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The properties of dust change during the transition from diffuse to denseclouds as a result of ice formation and dust coagulation, but much is stillunclear about this transformation. We present 2-20 micron spectra of 49 fieldstars behind the Perseus and Serpens Molecular Clouds and establishrelationships between the near-infrared continuum extinction (AK) and thedepths of the 9.7 micron silicate (tau97) and 3.0 micron H2O ice (tau30)absorption bands. The tau97/AK ratio varies from large, diffuse interstellarmedium-like values (~0.55), to much lower ratios (~0.26). Above extinctions ofAK~1.2 (AV~10; Perseus, Lupus, dense cores) and ~2.0 (AV~17; Serpens), thetau97/AK ratio is lowest. The tau97/AK reduction from diffuse to dense cloudsis consistent with a moderate degree of grain growth (sizes up to ~0.5 micron),increasing the near-infrared color excess (and thus AK), but not affecting iceand silicate band profiles. This grain growth process seems to be related tothe ice column densities and dense core formation thresholds, highlighting theimportance of density. After correction for Serpens foreground extinction, theH2O ice formation threshold is in the range of AK=0.31-0.40 (AV=2.6-3.4) forall clouds, and thus grain growth takes place after the ices are formed.Finally, abundant CH3OH ice (~21% relative to H2O) is reported for2MASSJ18285266+0028242 (Serpens), a factor of >4 larger than for the othertargets.

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