[en] The properties of the ices that form in dense molecular clouds represent an important set of initial conditions in the evolution of interstellar and preplanetary matter in regions of active star formation. Of the various spectral features available for study, the bending mode of solid CO₂ near 15 μm has proven to be a particularly sensitive probe of physical conditions, especially temperature. We present new observations of this absorption feature in the spectrum of Q21-1, a background field star located behind a dark filament in the Cocoon Nebula (IC 5146). We show the profile of the feature to be consistent with a two-component (polar + nonpolar) model for the ices, based on spectra of laboratory analogs with temperatures in the range 10-20 K. The polar component accounts for ∼85% of the CO₂ in the line of sight. We compare for the first time 15 μm profiles in three widely separated dark clouds (Taurus, Serpens, and IC 5146), and show that they are indistinguishable to within observational scatter. Systematic differences in the observed CO₂/H₂O ratio in the three clouds have little or no effect on the 15 μm profile. The abundance of elemental oxygen in the ices appears to be a unifying factor, displaying consistent behavior in the three clouds. We conclude that the ice formation process is robust and uniformly efficient, notwithstanding compositional variations arising from differences in how the O is distributed between the primary species (H₂O, CO₂, and CO) in the ices.

[en] Recent surface chemistry experiments have shown that the hydrogenation of molecular oxygen on interstellar dust grains is a plausible formation mechanism, via hydrogen peroxide (H₂O₂), for the production of water (H₂O) ice mantles in the dense interstellar medium. Theoretical chemistry models also predict the formation of a significant abundance of H₂O₂ ice in grain mantles by this route. At their upper limits, the predicted and experimental abundances are sufficiently high that H₂O₂ should be detectable in molecular cloud ice spectra. To investigate this further, laboratory spectra have been obtained for H₂O₂/H₂O ice films between 2.5 and 200 μm, from 10 to 180 K, containing 3%, 30%, and 97% H₂O₂ ice. Integrated absorbances for all the absorption features in low-temperature H₂O₂ ice have been derived from these spectra. For identifying H₂O₂ ice, the key results are the presence of unique features near 3.5, 7.0, and 11.3 μm. Comparing the laboratory spectra with the spectra of a group of 24 protostars and field stars, all of which have strong H₂O ice absorption bands, no absorption features are found that can definitely be identified with H₂O₂ ice. In the absence of definite H₂O₂ features, the H₂O₂ abundance is constrained by its possible contribution to the weak absorption feature near 3.47 μm found on the long-wavelength wing of the 3 μm H₂O ice band. This gives an average upper limit for H₂O₂, as a percentage of H₂O, of 9% ± 4%. This is a strong constraint on parameters for surface chemistry experiments and dense cloud chemistry models.

[en] 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. 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 × 10^–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 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 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 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_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

[en] This paper presents spectra in the 2 to 20 μm range of quiescent cloud material located in the IC 5146 cloud complex. The spectra were obtained with NASA's Infrared Telescope Facility SpeX instrument and the Spitzer Space Telescope's Infrared Spectrometer. We use these spectra to investigate dust and ice absorption features in pristine regions of the cloud that are unaltered by embedded stars. We find that the H₂O-ice threshold extinction is 4.03 ± 0.05 mag. Once foreground extinction is taken into account, however, the threshold drops to 3.2 mag, equivalent to that found for the Taurus dark cloud, generally assumed to be the touchstone quiescent cloud against which all other dense cloud and embedded young stellar object observations are compared. Substructure in the trough of the silicate band for two sources is attributed to CH₃OH and NH₃ in the ices, present at the ∼2% and ∼5% levels, respectively, relative to H₂O-ice. The correlation of the silicate feature with the E(J - K) color excess is found to follow a much shallower slope relative to lines of sight that probe diffuse clouds, supporting the previous results by Chiar et al.