New models of interstellar gas–grain chemistry – II. Surface photochemistry in quiescent cores

The photodissociation of surface species, caused by photons from the cosmic-ray-induced and background interstellar radiation fields, is incorporated into our combined gas-phase and grain-surface chemical models of quiescent dense interstellar cores. For the cores studied here, only cosmic-ray-induced photons are important. We find that photodissociation alters gas-phase and surface abundances mainly at large cloud ages (≳ 10^6–7 yr). The abundances of those surface species, such as H₂O, that are readily reproduced on the surface following photodissociation are not strongly affected at any time. The abundances of surface species that are, on the other hand, reformed slowly via surface reactions possessing activation energy (e.g. CH₃OH) are reduced, while the abundances of associated surface photoproducts (e.g. CO) increase. In the gas phase, inclusion of surface photodissociation tends to increase molecular abundances at late times, slightly improving the agreement with observation for TMC-1.     

New models of interstellar gas–grain chemistry – I. Surface diffusion rates

In recent years it has become evident that large differences can exist between model results of grain-surface chemistry obtained from a rate equation approach and from a Monte Carlo technique. This dichotomy has led to the development of a modified rate equation method, in which a key element is the artificial slowing down of the diffusion rate of surface hydrogen atoms. Recent laboratory research into the surface diffusion rate of atomic hydrogen suggests that atomic hydrogen moves more slowly on grains than heretofore assumed. This research appears to lessen the need for modifications to the rate equation method. Based on the new laboratory work, we have developed appropriate models of gas-phase and grain-surface chemistry in quiescent dense cloud cores to examine the chemical effects of slowing down the rate at which atomic H can scan over dust surfaces. Furthermore, we have investigated the effect of slowing down the rate at which all species can move over grain surfaces.     

Interstellar deuteroammonia

Close to 30 deuterated molecules have now been detected in the ISM, including doubly-deuterated species D₂H⁺, ND₂H, D₂CO, CHD₂OH, D₂S, and D₂CS, as well as triply-deuterated ammonia and methanol. We review the current understanding of depletion and deuteration processes in cold, dense interstellar medium (ISM) and discuss the utility of deuteroammonia as a tracer of the physical conditions and kinematics of cold, dense gas.     

New models of interstellar gas–grain chemistry – III. Solid CO2

Solid CO₂ is observed to be an abundant interstellar ice component towards both quiescent clouds and active star-forming regions. Our recent models of gas–grain chemistry, appropriate for quiescent regions, severely underproduce solid CO₂ at the single assumed gas density and temperature. In this paper, we investigate the sensitivity of our model results to changes in these parameters. In addition, we examine how the nature of the grain surface affects the results and also consider the role of the key surface reaction between O and CO. We conclude that the observed high abundance of solid CO₂ can be reproduced at reasonable temperatures and densities by models with diffusive surface chemistry, provided that the diffusion of heavy species such as O occurs efficiently.     

On the CH B‐X (1, 0) band in translucent clouds

We present observations of CH B‐X (1, 0) band toward 13 translucent sightlines. The given precise positions (at 3627.403, 3633.289, 3636.222 Å) are derived by means of shifting our spectra to the rest velocity frame using B‐X (0, 0) features (at 3878, 3886 and 3890 Å). The proposed oscillator strengths (equal to 0.00035, 0.00104, 0.00069, respectively) are based on f ‐values of the CH B‐X (0, 0) system, i.e. 0.00110, 0.00320, 0.00210 (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interstellar Na i D lines towards the Southern Coalsack

The interstellar Na  i D absorption-line profiles observed for 15 stars with lines of sight towards the Southern Coalsack are analysed. The method of profile fitting was used in an attempt to determine column densities, linewidths and velocities for the individual interstellar clouds contributing to the observed absorption lines. In common, the observed spectra show a prominent component which is probably associated with the nearest absorbing material composing the Coalsack. The obtained spatial velocity distribution shows great similarity with earlier results from CO emission. In addition, the Na  i D data reveal evidence for the existence of two or three other structures with radial velocities of about −22, −33 and −40 km s⁻¹. Such components may be the counterparts of interstellar structures observed in diffuse H α and CO emission. The assumption that at least one of these components originated in the Carina arm imposes ∼0.9–1.0 kpc as the maximum distance to the near side of that arm.     

Molecular line intensities as measures of cloud masses – I. Sensitivity of CO emissions to physical parameter variations

A reliable estimate of the molecular gas content in galaxies plays a crucial role in determining their dynamical and star-forming properties. However, H₂, the dominant molecular species, is difficult to observe directly, particularly in the regions where most molecular gas is thought to reside. Its mass is therefore commonly inferred by assuming a direct proportionality with the integrated intensity of the  ¹²CO( J = 1 → 0)  emission line, using a CO-to-H₂ conversion factor, X . Although a canonical value for X is used extensively in such estimates, there is increasing evidence, both theoretical and observational, that the conversion factor may vary by over an order of magnitude under conditions different from those of the local neighbourhood. In an effort to understand the influence of changing environmental conditions on the conversion factor, we derive theoretical estimates of X for a wide range of physical parameters using a photon-dominated region (PDR) time-dependent chemical model, benchmarking key results against those of an independent PDR code to ensure reliability. Based on these results, the sensitivity of the X factor to change in each physical parameter is interpreted in terms of the chemistry and physical processes within the cloud. In addition to confirming previous observationally derived trends, we find that the time-dependence of the chemistry, often neglected in such models, has a considerable influence on the value of the conversion factor.     

Molecular line mapping of the giant molecular cloud associated with RCW 106 – II. Column density and dynamical state of the clumps

We present a fully sampled C¹⁸O (1–0) map towards the southern giant molecular cloud (GMC) associated with the H  ii region RCW 106, and use it in combination with previous ¹³CO (1–0) mapping to estimate the gas column density as a function of position and velocity. We find localized regions of significant ¹³CO optical depth in the northern part of the cloud, with several of the high-opacity clouds in this region likely associated with a limb-brightened shell around the H  ii region G333.6−0.2. Optical depth corrections broaden the distribution of column densities in the cloud, yielding a lognormal distribution as predicted by simulations of turbulence. Decomposing the ¹³CO and C¹⁸O data cubes into clumps, we find relatively weak correlations between size and linewidth, and a more sensitive dependence of luminosity on size than would be predicted by a constant average column density. The clump mass spectrum has a slope near −1.7, consistent with previous studies. The most massive clumps appear to have gravitational binding energies well in excess of virial equilibrium; we discuss possible explanations, which include magnetic support and neglect of time-varying surface terms in the virial theorem. Unlike molecular clouds as a whole, the clumps within the RCW 106 GMC, while elongated, appear to show random orientations with respect to the Galactic plane.     

Dynamical Molecular Regions in Interstellar Clouds

Work on three problems concerning the chemistry and dynamics of star forming fregions is addressed. Most of the work was done in collaboration with David A. Williams. A chemical model of core D ofTMC-1 is described; it is consistent with that dense core having evolved to its present physical state in the last 10⁵years. Slow-mode wave excitation by the nonlinear evolution of fast-mode waves is examined as a possible mechanism for the production of structures, including densecores, in molecular sources. Sufficient atomic hydrogen may be present at the births of some dense cores that the radiative association of O with H is important in the initiation of the gas phase chemistry. This revised version was published online in July 2006 with corrections to the Cover Date.     

Molecular line intensities as measures of cloud masses – II. Conversion factors for specific galaxy types

We present theoretically established values of the CO-to-H₂ and C-to-H₂ conversion factors that may be used to estimate the gas masses of external galaxies. We consider four distinct galaxy types, represented by M51, NGC 6946, M82 and SMC N27. The physical parameters that best represent the conditions within the molecular clouds in each of the galaxy types are estimated using a χ² analysis of several observed atomic fine structure and CO rotational lines. This analysis is explored over a wide range of density, radiation field, extinction and other relevant parameters. Using these estimated physical conditions in methods that we have previously established, CO-to-H₂ conversion factors are then computed for CO transitions up to J = 9 → 8. For the conventional CO(1–0) transition, the computed conversion factor varies significantly below and above the canonical value for the Milky Way in the four galaxy types considered. Since atomic carbon emission is now frequently used as a probe of external galaxies, we also present, for the first time, the C-to-H₂ conversion factor for this emission in the four galaxy types considered.     

Detection of HCO+ towards Cygnus OB2 No. 12

HCO⁺ has been detected for the first time towards the star Cygnus OB2 No. 12 through emission of the 1–0 rotational transition at 89 GHz. The CO( J =2−1) transition has also been observed. The observations are consistent with a model of dense regions embedded in a low-density clump gas. If actually present, the dense component would have an aggregate size L 1300 au, in agreement with estimates of small-scale density fluctuations observed along diffuse lines of sight.     

HE 1127–1304: the largest radio quasar

By considering the propagation of low-amplitude magnetohydrodynamic waves in partially ionized plasmas, it is shown that the ion-neutral drift (ambipolar diffusion) induced by the waves can have specific effects on the molecular chemistry of cold material. The chemistry occurring in gas swept by Alfvén waves is described and it is shown that this leads to spatial variations in the deuterium fractionation ratios of, for example, HCO⁺ and N₂H⁺, on spatial scales of a few hundredths of a parsec, depending upon the fractional ionization of the ambient medium. The possibility of detecting interstellar Alfvén waves by molecular spectroscopy and their effect of producing small-scale chemical abundance gradients in molecular clouds are briefly discussed.     

Hydrodynamical simulations of Galactic fountains – I. Evolution of single fountains

The ejection of the gas out of the disc in late-type galaxies is related to star formation and is due mainly to Type II supernovae. In this paper, we studied in detail the development of the Galactic fountains in order to understand their dynamical evolution and their influence on the redistribution of the freshly delivered metals over the disc. To this aim, we performed a number of 3D hydrodynamical radiative cooling simulations of the gas in the Milky Way where the whole Galaxy structure, the Galactic differential rotation and the supernova explosions generated by a single OB association are considered. A typical fountain powered by 100 Type II supernovae may eject material up to ∼2 kpc which than collapses back mostly in the form of dense, cold clouds and filaments. The majority of the gas lifted up by the fountains falls back on the disc remaining within a radial distance  Δ R = 0.5 kpc  from the place where the fountain originated. This localized circulation of disc gas does not influence the radial chemical gradients on large scale, as required by the chemical models of the Milky Way which reproduce the metallicity distribution without invoking large fluxes of metals. Simulations of multiple fountains fuelled by Type II supernovae of different OB associations will be presented in a companion paper.     

Molecular line mapping of the giant molecular cloud associated with RCW 106 – III. Multimolecular line mapping

We present multimolecular line maps obtained with the Mopra telescope towards the southern giant molecular cloud (GMC) complex G333, associated with the H  ii region RCW 106. We have characterized the GMC by decomposing the 3D data cubes with gaussclumps , and investigated spatial correlations among different molecules with principal component analysis (PCA). We find no correlation between clump size and linewidth, but a strong correlation between emission luminosity and linewidth. PCA classifies molecules into high- and low-density tracers, and reveals that HCO⁺ and N₂H⁺ are anticorrelated.     

Photoionization modelling of planetary nebulae – II. Galactic bulge nebulae, a comparison with literature results

We have constructed photoionization models of five galactic bulge planetary nebulae using our automatic method, which enables a fully self-consistent determination of the physical parameters of a planetary nebula. The models are constrained using the spectrum, the IRAS and radio fluxes and the angular diameter of the nebula. We also conducted a literature search for physical parameters determined with classical methods for these nebulae. Comparison of the distance-independent physical parameters with published data shows that the stellar temperatures generally are in good agreement and can be considered reliable. The literature data for the electron temperature, electron density and also for the abundances show a large spread, indicating that the use of line diagnostics is not reliable and that the accuracy of these methods needs to be improved. Comparison of the various abundance determinations indicates that the uncertainty in the electron temperature is the main source of uncertainty in the abundance determination. The stellar magnitudes predicted by the photoionization models are in good agreement with observed values.     

Calculations on the rates, mechanisms, and interstellar importance of the reactions between C and NH2 and between N and CH2

The attempt to understand the temperature dependence of the HNC/HCN abundance ratio in interstellar clouds has been long standing and indecisive. In this paper we report quantum chemical and dynamical studies of two neutral–neutral reactions thought to be important in the formation of HNC and HCN, respectively – C+NH₂→HNC+H, and N+CH₂→HCN+H. We find that although these reactions do lead initially to the products suggested by astronomers, there is so much excess energy available in both reactions that the HCN and HNC products are able to undergo efficient isomerization reactions after production. The isomerization leads to near equal production rates of the two isomers, with HNC slightly favoured if there is sufficient rotational excitation. This result has been incorporated into our latest chemical model network of dense interstellar clouds.