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Wednesday 18 Jul 2018[Seminar] The Effect of Magnetic Geometry on Angular Momentum Loss From the Sun and Other Stars

Adam Finley - University of Exeter

4th Floor, Physics Building 14:00-15:00

The rotational evolution of cool stars is governed by magnetised stellar winds which slow the stellar rotation during their main sequence lifetimes. During the 1st year of my PhD I computed a grid of MHD simulations for a range of stellar wind parameters, Finley & Matt (2017, 2018). My current work focusses on applying and developing these models to better model the angular momentum loss rates from Sun-like stars. Magnetic variability is commonly observed in Sun-like stars, and the changing strength and topology of the global field is expected to effect the torque exerted by the stellar wind. I present three different methods for computing the angular momentum loss in the solar wind. Two are based on MHD simulations and scaling relationships from Finley & Matt (2018), plus observed mass loss rates for the solar wind. One uses the open flux measured in-situ in the solar wind, and one uses remotely-observed surface magnetograms. Both methods agree in the variation of the solar torque seen through the solar cycle and show a 30 ? 40% decrease in the average torque between cycle 23 and 24. The third method is based on the observed spin-rates of Sun-like stars, which decrease with age and thus directly probe the average angular momentum loss. This method gives 6.2 ? 1030 erg for the averaged solar torque, larger than the other two methods, which could have a few explanations, but notably may be indicative of further variability in the solar torque on timescales much longer than the magnetic cycle. We discuss the implications for applying the formula to other Sun-like stars, where only surface field measurements are available, and where the magnetic variations are ill-constrained. Through the use of Zeeman Doppler Imaged stars I construct models of their magnetic variability and compare these to the spin-evolution models of Matt et al. (2015).

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