Misc,
Magnetic braking of Sun-like and low-mass stars: Dependence on coronal temperature
(Oct 3, 2017)
Abstract
Sun-like and low-mass stars possess high temperature coronae and lose mass in
the form of stellar winds, driven by thermal pressure and complex
magnetohydrodynamic processes. These magnetized outflows probably do not
significantly affect the star's 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, and the rotation rate of the star, assuming a fixed coronal temperature.
For this study we explore how different coronal temperatures can influence the
stellar torque. We employ 2.5D, axisymmetric, magnetohydrodynamic simulations,
computed with the PLUTO code, to obtain steady-state wind solutions from
rotating stars with dipolar magnetic fields. Our parameter study includes 30
simulations with variations in coronal temperature and surface-magnetic-field
strength. We consider a Parker-like (i.e. thermal-pressure-driven) wind, and
therefore coronal temperature is the key parameter determining the velocity and
acceleration profile of the flow. Since the mass loss rates for these types of
stars are not well constrained, we determine how torque scales for a vast range
of stellar mass loss rates. Hotter winds lead to a faster acceleration, and we
show that (for a given magnetic field strength and mass-loss rate) a hotter
outflow leads to a weaker torque on the star. We derive new predictive torque
formulae for each temperature, which quantifies this effect over a range of
possible wind acceleration profiles.
