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Evaluating Possible Heating Mechanisms Using the Transition Region Line Profiles of Late-Type Stars

机译:使用晚型恒星的过渡区域线轮廓评估可能的加热机制

摘要

Our analysis of high-resolution Goddard High-Resolution Spectrograph (GHRS) spectra of late-type stars shows that the Si IV and C IV lines formed near 10(exp 5) K can be decomposed into the sum of two Gaussians, a broad component and a narrow component. We find that the flux contribution of the broad components is correlated with both the C IV and X-ray surface fluxes. For main-sequence stars, the widths of the narrow components suggest subsonic nonthermal velocities, and there appears to be a tight correlation between these nonthermal velocities and stellar surface gravity [xi(sub nc) varies as g(sup (-.68 +/-.07))]. For evolved stars with lower surface gravities, the nonthermal velocities suggested by the narrow components are at or just above the sound speed. Nonthermal velocities computed from the widths of the broad components are always highly supersonic. We propose that the broad components are diagnostics for microflare heating. Turbulent dissipation and Alfven waves are both viable candidates for the narrow component heating mechanism. A solar analog for the broad components might be the 'explosive events' detected by the High-Resolution Telescope and Spectrograph (HRTS) experiment. The broad component we observe for the Si IV lambda 1394 line of alpha Cen A, a star that is nearly identical to the Sun, has a FWHM of 109 +/- 10 km/s and is blueshifted by 9 +/- 3 km/s relative to the narrow component. Both of these properties are consistent with the properties of the solar explosive events. However, the alpha Cen A broad component accounts for 25% +/- 4% of the total Si IV line flux, while solar explosive events are currently thought to account for no more than 5% of the Sun's total transition region emission. This discrepancy must be resolved before the connection between broad components and explosive events can be positively established. In addition to our analysis of the Si IV and C IV lines of many stars, we also provide a more thorough analysis of all of the available GHRS data for alpha Cen A (G2 V) and alpha Cen B (K1 V). We find that the transition region lines of both stars have redshifts almost identical to those observed on the Sun: showing an increase with line formation temperature up to about log T = 5.2 and then a rapid decrease. Using the O IV] lines as density diagnostics, we compute electron densities of log n(sub e) = 9.65 +/- 0.20 and log n(sub e) = 9.50 +/- 0.30 for alpha Cen A and alpha Cen B, respectively.
机译:我们对后期型恒星的高分辨率戈达德高分辨率光谱(GHRS)光谱的分析表明,在10(exp 5)K附近形成的Si IV和C IV线可以分解为两个高斯的总和,这是一个宽泛的分量和一个狭窄的组成部分。我们发现,宽分量的通量贡献与C IV和X射线表面通量都相关。对于主序星,窄分量的宽度表明亚音速非热速度,并且这些非热速度与恒星表面重力之间似乎存在紧密的相关性[xi(sub nc)随着g(sup(-.68 + / -.07))]。对于具有较低表面重力的演化恒星,窄分量所建议的非热速度等于或刚好高于声速。从广义分量的宽度计算出的非热速度始终是高度超音速的。我们建议将广泛的组件用于微火炬加热的诊断。湍流耗散和Alfven波都是狭窄组件加热机制的可行选择。高分辨率望远镜和光谱仪(HRTS)实验检测到的“爆炸事件”可能是构成广泛组件的太阳模拟物。我们观察到的Al Cen A的Si IV lambda 1394线的广泛成分,这颗恒星几乎与太阳相同,FWHM为109 +/- 10 km / s,并且蓝移了9 +/- 3 km /相对于窄分量。这两个性质都与太阳爆炸事件的性质一致。但是,αCen A广泛成分占Si IV线通量的25%+/- 4%,而目前据认为太阳爆炸事件不超过太阳过渡区总排放量的5%。必须先解决这一差异,才能积极地建立广泛的组成部分与爆炸事件之间的联系。除了分析许多恒星的Si IV和C IV线外,我们还对alpha Cen A(G2 V)和alpha Cen B(K1 V)的所有可用GHRS数据进行了更彻底的分析。我们发现,两颗恒星的过渡区线都具有与在太阳上观察到的几乎相同的红移:显示出随着线形成温度的升高而升高,直至约log T = 5.2,然后迅速降低。使用O IV]线作为密度诊断程序,我们分别计算alpha Cen A和alpha Cen B的log n(sub e)= 9.65 +/- 0.20和log n(sub e)= 9.50 +/- 0.30的电子密度。

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