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Tuesday 17 Oct 2017Investigating the effects of rotation and magnetism on stellar radii in 1D stellar structure models

Lewis Ireland - University of Exeter

4th Floor Interaction Area 11:15-11:45

Some low-mass stars appear to have larger radii than predicted by standard 1D stellar models. The origins of these inflated radii are unknown, but several authors suggest that inefficient convective heat transport is ultimately to blame. We aim in particular to understand whether the unexpectedly large radii measured in some low-mass stars can be explained by a decrease in convective efficiency, due to rotation or magnetism. Standard 1D structure models do not explicitly include the effects of rotation and magnetism on convection, but these may be modelled simply by varying the mixing-length parameter αMLT. Using the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code, we modify the standard MLT formulation to mimic rotational and magnetic effects on the stellar structure, using formulations suggested by Stevenson (1979) and MacDonald & Mullan (2014) respectively. We find rapid rotation in our models to have a negligible effect on the stellar structure, essentially because the overall radius is affected most by layers near the surface, where convection tends to be rapid and thus largely uninfluenced by rotation. Magnetic fields, if they indeed influence the convective transport in the manner described by MacDonald & Mullan (2014), could lead to much more noticeable inflation of the radius. We show that these effects on the stellar structure can be reproduced using a depth-dependent αMLT: that is, a non-magnetic non-rotating model can be constructed that almost reproduces the solution to these “rotating” and “magnetic” models.

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