CARMENES寻找M矮星周围的系外行星：附近冷恒星中的铷丰度
The CARMENES search for exoplanets around M dwarfs: Rubidium abundances in nearby cool stars
摘要：
在这项研究中，中子捕获元素Rb，Sr和Zr的丰度是第一次，在附近的M矮星的样本。我们关注的是金属丰度范围为-0.5<[Fe/H]<+0.3的恒星，这是以前分析中对Rb丰度探索不足的区间。为此，我们使用高分辨率，高信噪比，光学和近红外光谱的57 M矮星与CARMENES观察。大多数M矮星的[Sr/Fe]和[Zr/Fe]比值几乎恒定在太阳值附近，与相同金属丰度的GK矮星相同。然而，对于Rb，我们发现系统丰度不足（[Rb/Fe]<0.0）平均为2倍。此外，发现Rb有随金属丰度增加而增加的趋势，而其它重元素（Sr，Zr）则无此趋势，当金属丰度高于太阳时，[Rb/Fe]>0.0。这是令人惊讶的结果，从未见过任何其他重元素，并且很难在s-和r-过程的公式中理解，这两个过程都是银河Rb丰度的来源。我们讨论了这些发现的可靠性Rb在非LTE效应，恒星活动，或在太阳系中的异常Rb丰度，但没有找到解释，然后我们解释了完整的观察到的[Rb/Fe]与[Fe/H]的趋势的理论预测框架内，从国家的最先进的重元素化学演化模型，但一个简单的解释也没有找到。特别是，[Rb/Fe]比在超太阳金属丰度的可能的二次行为将需要一个更大的生产Rb比目前预测的AGB星通过s-过程没有过量生产Sr和Zr。
英文摘要：
In this study, abundances of the neutron-capture elements Rb, Sr, and Zr are derived, for the first time, in a sample of nearby M dwarfs. We focus on stars in the metallicity range -0.5<[Fe/H]<+0.3, an interval poorly explored for Rb abundances in previous analyses. To do this we use high-resolution, high-signal-to-noise-ratio, optical and near-infrared spectra of 57 M dwarfs observed with CARMENES. The resulting [Sr/Fe] and [Zr/Fe] ratios for most M dwarfs are almost constant at about the solar value, and are identical to those found in GK dwarfs of the same metallicity. However, for Rb we find systematic underabundances ([Rb/Fe]<0.0) by a factor two on average. Furthermore, a tendency is found for Rb-but not for other heavy elements (Sr, Zr) -to increase with increasing metallicity such that [Rb/Fe]>0.0 is attained at metallicities higher than solar. These are surprising results, never seen for any other heavy element, and are difficult to understand within the formulation of the s- and r-processes, both contributing sources to the Galactic Rb abundance. We discuss the reliability of these findings for Rb in terms of non-LTE effects, stellar activity, or an anomalous Rb abundance in the Solar System, but no explanation is found. We then interpret the full observed [Rb/Fe] versus [Fe/H] trend within the framework of theoretical predictions from state-of-the-art chemical evolution models for heavy elements, but a simple interpretation is not found either. In particular, the possible secondary behaviour of the [Rb/Fe] ratio at super-solar metallicities would require a much larger production of Rb than currently predicted in AGB stars through the s-process without overproducing Sr and Zr.
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