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This study assesses the contribution of neutral polycyclic aromatic hydrocarbons (PAHs) and hydrogenated PAHs (Hn-PAHs) in H2O-ices to the 3 μm and 5–8 μm regions of the infrared absorption spectrum of the Young Stellar Object, Mon R2 IRS 3, and the Taurus dense cloud in the direction of Elias 16. We compare the astronomical spectra to the previously published laboratory data of matrix-isolated PAHs and Hn-PAHs in H2O-ices. For the molecules in this study, the band positions, FWHMs, and integrated band strengths have been measured. For the PAHs considered here, the ratio of the experimental A(C–H) stretch (in H2O ices) to the theoretical A(C–H) stretch (in the gas phase) is consistently about 0.10, meaning that the trove of theoretical data can be reliably scaled to compare to the astronomical spectra. We find that the fractional percentage contribution to the 5–8 μm region for Mon R2 IRS 3 ranges between 2.7 and 3.9 for neutral PAHs and 0.25–1.2 for Hn-PAHs. The best match to the observed 3.25 μm profile in the Mon R2 IRS 3 spectrum is accomplished with the laboratory-measured coronene:H2O spectrum. Using this spectrum we estimate that neutral PAHs contribute up to 12.0% of the cosmic carbon budget for Mon R2 IRS 3 and <14.5% for Elias 16. Neutral hydrogenated PAHs contribute mainly to the 3.47 μm absorption and very little to the 5 to 8 μm region. For neutral Hn-PAHs, we estimate that they contribute up to 0.6% of the cosmic carbon budget for Mon R2 IRS 3 and <1% for Elias 16. [ABSTRACT FROM AUTHOR]

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 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 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 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 τ9.7/AK 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 τ9.7/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. [ABSTRACT FROM AUTHOR]

Many materials have been considered for the carrier of the hydrocarbon absorption bands observed in the diffuse interstellar medium (ISM). In order to refine the model for ISM hydrocarbon grains, we analyze the observed aromatic (3.28, 6.2 μm) and aliphatic (3.4 μm) hydrocarbon absorption features in the diffuse ISM along the line of sight toward the Galactic center Quintuplet Cluster. Observationally, sp2 bonds can be measured in astronomical spectra using the 6.2 μm CC aromatic stretch feature, whereas the 3.4 μm aliphatic feature can be used to quantify the fraction of sp3 bonds. The fractional abundance of these components allows us to place the Galactic diffuse ISM hydrocarbons on a ternary phase diagram. We conclude that the Galactic hydrocarbon dust has, on average, a low H/C ratio and sp3 content and is highly aromatic. We have placed the results of our analysis within the context of the evolution of carbon dust in the ISM. We argue that interstellar carbon dust consists of a large core of aromatic carbon surrounded by a thin mantle of hydrogenated amorphous carbon (a-C:H), a structure that is a natural consequence of the processing of stardust grains in the ISM. [ABSTRACT FROM AUTHOR]

The properties of dust change during the transition from diffuse to dense clouds as a result of ice formation and dust coagulation, but much is still unclear about this transformation. We present 2â€"20 ÎĽ m spectra of 49 field stars behind the Perseus and Serpens Molecular Clouds and establish relationships between the near-infrared continuum extinction (A K) and the depths of the 9.7 ÎĽ m silicate (Ď„ 9.7) and 3.0 ÎĽ m H2O ice (Ď„ 3.0) absorption bands. The Ď„ 9.7/ A K ratio varies from large, diffuse interstellar medium-like values (∼0.55), to much lower ratios (∼0.26). Above extinctions of A K ∼ 1.2 (A V ∼ 10; Perseus, Lupus, dense cores) and ∼2.0 (A V ∼ 17; Serpens), the Ď„ 9.7/ A K ratio is lowest. The Ď„ 9.7/ A K reduction from diffuse to dense clouds is consistent with a moderate degree of grain growth (sizes up to ∼0.5 ÎĽ m), increasing the near-infrared color excess (and thus A K), but not affecting the ice and silicate band profiles. This grain growth process seems to be related to the ice column densities and dense core formation thresholds, highlighting the importance of density. After correction for Serpens foreground extinction, the H2O ice formation threshold is in the range of A K = 0.31â€"0.40 (A V = 2.6â€"3.4) for all clouds, and thus grain growth takes place after the ices are formed. Finally, abundant CH3OH ice (∼21% relative to H2O) is reported for 2MASSJ18285266+0028242 (Serpens), a factor of >4 larger than for the other targets. [ABSTRACT FROM AUTHOR]

Dust grains are nucleation centers and catalysts for the growth of icy mantles in quiescent interstellar clouds, the products of which may accumulate into preplanetary matter when new stars and solar systems form within the clouds. In this paper, we present the first spectroscopic detections of silicate dust and the molecular ices H2O, CO, and CO2 in the vicinity of the prestellar core L183 (L134N). An infrared photometric survey of the cloud was used to identify reddened background stars, and we present spectra covering solid-state absorption features in the wavelength range 2-20μm for nine of them. The mean composition of the ices in the best-studied line of sight (toward J15542044-0254073) is H2O:CO:CO2 ≈ 100:40:24. The ices are amorphous in structure, indicating that they have been maintained at low temperature (≲15 K) since formation. The ice column density N(H2O) correlates with reddening by dust, exhibiting a threshold effect that corresponds to the transition from unmantled grains in the outer layers of the cloud to ice-mantled grains within, analogous to that observed in other dark clouds. A comparison of results for L183 and the Taurus and IC 5146 dark clouds suggests common behavior, with mantles first appearing in each case at a dust column corresponding to a peak optical depth τ9.7 = 0.15 ± 0.03 in the silicate feature. Our results support a previous conclusion that the color excess EJ-K does not obey a simple linear correlation with the total dust column in lines of sight that intercept dense clouds. The most likely explanation is a systematic change in the optical properties of the dust as the density increases. [ABSTRACT FROM AUTHOR]

Methanol (CH3OH) is thought to be an important link in the chain of chemical evolution that leads from simple diatomic interstellar molecules to complex organic species in protoplanetary disks that may be delivered to the surfaces of Earthlike planets. Previous research has shown that CH3OH forms in the interstellar medium predominantly on the surfaces of dust grains. To enhance our understanding of the conditions that lead to its efficient production, we assemble a homogenized catalog of published detections and limiting values in interstellar and preplanetary ices for both CH3OH and the other commonly observed C- and O-bearing species, H2O, CO, and CO2. We use this catalog to investigate the abundance of ice-phase CH3OH in environments ranging from dense molecular clouds to circumstellar envelopes around newly born stars of low and high mass. Results show that CH3OH production arises during the CO freezeout phase of ice-mantle growth in the clouds, after an ice layer rich in H2O and CO2 is already in place on the dust, in agreement with current astrochemical models. The abundance of solid-phase CH3OH in this environment is sufficient to account for observed gas-phase abundances when the ices are subsequently desorbed in the vicinity of embedded stars. CH3OH concentrations in the ices toward embedded stars show order-of-magnitude object-to-object variations, even in a sample restricted to stars of low mass associated with ices lacking evidence of thermal processing. We hypothesize that the efficiency of CH3OH production in dense cores and protostellar envelopes is mediated by the degree of prior CO depletion. [ABSTRACT FROM AUTHOR]