缺金属场巨星系色球的HST观测
HST Observations of Chromospheres in Metal Deficient Field Giants
摘要：
缺乏金属的场巨星的HST高分辨率光谱是以前研究中的两倍多，在红巨星分支上跨越约3个星等，并采样[Fe/H]= -1到-3的丰度范围。这些恒星，尽管他们的年龄和低金属量，拥有色球镁II（2800埃）的流量是在一个因素的4人口I星，并给出了迹象的依赖于金属丰度在最低的金属。Mg II的K线宽度取决于光度，并与金属丰度相关。谱线轮廓的不对称性揭示了在低光度（M_V = -0.8）下发生的外流，比在贫金属巨星中的Ca K和H α谱线所检测到的要低，这表明质量外流发生在红巨星分支的更大跨度上，比以前认为的要大，并证实Mg II谱线是很好的风诊断。这些结果并不支持色球层以磁性为主，但似乎更符合某种流体动力学，或外部大气的声学加热。
英文摘要：
HST high resolution spectra of metal-deficient field giants more than double the stars in previous studies, span about 3 magnitudes on the red giant branch, and sample an abundance range [Fe/H]= -1 to -3. These stars, in spite of their age and low metallicity, possess chromospheric fluxes of Mg II (2800 Angstrom) that are within a factor of 4 of Population I stars, and give signs of a dependence on the metal abundance at the lowest metallicities. The Mg II k-line widths depend on luminosity and correlate with metallicity. Line profile asymmetries reveal outflows that occur at lower luminosities (M_V = -0.8) than detected in Ca K and H-alpha lines in metal-poor giants, suggesting mass outflow occurs over a larger span of the red giant branch than previously thought, and confirming that the Mg II lines are good wind diagnostics. These results do not support a magnetically dominated chromosphere, but appear more consistent with some sort of hydrodynamic, or acoustic heating of the outer atmospheres.
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