Stellar Winds and Coronae of Low-mass Pop. II/III Stars

TL;DR

This study uses magnetohydrodynamical simulations to explore how low-metallicity, low-mass stars generate stellar winds and coronae, revealing that metal-poor stars have denser coronae, higher mass loss rates, and stronger X-ray emissions than solar-metallicity stars.

Contribution

It provides the first detailed modeling of stellar winds and coronae for Pop. II/III stars, highlighting the impact of metallicity on wind properties and coronal activity.

Findings

Coronal density in low-metallicity stars is 10-100 times higher than in solar-metallicity stars.

Mass loss rates are 4.5 to 20 times larger in low-metallicity stars.

X-ray flux can be 1 to 30 times stronger in Pop. II/III stars.

Abstract

We investigated stellar winds from zero/low-metallicity low-mass stars by magnetohydrodynamical simulations for stellar winds driven by \Alfven waves from stars with mass $M_{\star}=(0.6-0.8)M_{\odot}$ and metallicity $Z=(0-1)Z_{\odot}$, where $M_{\odot}$ and $Z_{\odot}$ are the solar mass and metallicity, respectively. \Alfvenic waves, which are excited by the surface convection, travel upward from the photosphere and heat up the corona by their dissipation. For lower $Z$, denser gas can be heated up to the coronal temperature because of the inefficient radiation cooling. The coronal density of Pop.II/III stars with $Z\le 0.01Z_{\odot}$ is 1-2 orders of magnitude larger than that of the solar-metallicity star with the same mass, and as a result, the mass loss rate, $\dot{M}$, is $(4.5-20)$ times larger. This indicates that metal accretion on low-mass Pop.III stars is negligible. The soft X-ray flux of the Pop.II/III stars is also expected to be $\approx (1-30)$ times larger than that of the solar-metallicity counterpart owing to the larger coronal density, even though the radiation cooling efficiency is smaller. A larger fraction of the input \Alfvenic wave energy is transmitted to the corona in low $Z$ stars because they avoid severe reflection owing to the smaller density difference between the photosphere and the corona. Therefore, a larger fraction is converted to the thermal energy of the corona and the kinetic energy of the stellar wind. From this energetics argument, we finally derived a scaling of $\dot{M}$ as $\dot{M}\propto L R_{\star}^{11/9}M_{\star}^{-10/9}T_{\rm eff}^{11/2}\left[\max (Z/Z_{\odot},0.01)\right]^{-1/5}$, where $L$, $R_{\star}$, and $T_{\rm eff}$ are stellar luminosity, radius, and effective temperature, respectively.