Line shift, line asymmetry, and the 6Li/7Li isotopic ratio determination
Line shift, line asymmetry, and the 6Li/7Li isotopic ratio determination
复制标题
线位移、线不对称性和 6Li/7Li 同位素比测定
DOI：
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发表时间：
2007
期刊：
影响因子：
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通讯作者：
E. Caffau
中科院分区：
文献类型：
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作者：
R. Cayrel;M. Steffen;Hum Chand;P. Bonifacio;M. Spite;F. Spite;P. Petitjean;H. Ludwig;E. Caffau
上下文线不对称是由恒星大气中的对流多普勒频移产生的，特别是在贫金属恒星中，对流运动穿透到更高的大气层。这种不对称性在丰度分析中通常被忽略。6 Li/ 7 Li同位素比的测定容易受到这种不对称性的影响，因为6 Li的贡献是相应7 Li线的每个组分的红翼相对于其蓝翼的轻微混合增强。目标。本文研究晕星星HD 74000，并估计对流相关的不对称性对Li同位素比值测定的影响。方法.有两种方法可以达到这个目的。第一，这是纯粹的经验，包括在推导出一个模板配置文件从另一个元素，可以假设起源于相同的恒星大气层作为锂i，产生吸收线的近似相同的等效宽度的个别组件的7锂i共振线。第二种方法包括在进行丰度分析的基础上NLTE线形成的三维流体动力学模型大气，考虑到光球对流的影响。结果第一种方法的结果表明，对流的不对称性产生了过量的吸收在红翼的7锂吸收功能，模仿6锂的存在下，在一个相当的水平迄今公布的值。这开启了这样的可能性，即到目前为止仅导出了6 Li/ 7 Li的上限。第二种方法证实了这些发现。结论.从这项工作来看，似乎对以前晕星中Li丰度的测定进行了系统的重新评价，
Context. Line asymmetries are generated by convective Doppler shifts in stellar atmospheres, especially in metal-poor stars, where convective motions penetrate to higher atmospheric levels. Such asymmetries are usually neglected in abundance analyses. The determination of the 6 Li/ 7 Li isotopic ratio is prone to su ering from such asymmetries, as the contribution of 6 Li is a slight blending reinforcement of the red wing of each component of the corresponding 7 Li line, with respect to its blue wing. Aims. The present paper studies the halo star HD 74000 and estimates the impact of convection-related asymmetries on the Li isotopic ratio determination. Methods. Two methods are used to meet this aim. The first, which is purely empirical, consists in deriving a template profile from another element that can be assumed to originate in the same stellar atmospheric layers as Li i, producing absorption lines of approximately the same equivalent width as individual components of the 7 Li i resonance line. The second method consists in conducting the abundance analysis based on NLTE line formation in a 3D hydrodynamical model atmosphere, taking into account the e ects of photospheric convection. Results. The results of the first method show that the convective asymmetry generates an excess absorption in the red wing of the 7 Li absorption feature that mimics the presence of 6 Li at a level comparable to the hitherto published values. This opens the possibility that only an upper limit on 6 Li/ 7 Li has thus far been derived. The second method confirms these findings. Conclusions. From this work, it appears that a systematic reappraisal of former determinations of 6 Li abundances in halo stars is