J. E. Chiar

The composition and distribution of dust along the line of sight toward the Galactic center

We discuss the composition of dust and ice along the line of sight to the Galactic center (GC) based on analysis of mid-infrared spectra (2.4-13 μm) from the Short Wavelength Spectrometer on the Infrared Space Observatory (ISO). We have analyzed dust absorption features arising in the molecular cloud material and the diffuse interstellar medium along the lines of sight toward Sgr A* and the Quintuplet sources, GCS 3 and GCS 4. It is evident from the depth of the 3.0 μm H2O and the 4.27 μm CO2 ice features that there is more molecular cloud material along the line of sight toward Sgr A* than toward GCS 3 and GCS 4. In fact, Sgr A* has a rich infrared ice spectrum with evidence for the presence of solid CH4, NH3, and possibly HCOOH. Hydrocarbon dust in the diffuse interstellar medium along the line of sight to the GC is characterized by absorption features centered at 3.4, 6.85, and 7.25 μm. Ground-based studies have identified the 3.4 μm feature with aliphatic hydrocarbons, and ISO has given us the first meaningful observations of the corresponding modes at longer wavelengths. The integrated strengths of these three features suggest that hydrogenated amorphous carbon is their carrier. We attribute an absorption feature centered at 3.28 μm in the GCS 3 spectrum to the C–H stretch in aromatic hydrocarbons. This feature is not detected, and its C–C stretch counterpart appears to be weaker, in the Sgr A* spectrum. A key question now is whether or not aromatics are a widespread component of the diffuse interstellar medium, analogous to aliphatic hydrocarbons.

The Abundance of Carbon Dioxide Ice in the Quiescent Intracloud Medium

We present new observations with the Infrared Spectrograph on board the Spitzer Space Telescope of the solidCO2 absorption feature near 15 � m in the spectra of eight field stars behind the Taurus complex of dark clouds. Solid CO2isdetectedinsixlinesofsight.Newresultsarecombinedwithpreviousdatatoinvestigatethecorrelationof CO2 columndensitywiththoseofother majoriceconstituents(H2OandCO)andwithextinction.CO2isshowntodisplaya ‘‘thresholdextinction’’ effect, i.e., a minimumextinction(A0 ¼ 4:3 � 1:0 mag)requiredfordetection,behaviorsimilar to that previously reported for H2O and CO. We find a particularly tight correlation through the origin between N(CO2) and N(H2O), confirming that these species form in tandem and coexist in the same (polar) ice layer on the grains. The observedcomposition of themantlesisbroadlyconsistentwiththepredictionsof photochemical modelswithdiffusive surface chemistry proposed by Ruffle & Herbst. Comparison of our results for Taurus with published data for Serpens indicatessignificantdifferencesinicecomposition consistentwithenhancedCO2productioninthe lattercloud.Ourresultsalsoplaceconstraintsonthedistributionofelementaloxygenbetweenicesandotherpotentialreservoirs.Assuming aconstant N(H)toextinctionratio,weshowthat ~65%of thesolarOabundanceisaccountedforbysummingthecontributions of ices (~26%), refractory dust (~30%) and gas-phase CO (~9%). If the Sun is an appropriate standard for the interstellar medium, the ‘‘missing’’ oxygen may reside in atomic O i gas and/or (undetected) O2 within the ices.

FIRE AND ICE: SPITZER INFRARED SPECTROGRAPH (IRS) MID-INFRARED SPECTROSCOPY OF IRAS F00183-7111

We report the detection of strong absorption and weak emission features in the 4-27 μm Spitzer Infrared Spectrograph (IRS) spectrum of the distant ultraluminous infrared galaxy IRAS F00183-7111 (z = 0.327). The absorption features of CO2 and CO gas, water ice, hydrocarbons, and silicates are indicative of a strongly obscured (A9.6 ≥ 5.4; AV ≥ 90) and complex line of sight through both the hot diffuse interstellar medium and shielded cold molecular clouds toward the nuclear power source. From the profile of the 4.67 μm CO fundamental vibration mode, we deduce that the absorbing gas is dense (n ~ 106 cm-3) and warm (720 K) and has a CO column density of ~1019.5 cm-2, equivalent to NH ~ 1023.5 cm-2. The high temperature and density, as well as the small inferred size (<0.03 pc), locates this absorbing gas close to the power source of this region. Weak emission features of molecular hydrogen, polycyclic aromatic hydrocarbons (PAHs), and Ne+, likely associated with star formation, are detected against the 9.7 μm silicate feature, indicating an origin away from the absorbing region. Based on the 11.2 μm PAH flux, we estimate the star formation component to be responsible for up to 30% of the IR luminosity of the system. While our mid-infrared spectrum shows no telltale signs of active galactic nucleus (AGN) activity, the similarities to the mid-infrared spectra of deeply obscured sources (e.g., NGC 4418) and AGN hot dust (e.g., NGC 1068), as well as evidence from other wavelength regions, suggest that the power source hiding behind the optically thick dust screen may well be a buried AGN.

SOLID CARBON DIOXIDE IN REGIONS OF LOW-MASS STAR FORMATION

We present high-resolution (R D 1500¨2000) spectra of the 4.27 km asymmetric stretching feature of solid in eight lines of sight observed with the Short Wavelength Spectrometer of the Infrared Space CO 2 Observatory. Two of the sources are —eld stars located behind the Taurus molecular cloud; the others are young stellar objects (YSOs) of predominantly low-to-intermediate mass. We —nd a signi—cant source-tosource variation in the solid abundance ratio in our sample: two lines of sight, Elias 18 and CO 2 /H 2 O RAFGL 989, have abundances of D 34%¨37%, considerably higher than in other lines of sight CO 2 studied to date. In agreement with a previous study of Elias 16, we con—rm a substantial (D20%) abundance of solid relative to in the quiescent intracloud medium. We compare the pro—les CO 2 H 2 OC O 2 with laboratory spectra of interstellar ice analogs from the Leiden Observatory Laboratory database. Results show that the 4.27 km pro—les toward —eld stars and embedded low-mass objects are remarkably similar to each other and seem to originate mostly in cold ice. In two higher mass YSOs H 2 O-rich (RAFGL 989 and S255 IRS1), the pro—les are clearly diUerent, and at least the latter source shows signs of thermal processing. . .. . .. . ..

Ice and dust in the quiescent medium of isolated dense cores

The relation between ices in the envelopes and disks surrounding young stellar objects (YSOs) and those in the quiescent interstellar medium (ISM) is investigated. For a sample of 31 stars behind isolated dense cores, ground-based and Spitzer spectra and photometry in the 1-25 μm wavelength range are combined. The baseline for the broad and overlapping ice features is modeled, using calculated spectra of giants, H_2O ice and silicates. The adopted extinction curve is derived empirically. Its high resolution allows for the separation of continuum and feature extinction. The extinction between 13 and 25 μm is ~50% relative to that at 2.2 μm. The strengths of the 6.0 and 6.85 μm absorption bands are in line with those of YSOs. Thus, their carriers, which, besides H_2O and CH_3OH, may include NH^+_4, HCOOH, H_2CO, and NH_3, are readily formed in the dense core phase, before stars form. The 3.53 μm C-H stretching mode of solid CH_3OH was discovered. The CH_3OH/H_2O abundance ratios of 5%-12% are larger than upper limits in the Taurus molecular cloud. The initial ice composition, before star formation occurs, therefore depends on the environment. Signs of thermal and energetic processing that were found toward some YSOs are absent in the ices toward background stars. Finally, the peak optical depth of the 9.7 μm band of silicates relative to the continuum extinction at 2.2 μm is significantly shallower than in the diffuse ISM. This extends the results of Chiar et al. to a larger sample and higher extinctions.

ICES IN THE QUIESCENT IC 5146 DENSE CLOUD

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 NASAs Infrared Telescope Facility SpeX instrument and the Spitzer Space Telescopes 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 H_2O-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 CH_3OH and NH_3 in the ices, present at the ~2% and ~5% levels, respectively, relative to H_2O-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.

OBSERVATIONAL CONSTRAINTS ON METHANOL PRODUCTION IN INTERSTELLAR AND PREPLANETARY ICES

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.

The Relationship between the Optical Depth of the 9.7 μm Silicate Absorption Feature and Infrared Differential Extinction in Dense Clouds

We have examined the relationship between the optical depth of the 9.7 μm silicate absorption feature (τ_(9.7)) and the near-infrared color excess, E(J - K_s), in the Serpens, Taurus, IC 5146, Chameleon I, Barnard 59, and Barnard 68 dense clouds/cores. Our data set, based largely on Spitzer IRS spectra, spans E(J - K_s) = 0.3-10 mag (corresponding to visual extinction between about 2 and 60 mag). All lines of sight show the 9.7 μm silicate feature. Unlike in the diffuse ISM where a tight linear correlation between the 9.7 μm silicate feature optical depth and the extinction (A_V) is observed, we find that the silicate feature in dense clouds does not show a monotonic increase with extinction. Thus, in dense clouds, τ_(9.7) is not a good measure of total dust column density. With few exceptions, the measured τ_(9.7) values fall well below the diffuse ISM correlation line for E(J - K_s) > 2 mag (A_V > 12 mag). Grain growth via coagulation is a likely cause of this effect.

SEARCHING FOR AMMONIA IN GRAIN MANTLES TOWARD MASSIVE YOUNG STELLAR OBJECTS

We present a study of the mid-infrared spectral regions of 17 objects observed with the Short-Wavelength Spectrometer on the Infrared Space Observatory (ISO SWS) in a search for the presence of the elusive NH3 solid-state absorption features at 2.96 and 9.0 μm. We compare the 3 μm profile with laboratory spectra of pure water and water/ammonia mixtures from the Leiden Molecular Astrophysics database. We determine that the shape of the 3 μm water feature alone does not present conclusive evidence of the presence or lack of NH3 ice on dust grain mantles because of the large number of unknown physical parameters that influence the 3 μm water profile: grain size distribution, grain shape, mantle composition, thermal history, and geometrical effects. In the 9 μm region, the presence of NH3 is characterized by a sudden change in slope at 8.5 μm and a peak at 9.0 μm within the silicate profile. Of the 16 silicate features studied, four show evidence of a 9 μm ammonia feature, including the previously reported detections in ground-based observations of NGC 7538 IRS 9, and SWS observations of W33 A. Upper limits are found for the remaining objects. Why NH3, which is an abundant gas-phase molecule in cold, dense cores, is not apparently present in significant amounts on most grain mantles, where it is expected to freeze out, is currently a mystery.

Observational Constraints on the Abundance and Evolution of “XCN” in Interstellar Grain Mantles*

The 4.62 km ii XCN ˇˇ absorption feature, attributed to CN-bearing molecules in solids, is potentially an important diagnostic of the evolution of organic matter in the interstellar medium and the envelopes of newly formed stars. We report quantitative limits on the strength of this feature in the diUuse interstellar medium toward the reddened B-type star Cyg OB2 No. 12 and in the dark cloud toward the young stellar object R CrA IRS 2. On the basis of an assumed band strength for the carrier species, we estimate that less than 0.3% and less than 0.1% of the elemental nitrogen is in CN bonds along these lines of sight, respectively; if they are typical of diUuse and dense environments, it follows that the carrier of XCN is no more than a trace constituent of either organic-refractory or icy interstellar grain mantles. Appreciable XCN abundances seem to occur only in the envelopes of certain young stellar objects (YSOs), most notably the high-mass objects W33A and AFGL 7009S. We con—rm the presence of XCN in the spectrum of the low-mass YSO R CrA IRS 7. The strengths of the XCN absorptions in R CrA IRS 7 and other low-mass YSOs indicate mean XCN concentrations relative to in the ices of H 2 O D1%, comparable with the abundance of CN-bearing species in comets. )