Implications of self-consistent H$_2$O ice optical constants on radiative transfer models of disks

Abstract

Interstellar water (H$_2$O) ice exhibits significantly varied profiles over its spectral features with temperature and thermal history. No previous radiative transfer model of a protoplanetary disk has fully accounted for these effects over 3 - 200$\mu$m simultaneously. A radiative transfer model with region-based distribution of ices and applicable ice optical constant composites (OCCs) is required to properly model the distribution and spectral signatures of water ice in disks. We build such H$_2$O ice OCCs by combining multiple experiment measurements of the imaginary refractive indices, $k$, to cover from 0.1 - 10,000$\mu$m and perform Kramers-Kronig (KK) integrations to derive updated real refractive indices, $n$. We construct H$_2$O ice OCCs for two thermal histories: first, direct deposit and measure at a fixed temperature ices (DT) and deposit and cool down (DC) ices, for five and eight distinct temperatures, respectively. Next, we self-consistently apply these within a radiative transfer model of a protoplanetary disk according to its thermal structure. This produces the first self-consistent RT profiles for the two most commonly used spectroscopic tracers of thermal processing of H$_2$O ice. To assess the impact that appropriate H$_2$O optical constants have on the 3$\mu$m absorption and 45 and 63$\mu$m emission profiles, we run four RADMC-3D radiative transfer models of edge-on class II disk, HH 48 NE. The four models each use different OCCs; the standard single temperature crystalline and amorphous H$_2$O ice OCCs available from OpTool and our own multi-temperature DT set of OCCs and DC only set of OCCs. We find that ices must have been thermally processed at temperatures higher than the local disk temperature in order to produce crystalline spectral profiles at 3, 45 or 63$\mu$m in disks, suggesting local heating events or outward transport to colder regions.