Infrared Spectroscopic Survey of the Quiescent Medium of Nearby Clouds: I. Ice Formation and Grain Growth in Lupus

Abstract

Infrared photometry and spectroscopy (1-25 µm) of background stars reddened by the Lupus molecular cloud complex are used to determine the properties of the grains and the composition of the ices before they are incorporated into circumstellar envelopes and disks. H_2O 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 H_2O ice (2.3 ± 0.1 x 10^(-5) relative to N_h) is typical for quiescent regions, but lower by a factor of 3-4 compared to dense envelopes of YSOs. The low solid CH_3OH abundance (<3-8% relative to H_2O) 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_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 ~ 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 2 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_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.

Files

Additional details