Stellar winds and coronae of low-mass Population II/III stars

PASJ: Publications of the Astronomical Society of Japan

Suzuki, Takeru K¹ 

[1] School of Arts & Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan

关键词: magnetohydrodynamics (MHD);    stars: coronae;    stars: low-mass;    stars: Population III;    stars: winds, outflows;   

DOI  :  10.1093/pasj/psy023

学科分类：天文学(综合)

来源: Oxford University Press

【 摘 要 】

We investigated stellar winds from zero-/low-metallicity low-mass stars by magnetohydrodynamical simulations for stellar winds driven by Alfvén waves from stars with mass M = (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. Alfvénic 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 Population II/III stars with Z ≤ 0.01 Z|$\odot$| is one to two orders of magnitude larger than that of a 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 ∼1–30 times larger than that of a solar-metallicity counterpart owing to the larger coronal density, even though the radiation cooling efficiency is smaller. A larger fraction of the input Alfvénic 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⋆, and Teff are the stellar luminosity, radius, and effective temperature, respectively.

【 授权许可】

CC BY