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

We investigated stellar winds from zero/low-metallicity low-mass stars by magnetohydrodynamical simulations for stellar winds driven by Alfv\'{e}n 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. Alfv\'{e}nic 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. The soft X-ray flux [erg cm$^{-2}$s$^{-1}$] 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 [erg cm$^{3}$s$^{-1}$] is smaller. A larger fraction of the input Alfv\'{e}nic 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.

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T. Suzuki
Fri, 13 Oct 17

Comments: 15 pages, 8 figures embedded, comments are welcome