Unveiling Grain Growth in Very Dense Galactic Cores

Interstellar dust provides the building blocks of planets and the initial grain size distribution in the parent molecular cloud is key to understanding how planets form. Under typical diffuse ISM conditions, dust grains do not grow beyond ~0.5 micron. However, within dense molecular clouds, when shielded from the harsh interstellar radiation field, ice mantles form, allowing grain growth through coagulation. Theoretical models predict that grain growth deforms the silicate band profiles at 9.7 and 18 micron and flattens the extinction curve between 5 and 26 micron. We propose to observe, with MIRI spectroscopy, 9 independent lines of sight through 3 very dense cores, from 5 to 40 mag of Av. Our proposed measurements, made possible by the exquisite JWST sensitivity, will be a paradigm shift in the field: the simultaneous observations of the 9.7 and 18 micron silicate absorption features at such high Av, of the 5-26 micron extinction curve, and of several ice features, coupled with state-of-the-art modelling, will allow us to resolve for the first time intrinsic degeneracies in grain size, shape and porosity. This will make it possible to set unprecedented constraints on astrochemical models with wide applications to planet and star formation. We will also deliver to the community a mini atlas of spectroscopic MIR extinction curves for the observed regions, which will be the very first publicly available for high Av sightlines. This program complements the current JWST ERS portfolio, by helping to bridge a critical gap between star and planet formation, making it an ideal addition to the Cycle 1 observations which promise to deliver revolutionary science.