Near Infrared Spectroscopy of M Dwarfs. II. H2O Molecule as an Abundance Indicator of Oxygen
Near Infrared Spectroscopy of M Dwarfs. II. H2O Molecule as an Abundance Indicator of Oxygen
复制标题
M 矮星的近红外光谱。
DOI：
10.1093/pasj/psu160
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发表时间：
2014
影响因子：
4.8
通讯作者：
Y. Takeda
中科院分区：
文献类型：
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作者：
T. Tsuji;T. Nakajima;Y. Takeda
根据M矮星的近红外光谱(R≈20000)，从H2O的振动线确定了氧的丰度。尽管M矮星中的H2O谱线相互混杂严重，而且大量H2O谱线本身的集体效应对连续能级也有明显的抑制作用，但我们还是参照在观测光谱和理论光谱上一致定义的伪连续谱，对H2O谱线进行了定量分析。为此，利用最新的高精度H2O线表，对理论光谱上的准连续谱进行了精确的估算。然后，我们提出了一种简单而灵活的方法来分析混合特征的等效宽度(即，不一定限于单线)，使用小型增长曲线(CG)，它是观察到的EW周围的通常CG的一小部分。迷你CG是通过使用从合成光谱评估的理论EWS来产生的，其方法与从观测光谱测量EWS完全相同。通过使用微型CG将观察到的EW转换为丰度，并对所有观察到的EW逐行或逐个混合重复该过程。在冷M矮星中，CO形成后留下的几乎所有氧原子都是稳定的H2O分子，由于光球模型的不完善，这些分子几乎没有受到不确定性的影响。此外，由于H2O的相对分子质量较低，其热速度很可能大于微湍流速度，微湍流速度的不确定性对丰度测定的影响相对较小。那么众多的H2O线就是氧气丰度的极佳指示器。在38M矮星中，氧丰度在-3.5到-3.0之间，但在早期的4颗M矮星中，氧丰度不是这样的，在这些矮星中只检测到少量的H2O谱线。在富碳的M矮星中，相对于logAC绘制的logAO/AC似乎系统地较小，这表明了氧和碳在银盘化学演化中的不同形成历史。此外，大多数M矮星的AO/AFE比值更接近于基于经典的高氧丰度的太阳AO/AFE比值，而不是最近向下修正的低值。
Based on the near infrared spectra (R≈ 20000) of M dwarfs, oxygen abundances are determined from the ro-vibrational lines of H2O. Although H2O lines in M dwarfs are badly blended each other and the continuum levels are depressed appreciably by the collective effect of numerous H2O lines themselves, quantitative analysis of H2O lines has been carried out by referring to the pseudo-continua consistently defined by the same way on the observed and theoretical spectra. For this purpose, the pseudo-continuum on the theoretical spectrum has been evaluated accurately by the use of the recent high-precision H2O line-list. Then, we propose a simple and flexible method of analyzing equivalent widths (EWs) of blended features (i.e., not necessarily limited to single lines) by the use of a mini curve-of-growth (CG), which is a small portion of the usual CG around the observed EW. The mini CG is generated by using the theoretical EWs evaluated from the synthetic spectrum by exactly the same way as the EWs are measured from the observed spectrum. The observed EW is converted to the abundance by the use of the mini CG, and the process is repeated for all the observed EWs line-by-line or blend-by-blend. In cool M dwarfs, almost all the oxygen atoms left after CO formation are in stable H2O molecules, which suffer little change for the uncertainties due to imperfect modelling of the photospheres. Moreover, the thermal velocity of H2O is most probably larger than the micro-turbulent velocity because of its lower molecular weight, and the uncertainty of the micro-turbulent velocity will have relatively minor effect on the abundance determination. Then the numerous H2O lines are excellent abundance indicators of oxygen. The oxygen abundances are determined to be logAO (AO = NO/NH) between -3.5 and -3.0 in 38 M dwarfs, but cannot in four early M dwarfs in which H2O lines are detected only marginally. The resulting logAO/AC plotted against logAC appears to be systematically smaller in the carbon-rich M dwarfs, showing the different formation histories of oxygen and carbon in the chemical evolution of the Galactic disk. Also, AO/AFe ratios in most M dwarfs are closer to the solar AO/AFe ratio based on the classical high oxygen abundance rather than on the recently downward revised low value.