2013ApJ...777...73B

Infrared photometry and spectroscopy (1-25 µm) 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. H₂ O 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 H₂ O ice (2.3±0.1x10^–5 relative to N_H) 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 CH₃ OH 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 µm) continuum extinction relative to A_Kincreases as a function of A_K. Most Lupus lines of sight are well fitted with empirically derived extinction curves corresponding to R_V∼ 3.5 (A_K= 0.71) and R_V∼ 5.0 (A_K= 1.47). For lines of sight with A_K> 1.0 mag, the τ_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 τ_9.7/A_Kratio. 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.