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PhD project overview
- Field of studies:
- Theoretical astrophysics; late stellar evolution of low-intermediate mass stars – these are stars with a mass in the range of about one to eight solar masses.
Subject keywords:-
Simulations & Modeling: method development, analysis
Numerical methods: adaptive grid, non-linear physics, Henyey method
Physics: radiation hydrodynamics (RHD); gas dynamics; computational fluid dynamics (CFD), radiative transfer
Stellar winds: AGB stars, dust-driven winds, drift; multi-phase flow
Dust formation: stellar dust; carbon-rich chemistry, dynamic dust formation; combustion processes, drift dependent dust formation
Project summary:- When stars in the range of 1-8 solar masses have consumed the nuclear fuel in their cores they form a massive outflow of matter that quickly destroys the star. As such the formation of the outflow is a complicated physical process involving e.g. pulsations, dust formation, and a strong radiation field; and models of it have traditionally made use of various simplifying approximations. In my thesis I have made a thorough study of the role of a relative motion between gas and dust grains to the formation of the outflow (gas-dust drift).
The results of this study are new and important since they have shown that the character of the outflow can be significantly different depending on the use of drift. In the end the amount of formed stellar (carbon based) dust is affected, dust that in turn is crucial to the formation of life.
Supervisor: S. Höfner
Assistant supervisor: B.Gustafsson
Earned and practiced skills:-
Problem solving
- Inclusion of previously unused physics in models. Work that included: carrying out an assessment of the physics requirements, sorting out a growing mess of various approximative formulations, and solving numerical issues.
Modeling and analysis of complex non-linear systems
- Time-dependent modeling of the outer envelope of stars that form a massive outflow of matter, this in a so-called stellar wind.
- Solving a system coupling the evolution of gas, dust, and a strong radiation field, conserving mass, momentum and energy.
- The wind is treated using a variant of a multi-phase flow – called two-fluid flow or three-component flow – with connective terms between all [choose the nomenclature that suits you the best :-)]:
- three phases – gas, dust, and radiation field
- three components – gas, dust, and radiation field
- two fluids – gas and dust – plus the radiation field
- The gas-dust drift that I have focused on in my studies is handled through a drag force term which must handle both subsonic and supersonic velocities.
Numerical methods
- Shocks resulting from steep density gradients are resolved using an adaptive mesh present in the form of a grid equation.
- Implicit scheme with thirteen (13!) equations – out of which ten are PDEs. The equations to be solved are not linearized. All equations are specified on a staggered mesh.
- An adaptive grid is used in the form of an additional equation.
- The flux between grid cells is described in either 1:st (Donor cell) or 2:nd order (monotonic) accurate advection schemes. A highly accurate flux description is important not to dissipate shocks and other structures that form in the wind. Very recently (September 2006) these flux terms have been extended to a 3:rd order accurate scheme (PPM).
- The Henyey method is used for solving the system of equations since it nicely manages huge gradients in physical quantities present in a stellar structure.
Courses
- A complete list of all courses I attended as a PhD student can be found here.
Programming
- Programming in the main part of the Radiation hydrodynamical code I used Fortran 77 – the language the original code is written. For increased scalability and user friendliness I have made a full conversion of the code to Fortran 95.
- To make the analysis easier, more clear, more consistent, and allow for a quicker comparison between different models I wrote a complete graphical tool using the language IDL.
 The graphical RHD-tool RAT in work.
- Primary additional supporting programs and tools used: bash, CVS.
- Platforms used for development: SunOS, HP-UX, Linux.
Relevant resources:- – Complete details on my thesis, including the full-text introduction and links to articles, can be found here. Also see this link.
– A list of my selected articles can be found here.
– A publicly available radiation hydrodynamical code with similar capabilities as the one used in my thesis work – TITAN – can be found here (Note that this code does not include any treatment of stellar dust).
– I was a PhD student within the industrially oriented research school Advanced Instrumentation and Measurements – AIM – at Uppsala University. As a part of my time in this school I carried out two months of practical work at the Swedish Meteorological and Hydrological Institute (SMHI) in Norrköping, Sweden.
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