This project is a continuation of Dr. Frank and Dr. Blackman's efforts better to understand the process of planetary nebula formation and the late stages of evolution of low and intermediate mass stars. As the final stage of mass loss for such stars, planetary nebulae represent a critical step in the mass and chemical evolution cycle for more than half the material ejected into the interstellar medium. The ubiquity of planetary nebulae and their ease of observation have also made them premier laboratories for testing new astrophysical theories.
Recently, high-resolution images of planetary nebulae and their progenitors (pre-planetary nebulae) have triggered a critical re-evaluation of the dominant model for both nebular shaping and stellar evolution. Studies of pre-planetary nebulae show them to be collimated and to drive energetic outflows that lack sufficient luminosity for radiative driving. The launching or collimation of pre-planetary nebulae, and by association planetary nebulae, therefore cannot be considered to be understood. In light of new data the theory of pre-planetary and planetary nebulae evolution and, by implication, ideas about processes at work in the late stages of stellar evolution are undergoing fundamental revision. A new picture is emerging in which binaries and magnetohydrodynamic (MHD) processes are responsible for the majority of pre-planetary and planetary nebulae. Previous work by Drs. Frank and Blackman has established the potential efficacy of magneto-centrifugal launching processes in these systems. They have shown the pathways by which binary companions can create conditions for MHD launching, including the efficacy of dynamo processes in binary stars and its limits in single stars.
This project will build on this progress with three specific goals: 1) to provide a more accurate account of the field conditions supplied by dynamos in binaries, 2) to explore the formation of disks, the outflows generated by such disks, and the morphology and observational signatures produced, and 3) to understand the generation of magnetic tower explosions by binary driven dynamos. Goals 2 and 3 will be achieved using a new adaptive mesh refinement MHD code (AstroBEAR) developed by Dr. Frank and collaborators. Additionally, the project will provide continued theoretical support for a series of laboratory astrophysics experiments at Imperial College in London, which are of broad interest to astrophysics and directly relevant to pre-planetary and planetary nebulae.
Collimated outflows from a central gravitating source are a ubiquitous phenomenon in astrophysics. Jets and wider bipolar outflows are observed in newly forming stars, highly evolved high mass stars, compact objects in binaries, starburst galaxies, and supermassive black holes powering active galactic nuclei. Collimated outflows are also invoked in models for supernovae and gamma ray bursts. While this work is focused on planetary nebulae it will be of direct relevance to these other fields both theoretically and observationally in the sense of allowing planetary nebula to act as a test-bed for theories of MHD outflows. The collaboration with the Imperial Collage group will help to deepen the rapidly growing field of High Energy Density Laboratory Astrophysics. The work developing the AstroBEAR code is of particular benefit in continuing research with new multi-physics methods as well as training the next generation of computational astrophysicists. Finally this program also includes an innovative outreach program involving the creation of Sci-Interactives: simulation based learning modules which will be posted on the nation's most popular science magazine websites.
Final Outcomes Project Description. In this project we developed and applied state of the art supercomputer simulation code to study the gas flows in space and their relevance to the life and death of stars. The code called AstroBEAR required development of what is called Adaptive Mesh Refinement techniques in which the computer automatically finds regions of interest in the simulation and zooms in on those regions. Like a digital camera that knows to cluster pixels around people faces AstroBEAR automatically provides more resolution where interesting things (like the explosion of a star) are happening and removes resolution in uninteresting domains (like a region of empty space). During the tenure of the grant we used AstroBEAR to study the flow of gas from dying stars (though our work was applicable to newly formed stars as well). Using the code we explored how powerful winds from a dying star can be focused in a beam of plasma – an astrophysical "jet". These kinds of jets appear in my astrophysical environments including massive black holes at center of galaxies. In one study we looked at how the jet might break up into a series of bullets and explored how the propagation of such a clumpy jet might appear in telescopes. In a second study we explored how a brief period of jet formation embedded in a longer sequence of spherical winds might change the appearance of clouds surrounding a dying stars. All our work was, ultimately, aimed at understanding the fate of stars like the sun. It is now clear that much of our previous theory surrounding the final stages of evolution for sun-like stars may need to be revised and our work contributed to moving the scientific community forward in seeking a new understanding. Publications Additional Information. The development of scientific computer code has now become so complex that teams working of extended periods must now find ways to propagate their work without losing expertise. To handle this challenge we developed a sophisticated WIKI for AstroBEAR that uses lessons learned from professional software development. With this WIKI in place, the code is now ready to be made available to the public
