arXiv subjects

William J. Henney

Publications and source records attributed to William J. Henney.

24 records · Page 2Linked to original sources

arXiv2007

Merged ionization/dissociation fronts in planetary nebulae

The hydrogen ionization and dissociation front around an ultraviolet radiation source should merge when the ratio of ionizing photon flux to gas density is sufficiently low and the spectrum is sufficiently hard. This regime is particularly relevant to the molecular knots that are commonly found in evolved planetary nebulae, such as the Helix Nebula, where traditional models of photodissociation regions have proved unable to explain the high observed luminosity in H_2 lines. In this paper we present results for the structure and steady-state dynamics of such advection-dominated merged fronts, calculated using the Cloudy plasma/molecular physics code. We find that the principal destruction processes for H_2 are photoionization by extreme ultraviolet radiation and charge exchange reactions with protons, both of which form H_2^+, which rapidly combines with free electrons to undergo dissociative recombination. Advection moves the dissociation front to lower column densities than in the static case, which vastly increases the heating in the partially molecular gas due to photoionization of He^0, H_2, and H^0. This causes a significant fraction of the incident bolometric flux to be re-radiated as thermally excited infrared H_2 lines, with the lower excitation pure rotational lines arising in 1000 K gas and higher excitation H_2 lines arising in 2000 K gas, as is required to explain the H_2 spectrum of the Helix cometary knots.

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arXiv2007

Enrichment of the ISM by metal-rich droplets and the abundance bias in HII regions

We critically examine a scenario for the enrichment of the interstellar medium (ISM) in which supernova ejecta follow a long (10^8 yr) journey before falling back onto the galactic disk in the form of metal-rich ``droplets'', These droplets do not become fully mixed with the interstellar medium until they become photoionized in HII regions. We investigate the hypothesis that the photoionization of these highly metallic droplets can explain the observed ``abundance discrepancy factors'' (ADFs), which are found when comparing abundances derived from recombination lines and from collisionally excited lines, both in Galactic and extragalactic HII regions. We derive bounds of 10^{13}--10^{15} cm on the droplet sizes inside HII regions in order that (1) they should not have already been detected by direct imaging of nearby nebulae, and (2) they should not be too swiftly destroyed by diffusion in the ionized gas. From photoionization modelling we find that, if this inhomogeneous enrichment scenario holds, then the recombination lines strongly overestimate the metallicities of the fully mixed HII regions. The abundances derived from collisionally excited lines also suffer some bias, although to a much lesser extent. In the absence of any recipe for correcting these biases, we recommend the discarding of all objects showing large ADFs from studies of galactic chemical evolution. These biases must also be kept in mind when comparing the galactic abundance gradients for elements derived from recombination lines with those derived from collisionally excited lines. Finally, we propose a set of observations that could be undertaken to test our scenario and improve our understanding of element mixing in the ISM.

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arXiv2006

Dynamical HII Region Evolution in Turbulent Molecular Clouds

We present numerical radiation-hydrodynamic simulations of the evolution of HII regions formed in an inhomogeneous medium resulting from turbulence simulations. We find that the filamentary structure of the underlying density distribution produces a highly irregular shape for the ionized region, in which the ionization front escapes to large distances in some directions within 80,000 years. In other directions, on the other hand, neutral gas in the form of dense globules persists within 1 parsec of the central star for the full duration of our simulation (400,000 years). Divergent photoablation flows from these globules maintain a root-mean-squared velocity in the ionized gas that is close to the ionized sound speed. Simulated images in optical emission lines show morphologies that are in strikingly detailed agreement with those observed in real HII regions.

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arXiv2006

How to move ionized gas: an introduction to the dynamics of HII regions

This review covers the dynamic processes that are important in the evolution and structure of galactic HII regions, concentrating on an elementary presentation of the physical concepts and recent numerical simulations of HII region evolution in a non-uniform medium. The contents are as follows: (1) The equations (Euler equations; Radiative transfer; Rate equations; How to avoid the dynamics; How to avoid the atomic physics). (2) Physical concepts (Static photoionization equilibrium; Ionization front propagation; Structure of a D-type front; Photoablation flows; Other ingredients - Stellar winds, Radiation pressure, Magnetic fields, Instabilities). (3) HII region evolution (Early phases: hypercompact and ultracompact regions; Later phases: compact and extended regions; Clumps and turbulence).

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arXiv2005

Self-Consistent Dynamic Models of Steady Ionization Fronts I. Weak-D and Weak-R Fronts

We present a method for including steady-state gas flows in the plasma physics code Cloudy, which was previously restricted to modeling static configurations. The numerical algorithms are described in detail, together with an example application to plane-parallel ionization-bounded HII regions. As well as providing the foundation for future applications to more complex flows, we find the following specific results regarding the effect of advection upon ionization fronts in HII regions: 1. Significant direct effects of advection on the global emission properties occur only when the ionization parameter is lower than is typical for HII regions. 2. The overheating of partially ionized gas in the front is not large, even for supersonic (R-type) fronts. 3. The most significant morphological signature of advective fronts is an electron density spike that occurs at the ionization front. Observational evidence for such a spike is found in images of the Orion bar. 4. Plane-parallel, weak-D fronts are found to show at best a shallow correlation between mean velocity and ionization potential for optical emission lines even when the flow velocity closely approaches the ionized sound speed.

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arXiv2001

An Efficient Monte Carlo Algorithm for a Restricted Class of Scattering Problems in Radiation Transfer

We describe an efficient Monte Carlo algorithm for a restricted class of scattering problems in radiation transfer. This class includes many astrophysically interesting problems, including the scattering of ultraviolet and visible light by grains. The algorithm correctly accounts for multiply-scattered light. We describe the algorithm, present a number of important optimizations, and explicity show how the algorithm can be used to estimate quantities such as the emergent and mean intensity. We present two test cases, examine the importance of the optimizations, and show that this algorithm can be usefully applied to optically-thin problems, a regime sometimes considered limited to explicit single-scattering plus attenuation approximations.

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