Numerical Simulations of Non-Equilibrium Plasmas and their Spectra Applications to Star-Forming Regions

Grants and Contracts Details


PROJECT SUMMARY Intellectual Merit Much of our understanding of the chemical and physical evolution of the universe comes from the analysis of spectral lines produced by interstellar gas. This project is a simultaneous attack on a range of star-forming environments, extending from nearby H II regions to the ultraluminous infrared galaxies and luminous quasars, with the common theme of developing the theoretical tools needed to chart their physical and chemical evolution. Interstellar matter is far from equilibrium and the observed spectrum is sensitive to a host of microphysical processes. Large-scale numerical simulations are the best way to decipher the message in the spectrum. This dependence on details is a complication, but it is also why the spectrum reveals so much about the properties of the gas. This work centers on the development and application of the large-scale plasma simulation code Cloudy, a multi-pronged effort that combines plasma simulations, numerical radiative transport theory, and atomic / molecular physics. The goal is to establish a theoretical understanding of the underlying physics that governs what we observe. The problems outlined below are strongly interwoven and progress from the relatively simple nearby Orion Nebula, (an important benchmark) to luminous H II regions within the galaxy and eventually to the ultraluminous infrared galaxies and quasars. The observed gas is often a flow from a molecular cloud irradiated by light from an active nucleus or newly-formed star cluster. Light heats and ionizes the gas, causing it to become overpressured and flow from Hz to HOand eventually into H+regions. Cloudy has long been able to fully simulate conditions within an ionized gas. Previous NSF support allowed the development of a complete model of the hydrogen molecule and the code's extension into fully molecular regions. Cloudy now predicts the entire spectrum, including lines of Hz, H I, ions, and thermal dust emission. This allows the full spectrum to be understood on a holistic basis since the Hz / HO/ H+regions constitute a single physical problem within a continuous variation in the gas properties and transmitted spectrum. This is a unique aspect of our work and will provide additional insights into the gas composition and energy source. Work continues on the grain physics, especially its emission and collisional interactions with the gas, and emission from other molecular species. A series of projects are undertaken applying this framework to increasingly complex environments. The Orion bar is a Hz / HO/ H+transition region that is viewed nearly edge on. Recent observations suggest that either the Hz level populations do not trace the kinetic temperature or that this temperature is surprisingly high. This is a fundamental test of the physics that will be applied to high luminosity and redshift objects. The variation of PAH emission features across the bar offers insight into PAH formation, destruction, and excitation mechanisms, but requires some enhancements to our current treatment of grain physics. The M17 complex is one of the few where the magnetic field within the HOregion has been mapped. Deep optical spectra allow us to directly measure the gas pressure in the H+region along the same sight line where 21 em Zeeman splitting has measured the HOregion magnetic field. The gas equation of state, the relationship between density and pressure, changes from magnetic/turbulent domination in the HOregion to gas pressure domination in the H+region, underscoring the importance of magnetic fields. The field cannot be ignored in predicting the relationships between molecular, atomic, and ionized gas. This work will be extended to extragalactic H II regions, the class of objects used to measure galactic abundance gradients and the primordial helium abundance. The work then progresses to the most luminous objects, the ultraluminous infrared galaxies and quasars, with their associated questions concerning chemical and stellar evolution during the first stages
Effective start/end date5/15/064/30/10


  • National Science Foundation: $469,953.00


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