2022ApJ...930....2M

The properties of dust change during the transition from diffuse to dense clouds as a result of ice formation and dust coagulation, but much is still unclear about this transformation. We present 2-20 µm spectra of 49 field stars behind the Perseus and Serpens Molecular Clouds and establish relationships between the near-infrared continuum extinction (A_K) and the depths of the 9.7 µm silicate (τ_9.7) and 3.0 µm H₂O ice (τ_3.0) absorption bands. The τ_9.7/A_K ratio varies from large, diffuse interstellar medium-like values (∼0.55), to much lower ratios (∼0.26). Above extinctions of A_K ∼ 1.2 (A_V ∼ 10; Perseus, Lupus, dense cores) and ∼2.0 (A_V ∼ 17; Serpens), the τ_9.7/A_K ratio is lowest. The τ_9.7/A_K reduction from diffuse to dense clouds is consistent with a moderate degree of grain growth (sizes up to ∼0.5 µm), increasing the near-infrared color excess (and thus A_K), but not affecting the ice and silicate band profiles. This grain growth process seems to be related to the ice column densities and dense core formation thresholds, highlighting the importance of density. After correction for Serpens foreground extinction, the H₂O ice formation threshold is in the range of A_K = 0.31-0.40 (A_V = 2.6-3.4) for all clouds, and thus grain growth takes place after the ices are formed. Finally, abundant CH₃OH ice (∼21% relative to H₂O) is reported for 2MASSJ18285266+0028242 (Serpens), a factor of >4 larger than for the other targets.