Magnetohydrostatic equilibrium in starspots: dependences on color (T_{eff}) and surface gravity (g)

TL;DR

This paper investigates how magnetic field strengths and temperature contrasts in starspots depend on stellar effective temperature and surface gravity, using observational data and stellar models to understand their scaling and underlying physics.

Contribution

It introduces a detailed analysis of the dependence of starspot magnetic fields and temperature contrasts on T_{eff} and g, incorporating recent observations and stellar atmosphere models.

Findings

Magnetic field strengths increase as T_{eff} decreases for similar temperature contrasts.

Opacity variations with T_{eff} influence the magnetic and thermal properties of starspots.

The relationship between temperature contrast and magnetic field strength is modulated by stellar parameters.

Abstract

Temperature contrasts and magnetic field strengths of sunspot umbrae broadly follow the thermal-magnetic relationship obtained from magnetohydrostatic equilibrium. Using a compilation of recent observations, especially in molecular bands, of temperature contrasts of starspots in cool stars, and a grid of Kurucz stellar model atmospheres constructed to cover layers of sub-surface convection zone, we examine how the above relationship scales with effective temperature T_{eff}, surface gravity g and the associated changes in opacity of stellar photospheric gas. We calculate expected field strengths in starpots and find that a given relative reduction in temperatures (or the same darkness contrasts) yield increasing field strengths against decreasing T_{eff} due to a combination of pressure and opacity variations against T_{eff}.