A. G. G. M. Tielens

Formation on grain surfaces

The most abundant interstellar molecule, H-2, is generally thought to form by recombination of H atoms on grain surfaces. On surfaces, hydrogen atoms can be physisorbed and chemisorbed and their mobility can be governed by quantum mechanical tunneling or thermal hopping. We have developed a model for molecular hydrogen formation on surfaces. This model solves the time-dependent kinetic rate equation for atomic and molecular hydrogen and their isotopes, taking the presence of physisorbed and chemisorbed sites, as well as quantum mechanical diffusion and thermal hopping, into account. The results show that the time evolution of this system is mainly governed by the binding energies and barriers against migration of the adsorbed species. We have compared the results of our model with experiments on the formation of HD on silicate and carbonaceous surfaces under irradiation by atomic H and D beams at low and at high temperatures. This comparison shows that including both isotopes, both physisorbed and chemisorbed wells, and both quantum mechanical tunneling and thermal hopping is essential for a correct interpretation of the experiments. This comparison allows us to derive the characteristics of these surfaces. For the two surfaces we consider, we determine the binding energy of H atoms and H-2 molecules, as well as the barrier against diffusion for the H atoms to move from one site to another. We conclude that molecular hydrogen formation is efficient until quite high (similar to 500 K) temperatures. At low temperatures, recombination between mobile physisorbed atoms and trapped chemisorbed atoms dominates. At higher temperatures, chemisorbed atoms become mobile, and this then drives molecular hydrogen formation. We have extended our model to astrophysically relevant conditions. The results show that molecular hydrogen formation proceeds with near unity efficiency at low temperatures ( T less than or equal to 20 K). While the efficiency drops, molecular hydrogen formation in the ISM can be very efficient even at high temperatures, depending on the physical characteristics of the surface.

Infrared Space Observatory with the observations of solid carbon dioxide in molecular clouds

Spectra of interstellar CO2 ice absorption features at a resolving power of lambda/Delta lambda approximate to 1500-2000 are presented for 14 lines of sight. The observations were made with the Short-Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO). Spectral coverage includes the primary stretching mode of CO2 near 4.27 mu m in all sources; the bending mode near 15.2 mu m is also detected in 12 of them. The selected sources include massive protostars (Elias 29 [in rho Oph], GL 490, GL 2136, GL 2591, GL 4176, NGC 7538 IRS 1, NCC 7538 IRS 9, S140, W3 IRS 5, and W33 A), sources associated with the Galactic Center (Sgr A*, GCS 3 I, and GCS 4), and a background star behind a quiescent dark cloud in Taurus (Elias 16); they thus probe a diverse range of environments. Column densities of interstellar CO2 ice relative to H2O ice fall in the range 10%-23%: this ratio displays remarkably little variation for such a physically diverse sample. Comparison of the observed profiles with laboratory data for CO2-bearing ice mixtures indicates that CO2 generally exists in at least two phases, one polar (H2O dominant) and one nonpolar (CO2 dominant). The observed CO2 profiles may also be reproduced when the nonpolar components are replaced with thermally annealed ices. Formation and evolutionary scenarios for CO2 and implications for grain mantle chemistry are discussed. Our results support the conclusion that thermal annealing, rather than energetic processing due to UV photons or cosmic rays, dominates the evolution of CO2-bearing ices.

Laboratory studies of the infrared spectral propertries of CO in astrophysical ices

Analysis of laboratory spectra of numerous astrophysical ice analogs demonstrates that the exact band position, width, and profile of the solid state CO fundamental near 2137 cm-1 (4.679 microns) can provide important information on the physical conditions present during the ice accretion phase as well as during any subsequent thermal processes and radiation exposure. In the ices studied, the CO peak position varies from 2134 to 2144 cm-1 (4.686 to 4.664 microns) and the band width from 2.1 to over 20 cm-1 depending on the composition of the ice. In an ice matrix dominated by H2O, the CO peak falls at 2136.7 cm-1, has a full width at half-maximum of about 9 cm-1, and shows a prominent sideband at 2152 cm-1. This sideband and minor structure superposed on the main band arise from CO trapped in different matrix sites. These features provide information concerning the thermal and radiation history of the ice. The solid CO band in interstellar spectra often has contributions from broad (12 cm-1) and narrow (5 cm-1) components. We identify the broad component with CO intimately mixed in matrices dominated by polar molecules, of which H2O is likely to be the major component. Examination of the interstellar and laboratory band profiles shows that either the abundance of nonpoplar impurities in these ices must be less than 10% or the ices have been thermally annealed or processed by ultraviolet radiation. The narrow component is likely to originate from grain mantles dominated by nonpolar molecules such as CO2. These components reflect differences in the physical and chemical conditions in regions of the cloud along the line of sight. Laboratory determination of the absorption strength of the CO fundamental in H2O-rich ices showed that the value used in the past was approximately 60% too low and that most previously determined solid-state CO column densities have been systematically overestimated. The rich spectral behavior of the CO band observed in the laboratory studies clearly indicates that future high-quality astronomical spectra in the 2200-2100 cm-1 range can produce a wealth of new information and provide deeper insights into the nature of astrophysical ices.

A 3-5 mu m VLT spectroscopic survey of embedded young low mass stars I. Structure of the CO ice

Medium resolution (λ/Δλ = 5000-10000) VLT-ISAAC M-band spectra are presented of 39 young stellar objects in nearby low-mass star forming clouds showing the 4.67 μm stretching vibration mode of solid CO. By taking advantage of the unprecedentedly large sample, high S/N ratio and high spectral resolution, similarities in the ice profiles from source to source are identified. It is found that excellent fits to all the spectra can be obtained using a phenomenological decomposition of the CO stretching vibration profile at 4.67 μm into 3 components, centered on 2143.7 cm^(-1),2139.9 cm^(-1), and 2136.5 cm^(-1) with fixed widths of 3.0, 3.5 and 10.6 cm ^(-1), respectively. All observed interstellar CO profiles can thus be uniquely described by a model depending on only 3 linear fit parameters, indicating that a maximum of 3 specific molecular environments of solid CO exist under astrophysical conditions. A simple physical model of the CO ice is presented, which shows that the 2139.9 cm^(-1) component is indistinguishable from pure CO ice. It is concluded, that in the majority of the observed lines of sight, 60-90% of the CO is in a nearly pure form. In the same model the 2143.7 cm^(-1) component can possibly be explained by the longitudinal optical (LO) component of the vibrational transition in pure crystalline CO ice which appears when the background source is linearly polarised. The model therefore predicts the polarisation fraction at 4.67 μm, which can be confirmed by imaging polarimetry. The 2152 cm^(-1) feature characteristic of CO on or in an unprocessed water matrix is not detected toward any source and stringent upper limits are given. When this is taken into account, the 2136.5 cm ^(-1) component is not consistent with the available water-rich laboratory mixtures and we suggest that the carrier is not yet fully understood. A shallow absorption band centered between 2165 cm^(-1) and 2180 cn^(1) is detected towards 30 sources. For low-mass stars, this band is correlated with the CO component at 2136.5 cm^(-1), suggesting the presence of a carrier different from XCN at 2175 cm^(-1). Furthermore the absorption band from solid ^(13)CO at 2092 cm^(-1) is detected towards IRS 51 in the ρ Ophiuchi cloud complex and an isotopic ratio of ^(12)CO/^(13)CO = 68 ± 10 is derived. It is shown that all the observed solid ^(12)CO profiles, along with the solid ^(13)CO profile, are consistent with grains with an irregularly shaped CO ice mantle simulated by a Continuous Distribution of Ellipsoids (CDE), but inconsistent with the commonly used models of spherical grains in the Rayleigh limit.

Molecular Hydrogen Formation in the Interstellar Medium

We have developed a model for molecular hydrogen formation under astrophysically relevant conditions. This model takes fully into account the presence of both physisorbed and chemisorbed sites on the surface, allows quantum mechanical diffusion as well as thermal hopping for absorbed H atoms, and has been benchmarked versus recent laboratory experiments on H2 formation on silicate surfaces. The results show that H2 formation on grain surfaces is efficient in the interstellar medium up to some 300 K. At low temperatures (≤100 K), H2 formation is governed by the reaction of a physisorbed H with a chemisorbed H. At higher temperatures, H2 formation proceeds through a reaction between two chemisorbed H atoms. We present simple analytical expressions for H2 formation that can be adopted to a wide variety of surfaces once their surface characteristics have been determined experimentally.

Detection of abundant CO2 ice in the quiescent dark cloud medium toward Elias 16

We report the first detection of solid carbon dioxide (CO2) in quiescent regions of a dark cloud in the solar neighborhood, a result that has important implications for models of ice formation and evolution in the interstellar medium. The K-type field star Elias 16 was previously known to display solid-state absorption features of H2O and CO ices arising in the Taurus Dark Cloud. Our detection of the CO2 feature at 4.27 μm in this line of sight implies a column density N(CO2)=4.6+ 1.3−0.6×1017 cm-2, equivalent to ~18% and 70% of the H2O and CO column densities, respectively. Comparison with laboratory data indicates that (unlike CO) the CO2 resides primarily in a polar (H2O-rich) component of the ices. CO2 is formed easily in the laboratory by the photolysis of ice mixtures containing CO, but the detection toward Elias 16 indicates that CO2 formation can occur in dark clouds in the absence of a local embedded source of radiation. Possible alternative mechanisms for CO2 production include grain surface reactions and energetic processing driven by the interstellar radiation field or cosmic rays.

Theoretical Modeling of Infrared Emission from Neutral and Charged Polycyclic Aromatic Hydrocarbons. II.

Since the discovery of interstellar infrared emission features in the 3.3-12.7 μm wavelength range three decades ago, the carriers of these features have been the subject of much debate. Recent observational work with the Infrared Space Observatory, experimental work, and quantum chemical calculations concerning positively charged polycyclic aromatic hydrocarbon (PAH) molecules point to the infrared fluorescence of such species. This paper presents a model of the interstellar infrared emission between 3.3 and 12.7 μm from a population of symmetric, condensed polycyclic aromatic hydrocarbons composed of up to 54 carbon atoms. We describe the infrared emission intensity in terms of the size of the emitting molecule, its charge, and its temperature probability distribution function. The model takes the charge state (anion, neutral, cation of charge state up to +3) into account self-consistently, employing the most recent quantum chemically calculated infrared cross sections of such species. This paper provides an exploratory study to illustrate the dependence of the interstellar infrared emission on the polycyclic aromatic hydrocarbon charge distribution. We conclude that the charge state of the PAH has an important effect on the emitted infrared spectrum. The 3.3 μm stretching mode and, to a lesser extent, the 11-15 μm C–H out-of-plane bending modes produce significant emission relative to the other infrared features and originate predominantly from neutral and anionic PAHs. The 6-8 μm emission from the C–C stretching modes, in contrast, originates mainly from charged PAHs with only a partial contribution from neutrals. For heavily ultraviolet irradiated regions such as the Orion Bar, multiply positively charged PAHs are the norm and contribute significantly in this wavelength region. However, because the total infrared emission is a sum over various charge states of different molecules, the ratios of the infrared emission bands do not vary much for Go/ne ≤ 103 cm3. This range includes conditions relevant to both the diffuse interstellar medium and typical reflection nebulae. Larger variations in the interstellar infrared emission features can be expected from photodissociation regions associated with dense H II regions such as the Orion Bar (Go/ne ~ 104).

The H2CO abundance in the inner warm regions of low mass protostellar envelopes

We present a survey of the formaldehyde emission in a sample of eight Class 0 protostars obtained with the IRAM and JCMT telescopes. The data have been analyzed with three different methods with increasing level of sophistication. We first analyze the observed emission in the LTE approximation, and derive rotational temperatures between 11 and 40 K, and column densities between 1 and 20 x 10^13 cm^-2. Second, we use a LVG code and derive larger kinetic temperatures, between 30 and 90 K, consistent with subthermally populated levels and densities from 1 to 6 x 10^5 cm^-3. The column densities from the LVG modeling are within a factor of 10 with respect to those derived in the LTE approximation. Finally, we analyze the observations based upon detailed models for the envelopes surrounding the protostars, using temperature and density profiles previously derived from continuum observations. We approximate the formaldehyde abundance across the envelope with a jump function, the jump occurring when the dust temperature reaches 100 K, the evaporation temperature of the grain mantles. The observed formaldehyde emission is well reproduced only if there is a jump, more than two orders of magnitude, in four sources. In the remaining four sources the data are consistent with a formaldehyde abundance jump, but the evidence is more marginal (~2 sigma). The inferred inner H2CO abundance varies between 1 x 10^-8 and 6 x 10^-6. We discuss the implications of these jumps for our understanding of the origin and evolution of ices in low mass star forming regions. Finally, we give predictions for the submillimeter H2CO lines, which are particularly sensitive to the abundance jumps.

The structure of the time-dependent interstellar shocks and grain destruction in the interstellar medium

A new theoretical analysis of the structure of interstellar shocks and the grain dynamics of these shocks is presented. The basic hydrodynamic equations for J-shocks in interstellar gas are given in which shock-driving pressure is allowed to be weakly time-dependent and the grains are treated as a separate two-dimensional fluid. Specific equations for the grain dynamics in the cooling postshock gas are derived. An analytic theory for the propagation of a shock driven into an interstellar cloud by the blast wave of a supernova remnant which sweeps over the cloud is developed. An improved calculation of the grain charge in postshock gas is described, giving a simple analytic approximation for the results. The consequences of including these processes in the numerical code of Seab and Shull (1983) are addressed, including the effect of realistic magnetic fields in low-density gas. 40 references.

Chemical Rates on Small Grains and PAHs: C+ Recombination and H2 Formation

We use observations of the C I, C II, H I, and -->H2 column densities along lines of sight in the Galactic plane to determine the formation rate of -->H2 on grains and to determine chemical reaction rates with polycyclic aromatic hydrocarbons (PAHs). Photodissociation region models are used to find the best-fit parameters to the observed columns. We find the -->H2 formation rate on grains has a low rate ( -->R ~ 1 × 10−17 cm3 s−1) along lines of sight with low column density ( -->AV 0.25) and low molecular fraction ( -->fH2 10−4). At higher column densities ( -->0.25 ≤ AV ≤ 2.13), we find a rate of -->R ~ 3.5 × 10−17 cm3 s−1. The lower rate at low column densities could be the result of grain processing by interstellar shocks, which may deplete the grain surface area or process the sites of -->H + H formation, thereby inhibiting -->H2 production. Alternatively, the formation rate may be normal, and the low molecular fraction may be the result of lines of sight that graze larger clouds. Such lines of sight would have a reduced -->H2 self-shielding compared to the line-of-sight column. We find the reaction -->C+ + PAH−→ C + PAH0 is best fit with a rate -->2.4 × 10−7ΦPAHT−0.52 cm3 s−1 with -->T2 = T/100 K, and the reaction -->C+ + PAH0→ C + PAH+ is best fit with a rate -->8.8 × 10−9ΦPAH cm3 s−1. In high-column-density gas, we find -->ΦPAH ~ 0.4. In low-column-density gas, -->ΦPAH is less well constrained, with -->ΦPAH ~ 0.2–0.4.