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arXiv 1710.04478astro-ph.SRastro-ph.COastro-ph.GA
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低质量Pop. II/III星的星风和日冕

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

Takeru K. Suzuki

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16 pages, 8 figures embedded, comments are welcome
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摘要：

我们研究了来自零/低金属丰度低质量恒星的恒星风，通过磁流体动力学模拟，对来自质量为$M_{\star}=(0.6-0.8)M_{\odot}$和金属丰度为$Z=(0-1)Z_{\odot}$的恒星的\Alfven波驱动的恒星风进行了研究，其中$M_{\odot}$和$Z_{\odot}$分别为太阳质量和金属丰度。由表面对流激发的\Alfvenic波从光球层向上传播，并通过耗散加热日冕。对于较低的$Z$，由于辐射冷却效率低，密度较高的气体可以被加热到日冕温度。具有$Z\le 0.01Z_{\odot}$的Pop.II/III型恒星的日冕密度比相同质量的太阳金属丰度恒星的日冕密度大1-2个数量级，因此，质量损失率$\dot{M}$是$(4.5-20)$倍。这表明在低质量的Pop.III星上的金属吸积是可以忽略的。Pop.II/III型恒星的软X射线通量预计也比太阳金属丰度对应物的大$\approx (1-30)$倍，这是因为它们的日冕密度更大，尽管辐射冷却效率更小。在低$Z$的恒星中，由于光球层和日冕层之间的密度差较小，因此大部分输入的\Alfvenic波能量被传输到日冕层，避免了严重的反射。因此，更大的部分被转换为日冕的热能和恒星风的动能。从这个能量学的论证，我们最终导出了$\dot{M}$的标度为$\dot{M}\propto L R_{\star}^{11/9}M_{\star}^{-10/9}T_{\rm eff}^{11/2}\left[\max (Z/Z_{\odot},0.01)\right]^{-1/5}$，其中$L$、$R_{\star}$和$T_{\rm eff}$分别是恒星的光度、半径和有效温度。

英文摘要：

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.

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