. Interstellar and circumstellar ices are largely composed of frozen water. Therefore, it is important to derive fundamental parameters for H2O ice such as absorption and scattering opacities, for which accurate complex refractive indexes are needed. Aims. The primary goal of the work presented here is to derive ice-grain opacities based on accurate H2O ice complex refractive indexes at low temperatures and to assess the impact this has on the derivation of water ice column densities and porosity in space. Methods. We used the optool code to derive ice-grain scattering and absorption opacity values based on new and previously reported mid-infrared (mid-IR) complex refractive index measurements of H2O ice, primarily in its amorphous form, but not exclusively. Next, we used those opacities in the RADMC-3D code to run a radiative transfer simulation of a protostellar envelope containing H2O ice, which was then used to calculate water ice column densities. Results. We found that the real refractive index in the mid-IR of H2O ice at 30 K is ~14% lower than previously reported in the literature. This has a direct impact on the ice column densities derived from the simulations of embedded protostars. Additionally, we found that ice porosity plays a significant role in the opacity of icy grains and that the H2O libration mode can be used as a diagnostic tool to constrain the porosity level. Finally, the refractive indexes presented here allowed us to estimate a grain size detection limit of 18 μm based on the 3 μm band, whereas the 6 μm band allowed us to trace grain sizes larger than 20 μm. Conclusions. Based on radiative transfer simulations using new mid-IR refractive indexes, we conclude that H2O ice leads to more absorption of infrared light than previously estimated. This implies that the 3 and 6 μm bands remain detectable in icy grains with sizes larger than 10 μm. Finally, we also propose that the H2O ice libration band can be used as a diagnostic tool to constrain the porosity level of the interstellar ice, in addition to the OH dangling bond, which is now routinely used for this purpose.