Testing a Predictive Theoretical Model for the Mass Loss Rates of Cool Stars (1108.4369v1)

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 Alfven 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, Schroeder, 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.

• The paper presents a predictive model linking MHD turbulence and Alfvén waves to forecast mass loss rates in cool stars.
• The methodology derives mass loss rates directly from fundamental stellar parameters without reliance on empirical normalization.
• Results differentiate between gas-pressure driven winds in hot coronae and wave-pressure influenced winds in evolved giants.

An Examination of Theoretical Mass Loss Rates in Cool Stars

The paper "Testing a Predictive Theoretical Model for the Mass Loss Rates of Cool Stars" by Steven R. Cranmer and Steven H. Saar provides a robust investigation into the mechanisms driving mass loss in cool stars, drawing parallels from solar wind acceleration phenomena. The authors develop a physically-grounded model to predict mass loss rates in cool main-sequence stars and evolved giants, aiming to eschew arbitrary normalization factors. They utilize the framework of magnetohydrodynamic (MHD) turbulence and Alfvén waves, thereby connecting subsurface convection zones to the dissipation and escape processes that occur through stellar magnetic flux tubes.

Alfvén Waves and MHD Turbulence

A central element of the model lies in the role of Alfvén waves and MHD turbulence. The waves, generated by turbulent convective motions, propagate through the star's magnetic flux tubes—partially reflecting, dissipating, and contributing to the energy and momentum necessary for wind acceleration. Critical insights are drawn from the model's ability to predict whether winds originate from a hot corona or extend from a cooler chromosphere based on the balance between heating and radiative cooling processes.

Model Implementation and Assumptions

The authors introduce a parameterized model mapping key stellar properties—mass, radius, luminosity, rotation period, and metallicity—to predict the star’s photospheric conditions. They derive mass loss rates directly from these fundamental properties without recourse to empirically-derived prescriptions. For illustrative purposes, they test several dimensionless parameters (e.g., $\theta$ and $h$, affecting coronal flux and TR filling factor scaling) that adjust the model predictions.

Results and Comparison

Cranmer and Saar evaluate the model against observational data from 47 stars, noting that their predictions show improved agreement over traditional scaling laws (e.g., those by Reimers, Schröder, and Cuntz). Numerical integration of the model's mass loss predictions, which are available through a self-contained computer code, shows how particularly well correlations hold when contrasted with established empirical data for solar-type winds.

The work identifies that for stars with sustained hot coronae, mass loss is predominantly gas-pressure-driven, whereas stars transitioning to giant stages exhibit mass loss largely influenced by wave-pressure. An iterative solution to account for the magnetic activity level—linked to rotation rate and other stellar parameters—concludes that evolutionary rotation-activity correlations can extend to the filling factor of photospheric magnetic fields.

Implications and Future Directions

This paper carries significant implications for astrophysical models of stellar evolution and dynamics of stellar populations. By presenting a more precise accounting of mass loss in cool stars—an essential component influencing stellar lifecycles and galactic chemical evolution—the model stands to be incorporated into larger grids of stellar evolution simulation codes.

Future developments will likely focus on refining the treatment of wave reflection and interaction in complex three-dimensional stellar atmospheres. Extending the models to non-solar metallicities and enhanced dynamo processes in differentially rotating stars presents yet another avenue of exploration. Incorporation of these results in confronting new data from space observatories could further validate the theoretical underpinnings and offer broader predictive power across diverse stellar environments.