A Comparison Between Physics-based and Polytropic MHD Models for Stellar Coronae and Stellar Winds of Solar Analogs

TL;DR

This paper compares physics-based and polytropic MHD models for stellar coronae and winds, showing that physics-based models better reproduce solar observations, while uncalibrated polytropic models may produce unphysical results.

Contribution

It provides a comparative analysis demonstrating the limitations of polytropic models and advocates for calibration with physics-based models or solar observations.

Findings

AWSOM model reproduces solar observations accurately

Polytropic models often fail to match observations

Uncalibrated polytropic models can produce unphysical results

Abstract

The development of the Zeeman-Doppler Imagine (ZDI) technique has provided synoptic observations of surface magnetic fields of low-mass stars. This led the Stellar Astrophysics community to adopt modeling techniques that have been used in solar physics using solar magnetograms. However, many of these techniques have been neglected by the solar community due to their failure to reproduce solar observations. Nevertheless, some of these techniques are still used to simulate the coronae and winds of solar analogs. Here we present a comparative study between two MHD models for the solar corona and solar wind. The first type of model is a polytropic wind model, and the second is the physics-based AWSOM model. We show that while the AWSOM model consistently reproduces many solar observations, the polytropic model fails to reproduce many of these observations and in the cases it does, its solutions are unphysical. Our recommendation is that polytropic models, which are used to estimate mass-loss rates and other parameters of solar analogs, must be first calibrated with solar observations. Alternatively, these models can be calibrated with models that capture more detailed physics of the solar corona (such as the AWSOM model), and that can reproduce solar observations in a consistent manner. Without such a calibration, the results of the polytropic models cannot be validated, but they can be wrongly used by others.