Mapping water ice with infrared broadband photometry

TL;DR

This paper introduces the ice color excess method, a photometric technique using infrared broadband data to map water ice in molecular clouds, enabling large-scale interstellar ice studies.

Contribution

The study develops and calibrates a new photometric approach to trace water ice, leveraging widely available infrared data and minimizing systematic errors.

Findings

Strong correlation between optical depth and photometric index

Method effectively maps water ice distribution in molecular clouds

Enables large-scale interstellar ice studies with existing data

Abstract

Interstellar ices play a fundamental role in the physical and chemical evolution of molecular clouds and star-forming regions, yet their large-scale distribution and abundance remain challenging to map. In this work, I present the ice color excess method, which parametrizes the peak optical depth ($\tau_{3.0}^{\mathrm{max}}$) of the prominent 3$\mu$m absorption feature, which is predominantly caused by the presence of solid H$_2$O. The method builds on well-established near-infrared color excess techniques and uses widely available infrared broadband photometry. Through detailed evaluation of passband combinations and a comprehensive error analysis, I construct the ice color excess metric $\Lambda(W_1 - I_1)$. This parameter emerges as the optimal choice that minimizes systematic errors while leveraging high-quality, widely available photometry from Spitzer and WISE data archives. To calibrate the method, I compile from the literature a sample of stars located in the background of nearby molecular clouds, for which spectroscopically measured optical depths are available. The empirical calibration yields a remarkably tight correlation between $\tau_{3.0}^{\mathrm{max}}$ and $\Lambda(W_1 - I_1)$. This photometric technique opens a new avenue for tracing the icy component of the interstellar medium on Galactic scales, providing a powerful complement to spectroscopic surveys and enabling new insights into the environmental dependence of the formation and evolution of icy dust grains.