Chris Wareing's PhD Abstract

In this thesis, a three-dimensional Computational Fluid Dynamics (CFD) program has been described, made parallel, tested and applied to astrophysical problems. The program, named CUBEMPI, was made parallel using the Message-Passing Interface (MPI) library and examined via simple efficiency tests and more complex performance profilers within the Multi-Processing Environment (MPE) library of MPI.

CUBEMPI was tested with various hydrodynamical problems. CUBEMPI successfully completed all tests and its performance was compared with that of several other CFD schemes. Comparing CUBEMPI with these other schemes, no one scheme performed consistently better than any of the others. CUBEMPI efficiently computes accurate solutions to the Euler equations and thus can be used to simulate fluid flow. CUBEMPI has been shown to be particularly capable of simulating shocks in astrophysical conditions and so was used to consider planetary nebulae (PNe) and their interaction with the interstellar medium (ISM).

Previous modelling of PNe moving through the ISM has been performed in two dimensions (2D) following a detailed evolutionary track of the PN-forming star moving at a low speed. Here CUBEMPI has been used to simulate the interaction in three dimensions (3D) with a range of proper motions using a generic simple ``triple-wind '' model for the formation of PNe. This modelling has considered the asymptotic giant branch (AGB) and post-AGB stages of evolution and led to the conclusion that to unde rstand the shape of a nebula inferred to be moving through the ISM, the AGB evolution must be considered as well as the post-AGB evolution as it has a strong shaping effect on the nebula. Signs of PN-ISM interaction are found not to be confined to ancient or extreme nebulae. In this thesis, simulated younger nebulae moving at low speeds show one-sided brightening and displacement of the central star from the geometric centre of the nebula. At higher speeds the nebulae are strongly affected and eventually no longer appear symmetric. The effect of ISM shaping on nebulae has been quantitatively shown in this thesis to be an explanation for the missing mass problem in PNe; much of the AGB material is swept downstream of the nebula.

New observations of the crescent shaped nebula Sh 2-188 have been compared to the PN-ISM simulations and the nebula has been understood in terms of a PN moving at 125 km/s. A proper motion study of the nebula has found it to be moving at 30 +- 10 mas yr^-1 in the direction of the bright crescent. The combination of these values has led to an estimate of the distance D = 850^+500_-420 pc, diameter d ~ 2.5 pc and age t_PN = 22,500 +- 2,500 years. The triple-wind model explains the geometric displacement of the central star and the simulation estimates ~2/3 of the mass expected in the region of the star has been swept downstream. The nebula surrounding the classical nova GK Persei has also been investigated in terms of the triple-wind model using an asymmetric AGB wind. The model has produced a bipolar nebula of the shape observed around GK Per and high density equatorial regions which could account for the brightened emission in the southwest of the nova. Finally, improvements of CUBEMPI and the models are suggested.