Dark cloud chemistry in initially H-rich regions

The chemistry in dark regions of dense cores is explored as a function of the initial abundance ratio of H to H 2, on the assumption that some cores form on a timescale and are younger than the time required for the H :H 2 ratio to attain its equilibrium value. Observational diagnostics of non-equilibrium values of the initial H :H 2 ratio are identified. In initially H-rich material, the abundances of OH, NH 3, CN, and HNC are for some time higher than they are in initially H-poor material. In initially H-poor regions, the abundances of CO, species containing multiple carbon atoms in each molecule, and CS are larger for an (observationally significant) period than in initially H-rich material

Nanodust shedding and its potential influence on dust related phenomena in the mesosphere

We explore the possibility that some meteoric smoke particles that collide with larger nanoparticles near the mesopause can escape from the larger particles by capturing surface electrons. If the process were sufficiently efficient, under certain conditions it would influence the responses of polar mesospheric summer echoes to artificial heating in a manner that is compatible with observations that are unexplained with previous models. The process would have a number of other possible consequences for nanoparticles near the mesopause

The generation of optical emission-line filaments in galaxy clusters

Recent data support the idea that the filaments observed in H_alpha emission near the centres of some galaxy clusters were shaped by bulk flows within their intracluster media. We present numerical simulations of evaporated clump material interacting with impinging winds to investigate this possibility. In each simulation, a clump falls due to gravity while the drag of a wind retards the fall of evaporated material leading to elongation of the tail. However, we find that long filaments can only form if the outflowing wind velocity is sufficiently large, 10^8 cm s^-1. Otherwise, the tail material sinks almost as quickly as the cloud. For reasonable values of parameters, the morphological structure of a tail is qualitatively similar to those observed in clusters. Under certain conditions, the kinematics of the tail resemble those reported in Hatch et al.(2006). A comparison of the observations with the numerical results indicates that the filaments are likely to be a few tens of Myrs old. We also present arguments which suggest that the momentum transfer, from an outflowing wind, in the formation of these filaments is probably significant. As a result, tail formation could play a role in dissipating some of the energy injected by a central AGN close to the cluster centre where it is needed most. The trapping of energy by the cold gas may provide an additional feedback mechanism that helps to regulate the heating of the central regions of galaxy clusters and couple the AGN to the ICM.Comment: 15 pages, 18 figures. Altered density values in table 1, which were a factor of 1000 too small. Also extended caption for table 1 to show the mass outflow rate. Accepted for publication in MNRA

The interaction of hydrodynamic shocks with self-gravitating clouds

We describe the results of 3D simulations of the interaction of hydrodynamic shocks with Bonnor-Ebert spheres performed with an Adaptive Mesh Refinement code. The calculations are isothermal and the clouds are embedded in a medium in which the sound speed is either four or ten times that in the cloud. The strengths of the shocks are such that they induce gravitational collapse in some cases and not in others and we derive a simple estimate for the shock strength required for this to occur. These results are relevant to dense cores and Bok globules in star forming regions subjected to shocks produced by stellar feedback

The modifcation by diffuse radiation of "cometary tail" formation behind globules

We study the evolution of a globule of neutral material immersed in the more tenuous hotter plasma of an H II region surrounding newly born OB stars. The neutral globule is illuminated by the direct ionizing radiation of OB stars, and by diffuse radiation emitted by recombination in the surrounding ionized gas. We perform 2D, time dependent axisymmetric hydrodynamic simulations, and find that, for values of the diffuse field of the order of 10% of the direct field, the evolution of the globule is completely different to its evolution when the diffuse field is neglected

The Formation of Broad Emission Line Regions in Supernova-QSO Wind Interactions

We show that a cooled region of shocked supernova ejecta forms in a type II supernova-QSO wind interaction, and has a density, an ionization parameter, and a column density compatible with those inferred for the high ionization component of the broad emission line regions in QSOs. The calculations are based on the assumption that the ejecta flow is described initially by a similarity solution investigated by Chevalier (1982) and Nadyozhin (1985) and is spherically symmetric. Heating and cooling appropriate for gas irradiated by a nearby powerful continuum source is included in our model, together with reasonable assumptions for the properties of the QSO wind. The model results are also in agreement with observational correlations and imply reasonable supernova rates.Comment: 13 pages, 7 figures, to be published in A&

Magnetic ionization fronts II: Jump conditions for oblique magnetization

We present the jump conditions for ionization fronts with oblique magnetic fields. The standard nomenclature of R- and D-type fronts can still be applied, but in the case of oblique magnetization there are fronts of each type about each of the fast- and slow-mode speeds. As an ionization front slows, it will drive first a fast- and then a slow-mode shock into the surrounding medium. Even for rather weak upstream magnetic fields, the effect of magnetization on ionization front evolution can be important. [Includes numerical MHD models and an application to observations of S106.]Comment: 9 pages, 10 figures, Latex, to be published in MNRA

Erratum: The chemistry of transient molecular cloud cores

We assume that some, but not all, of the structure observed in molecular clouds is associated with transient features which are not bound by self-gravity. We investigate the chemistry of a transient density fluctuation, with properties similar to those of a core within a molecular cloud. We run a multipoint chemical code through a core's condensation from a diffuse medium to its eventual dispersion, over a period of ∼1 Myr. The dynamical description adopted for our study is based on an understanding of a particular mechanism, involving slow-mode wave excitation, for transient structure formation which so far has been studied in detail only with plane-parallel models in which self-gravity has not been included. We find a significant enhancement of the chemical composition of the core material on its return to diffuse conditions, whilst the expansion of the core as it disperses moves this material out to large distances from the core centre. This process transports molecular species formed in the high-density regions out into the diffuse medium. Chemical enrichment of the cloud as a whole also occurs, as other cores of various sizes, life-spans and separations evolve throughout. Enrichment is strongly affected by freeze-out on to dust grains, which takes place in high-density, high visual extinction regions. As the core disperses after reaching its peak density and the visual extinction drops below a critical value, grain mantles are evaporated back into the gas phase, initiating more chemistry. The influence of the sizes, masses and cycle periods of cores will be large both for the level of chemical enrichment of a dark cloud and ultimately for the low-mass star formation rate. The cores in which stars form are almost certainly bound by their self-gravity and are not transient in the sense that the cores on which most of our study is focused are transient. Obviously, enrichment of the chemistry of low-density material will not take place if self-gravity prevents the re-expansion of a core. We also consider the case of a self-gravitating core, by holding its peak density conditions for a further 0.4 Myr. We find that the differences near the peak densities between transient and gravitationally bound cores are generally small, and the resultant column densities for objects near the peak densities do not provide definitive criteria for discriminating between transient and bound cores. However, increases in fractional abundances due to reinjection of mantle-borne species may provide a criterion for detection of a non-bound core

H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate

Using high resolution infrared spectroscopy we have surveyed twenty sightlines for H3+ absorption. H3+ is detected in eight diffuse cloud sightlines with column densities varying from 0.6x10^14 cm^-2 to 3.9x10^14 cm^-2. This brings to fourteen the total number of diffuse cloud sightlines where H3+ has been detected. These detections are mostly along sightlines concentrated in the Galactic plane, but well dispersed in Galactic longitude. The results imply that abundant H3+ is common in the diffuse interstellar medium. Because of the simple chemistry associated with H3+ production and destruction, these column density measurements can be used in concert with various other data to infer the primary cosmic-ray ionization rate, zeta_p. Values range from 0.5x10^-16 s^-1 to 3x10^-16 s^-1 with an average of 2x10^-16 s^-1. Where H3+ is not detected the upper limits on the ionization rate are consistent with this range. The average value of zeta_p is about an order of magnitude larger than both the canonical rate and rates previously reported by other groups using measurements of OH and HD. The discrepancy is most likely due to inaccurate measurements of rate constants and the omission of effects which were unknown when those studies were performed. We believe that the observed column density of H3+ is the most direct tracer for the cosmic-ray ionization rate due to its simple chemistry. Recent models of diffuse cloud chemistry require cosmic-ray ionization rates on the order of 10^-16 s^-1 to reproduce observed abundances of various atomic and molecular species, in rough accord with our observational findings.Comment: Accepted to ApJ, 35 pages, 5 figures, 5 table