Infrared Spectroscopic Survey of the Quiescent Medium of Nearby Clouds. II. Ice Formation and Grain Growth in Perseus and Serpens

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 mu m 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 mu m silicate (tau (9.7)) and 3.0 mu m H2O ice (tau (3.0)) absorption bands. The tau (9.7)/A (K) ratio varies from large, diffuse interstellar medium-like values (similar to 0.55), to much lower ratios (similar to 0.26). Above extinctions of A (K) similar to 1.2 (A (V) similar to 10; Perseus, Lupus, dense cores) and similar to 2.0 (A (V) similar to 17; Serpens), the tau (9.7)/A (K) ratio is lowest. The tau (9.7)/A (K) reduction from diffuse to dense clouds is consistent with a moderate degree of grain growth (sizes up to similar to 0.5 mu m), 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 (similar to 21% relative to H2O) is reported for 2MASSJ18285266+0028242 (Serpens), a factor of >4 larger than for the other targets.