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