Testing a Predictive Theoretical Model for the Mass Loss Rates of Cool Stars

Abstract

The basic mechanisms responsible for producing winds from cool, late-type stars are still largely unknown. We take inspiration from recent progress in understanding solar wind acceleration to develop a physically motivated model of the time-steady mass loss rates of cool main-sequence stars and evolved giants. This model follows the energy flux of magnetohydrodynamic turbulence from a subsurface convection zone to its eventual dissipation and escape through open magnetic flux tubes. We show how Alfvén waves and turbulence can produce winds in either a hot corona or a cool extended chromosphere, and we specify the conditions that determine whether or not coronal heating occurs. These models do not utilize arbitrary normalization factors, but instead predict the mass loss rate directly from a star's fundamental properties. We take account of stellar magnetic activity by extending standard age-activity-rotation indicators to include the evolution of the filling factor of strong photospheric magnetic fields. We compared the predicted mass loss rates with observed values for 47 stars and found significantly better agreement than was obtained from the popular scaling laws of Reimers, Schröder, and Cuntz. The algorithm used to compute cool-star mass loss rates is provided as a self-contained and efficient computer code. We anticipate that the results from this kind of model can be incorporated straightforwardly into stellar evolution calculations and population synthesis techniques.

Improved angular momentum evolution model for solar-like stars

“…In Eq. (3), B eq is the equipartition magnetic field strength (Cranmer & Saar 2011) with ρ * the photospheric mass density, k B the Boltzmann's constant, T eff the effective temperature, μ the mean atomic weight, and m H the mass of a hydrogen atom. The mass-loss rate prescription we inject in Eq.…”

“…The mass-loss rate prescription we inject in Eq. (2) also stems from Cranmer & Saar (2011) coronal wind modelṡ…”

“…In the Matt et al (2012a) prescription adopted here, together with the dynamo relationship and mass-loss estimates from Cranmer & Saar (2011), the predicted braking torque already includes the mass dependency. This is seen in Eqs.…”

The effects of stellar winds on the magnetospheres and potential habitability of exoplanets

“…The most up to date models are fully three dimensional and self-consistent (Vidotto et al 2009(Vidotto et al , 2012 and use more physically motivated arguments to determine mass loss rates (Holzwarth & Jardine 2007;Cohen et al 2007;Cranmer & Saar 2011). However many assumptions are used and scaling relations are still present in some models.…”

“…Since the HZ of Table 1. Numerical values used in this study for the present day Earth's magnetic moment (Tarduno et al 2010), solar coronal number density (Guhathakurta et al 1996), luminosity (Harmanec & Prsa 2011), X-ray luminosity (Judge et al 2003), chromospheric activity (Mamajek & Hillenbrand 2008), and mass loss rate (Cranmer & Saar 2011).…”