Astrospheres - Stellar wind bubbles as topological borders to the interstellar medium

Advisor: Dieter Nickeler (AI CAS)

Funding: basic scholarship; additional top-up funding might be available depending on the outcome of grant applications.


The existence of astrospheres has been subject to a lot of investigations within the last decades. Observations, showing stellar wind blown bubbles, reveal the border marked by bow shocks, where the interstellar material or slower stellar matter/wind is swept off by faster streams of the interstellar wind. A real border between different flows, where flows here should be understood as dynamical systems (generally associated with vector fields, e.g. magnetic fields or plasma flows), is determined by the existence of separatrix surfaces. These separatrices are defined by field lines, originating from the stagnation point (or magnetic null point in the case of magnetic stars) and emanating across the separatrix surface, the so called astropause. The flow is thus diverted around the stellar wind obstacle and is therefore separated into topologically disjoint field lines.

Within the frame of the project the PhD student should analyse the role of different fluid theories, like hydrodynamics and magnetohydrodynamics, dissipative and non-dissipative, by exploiting tools of differential topology, respectively topological fluid dynamics, on solutions and physical implications/effects. The used theories are originating in theoretical plasma physics, astrophysics and the theory of nonlinear partial differential equations and the theory of nonlinear dynamical systems, e.g. classification of linear flows in the vicinity of null points of vector fields (Poincare & Lyapunov). The eigenvalues of the Jacobians of the vector fields, namely plasma flow and magnetic field, are due to the fluid equations not independent. The aim is to calculate and understand the local and global structure and the dimensions, diameters and other physical variables of the plasma environment of astrospheres and astropauses and their local interstellar medium. While the priority of the project is on the development of theoretical concepts, the results may also be applied to available observations of astrosphere and bow shocks.

Figure description: Left: Bow shock structure around a massive star detected in the infrared. Observations were taken with the Spitzer Space Telescope. The color scheme is 24/8.0/4.5 μm in red/green/blue. Figure is taken from Kobulnicky et al. (2016). Right: Superposition of a homogeneous inflow from negative x-direction and a mono-polar outflow from a star at coordinates (0,0) with non-vanishing real circulation, leading to an asymmetrical shift of the stagnation point and a warped astropause separatrix layer. This image is taken from Nickeler et al. (2014).


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