Infrared Spectroscopic Survey of the Quiescent Medium of Nearby Clouds:
  I. Ice Formation and Grain Growth in Lupus
  Infrared photometry and spectroscopy (1-25 um) 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. H2O ices form at extinctions of
Ak=0.25+/-0.07 mag (Av=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*10^-5 relative
to Nh) is typical for quiescent regions, but lower by a factor of 3-4 compared
to dense envelopes of YSOs. 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 um) continuum
extinction relative to Ak increases as a function of Ak. Most Lupus lines of
sight are well fitted with empirically derived extinction curves corresponding
to Rv~ 3.5 (Ak=0.71) and Rv~5.0 (Ak=1.47). For lines of sight with Ak>1.0 mag,
the tau9.7/Ak 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 tau9.7/Ak
ratio. This process is likely related to grain growth by coagulation, as traced
by the A7.4/Ak continuum extinction ratio, but not to ice mantle formation.
Conversely, grains acquire ice mantles before the process of coagulation
starts.
