Monday

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, we will show results from a study that explores how different coronal temperatures can influence the torque. We consider Parker-like
(thermal-pressure driven) winds, and therefore, coronal temperature is the key
parameter determining 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 we will show that (for all else equal) a hotter outflow also leads to a
weaker torque on the star. Furthermore, we will present new predictive torque formulae for each temperature, which quantifies this effect over a range of
possible wind acceleration profiles.