INFRARED SPECTROSCOPIC SURVEY OF THE QUIESCENT MEDIUM OF NEARBY CLOUDS. I. ICE FORMATION AND GRAIN GROWTH IN LUPUS

Infrared photometry and spectroscopy (1-25 mu m) of background stars reddened by the Lupus molecular cloud complex are used to determine the properties of grains and the composition of ices before they are incorporated into circumstellar envelopes and disks. H2O ices form at extinctions of A(K) = 0.25 +/- 0.07 mag (A(V) = 2.1 +/- 0.6). Such a low ice formation threshold is consistent with the absence of nearby hot stars. Overall, the Lupus clouds are in an early chemical phase. The abundance of H2O ice (2.3 +/- 0.1 x 10(-5) relative to NH) is typical for quiescent regions, but lower by a factor of three to four compared to dense envelopes of young stellar objects. The low solid CH3OH abundance (<3%-8% relative to H2O) indicates a low gas phase H/CO ratio, which is consistent with the observed incomplete CO freeze out. Furthermore it is found that the grains in Lupus experienced growth by coagulation. The mid-infrared (>5 mu m) continuum extinction relative to A(K) increases as a function of A(K). Most Lupus lines of sight are well fitted with empirically derived extinction curves corresponding to R-V similar to 3.5 (A(K) = 0.71) and R-V similar to 5.0 (A(K) = 1.47). For lines of sight with A(K) > 1.0 mag, the tau(9.7)/A(K) ratio is a factor of two lower compared to the diffuse medium. Below 1.0 mag, values scatter between the dense and diffuse medium ratios. The absence of a gradual transition between diffuse and dense medium-type dust indicates that local conditions matter in the process that sets the tau(9.7)/A(K) ratio. This process is likely related to grain growth by coagulation, as traced by the A(7.4)/A(K) continuum extinction ratio, but not to ice mantle formation. Conversely, grains acquire ice mantles before the process of coagulation starts.