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.Comment: 4 pages, accepted by ApJL, scheduled December 20 issu

The Three-Dimensional Ionization Structure and Evolution of NGC 6720, The Ring Nebula

We have determined the gas kinematics, diagnostic and ionic radial profiles, spatial structure, and evolutionary phase of NGC 6720 (the Ring Nebula) by means of tomography and a three-dimensional recovery technique applied to long-slit high-resolution spectra. The main shell of the Ring Nebula is a triaxial ellipsoid (radii of 0.10, 0.13, and 0.20 pc) seen nearly pole-on and expanding in an approximately ballistic fashion (Vexp = 0.65 km s-1 arcsec-1). The central star characteristics [log(L*/L?) 2.3, T* 120,000 K], combined with the nebular age of 7000 yr, indicate that the M* 0.61-0.62 M? post-AGB star is approaching the white dwarf cooling sequence. The equator of the Ring Nebula is optically thick and much denser than the optically thin poles. The inner halo surrounding NGC 6720 represents the pole-on projection of the AGB wind at high latitudes (circumpolar) directly ionized by the central star, whereas the outer, fainter, and circular halo is the projection of the recombining AGB wind at mean to low latitudes, shadowed by the main nebula. The spatio-kinematical properties of the Ring Nebula and the origin of the dense knots commonly observed in late-stage planetary nebulae are critically compared with the predictions of radiation-hydrodynamic and wind interaction models

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.Comment: ApJ, main journal, in press 1 March 2005, v621 (22 pages, 3 appendices, 21 figures) PDF version with high-res figures available from http://www.astrosmo.unam.mx/~w.henney/astro-ph/0501034

Can solid body destruction explain abundance discrepancies in planetary nebulae?

In planetary nebulae, abundances of oxygen and other heavy elements derived from optical recombination lines are systematically higher than those derived from collisionally excited lines. We investigate the hypothesis that the destruction of solid bodies may produce pockets of cool, high-metallicity gas that could explain these abundance discrepancies. Under the assumption of maximally efficient radiative ablation, we derive two fundamental constraints that the solid bodies must satisfy in order that their evaporation during the planetary nebula phase should generate a high enough gas phase metallicity. A local constraint implies that the bodies must be larger than tens of meters, while a global constraint implies that the total mass of the solid body reservoir must exceed a few hundredths of a solar mass. This mass greatly exceeds the mass of any population of comets or large debris particles expected to be found orbiting evolved low- to intermediate-mass stars. We therefore conclude that contemporaneous solid body destruction cannot explain the observed abundance discrepancies in planetary nebulae. However, similar arguments applied to the sublimation of solid bodies during the preceding asymptotic giant branch (AGB) phase do not lead to such a clear-cut conclusion. In this case, the required reservoir of volatile solids is only one ten-thousandth of a solar mass, which is comparable to the most massive debris disks observed around solar-type stars, implying that this mechanism may contribute to abundance discrepancies in at least some planetary nebulae, so long as mixing of the high metallicity gas is inefficient.Comment: 8 pages, no figures, ApJ in pres

Video Relay Service for Deaf people using WebRTC

This paper reports on an experimental open source video relay service prototype that helps Deaf people communicate with hearing people by accessing a third party sign language interpreter on a mobile device. Deaf people are disadvantaged in many ways when communicating with the hearing world in real world scenarios, such as hospital visits and in cases of emergency. When possible, Deaf people can enlist the assistance of a family member, community worker or sign language interpreter to assist with such scenarios, however this assistance is pre-arranged and Deaf people would prefer on-the-fly assistance. Our application will assist Deaf people to contact any available sign language interpreter to facilitate communication between the Deaf person and a hearing person using a split screen model, effectively creating a three-way conversation between the Deaf person, the hearing person and the sign language interpreter. The prototype was developed using the WebRTC platform, with JavaScript for browser operability and hardware platform independence. Our hope is that the research can be used to persuade mobile network operators of the need for free or heavily discounted data connection to relay services for Deaf mobile customers

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.Comment: Minor changes to sync with accepted version. 7 pages, ApJ in press. Accompanying video available at http://ifront.org/wiki/Turbulent_Hii_Regions/Paper