This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
Dr. Townsend will undertake an exploration of the incidence, characteristics and evolutionary impact of wave transport by oscillation modes in massive stars. The foundation for this exploration will lie in a new Self-Consistent Spectrum (SCS) method for incorporating the transport into stellar evolution calculations. At the heart of the SCS method is a pair of numerical codes: a non-radial, non-adiabatic pulsation code, which will be developed as part of the project's initial activities, and an evolution code, which will be derived from an already-existing code. As massive stars evolve across the main sequence, they pass through one or more instability strips where oscillation modes are excited by an opacity-regulated heat engine. During these phases, the unstable modes can alter a star's rotation profile by transporting angular momentum from one part of the interior to another. This wave transport has potentially far-reaching consequences for the star's subsequent evolution and ultimate fate, but in studies so far it has largely been overlooked. The SCS method will first be used to explore various wave-transport phenomena discovered during an earlier pilot study: the self-limitation of mode amplitudes via the angular velocity gradients that they establish; the acceleration of surface layers by retrograde gravity modes, and its relation to the Be-star phenomenon; and the mixing of molecular weight gradient zones due to the interplay between wave transport and shear instability. Subsequently, the method will be applied in a comprehensive survey of massive-star evolution with the full inclusion of wave transport.
This survey, based around the calculation of a large grid of evolutionary tracks, will be used to address many important questions relating to the wave transport, ranging from its observational diagnostics through to its impact on final evolutionary outcomes. The project results will be disseminated in the form of journal papers, presentations, calculation data, parametric models, and a browser-based stellar evolution code for research and education. The pulsation code will be released under an open-source license, to encourage other groups' involvement in exploring wave transport, and also as an analysis tool for asteroseismology of massive stars. A graduate student and postdoctoral researcher composing part of the project team will benefit from training in topics with steadily growing scientific visibility and relevance. This training will in turn stimulate the development of new curriculum materials for undergraduate/graduate courses. The project results will be broadly disseminated through papers, presentations, data and models. Moreover, with the release of source code and the development of on-line tools, the project will enhance infrastructure for research and education.
