Tuesday 14 Nov 2017: Magnetic Braking Of Cool Stars: Dependence On Coronal Temperature
Georgios Pantolmos - University of Exeter
4th Floor Interaction Area 11:15-11:45
Cool star (i.e. solar- and late-type stars) possess high temperature coronae and lose mass in the form of stellar winds, driven by thermal pressure and complex magnetohydrodynamic MHD) processes. These magnetized outflows do not significantly affect the structural evolution on the main-sequence, but they brake the stellar rotation by removing angular momentum, a mechanism known as magnetic braking. Previous studies have shown how the braking torque depends on magnetic field strength and geometry, stellar mass and radius, mass loss rate, as well as the rotation rate of the star, assuming a fixed coronal temperature. In this presentation, I will show results from a new study that explores how different coronal Temperatures can influence the torque. Considering Parker-like (thermal-pressure driven) winds, the stellar coronal temperature is the key parameter that determines the velocity and acceleration profile of the flow. The fact that the outflow mass flux is significantly affected by the wind driving, and since the mass loss rates for such type of stars are not well constrained, we treat this quantity as a free parameter and we determine how torque scales for a vast range of stellar mass loss rates. Using 2.5D, ideal MHD, axisymmetric, simulations, computed with PLUTO code, our parameter study comprises 30 steady-state wind solutions, from rotating stars with dipolar magnetic fields, with variations in stellar coronal temperature and a wide range of surface field strengths. Hotter winds lead to a faster acceleration, and I will show that (for all else equal) a hotter outflow also leads to a weaker torque on the star. Furthermore, I will present new predictive torque formulae for each temperature, which quantifies this effect over a range of possible wind acceleration profiles.