Abstract

A study of the activity of a bright M-dwarf L 98-59, located at a distance of 10.6 pc from Earth is reported. The star has a multiplanetary system, where one of the planets, L 98-59 b, is a rocky planet with a mass not exceeding the mass of Venus. It was found that the level of chromospheric activity of L 98-59 does not exceed the average value characteristic of other cold dwarfs with similar color index values \((B-V)\) . This level is below solar, the mean \(\log R{{}^{\prime}}_{\textrm{HK}}\) parameter for L 98-59 is equal to \(-\) 5.297 dex. The data about the parent star and its activity was used to estimate the magnitude of the atmospheric escape of five planets (including the planet candidate 05). To calculate the loss of matter from the planetary atmospheres (without a detailed modeling of the processes in the ‘‘star–planet’’ system), we applied an approximation formula corresponding to the energy-limited atmospheric loss model. Estimates of the XUV photon flux—extreme ultraviolet (EUV) and X-ray photons for the medium and high activity levels, as well as the estimates of the tidal parameter \(K_{\textrm{tide}}(\xi)\) were found. Variations of the \(\dot{M}\) parameter have been specified for five planets of the L 98-59 system, taking into account the possible activity level variations of the star. The \(\dot{M}\) parameter variations for planets c–e range from \(7.6\times 10^{6}\) g s \({}^{-1}\) to \(3.8\times 10^{7}\) g s \({}^{-1}\) (average activity level) and \(1.2\) – \(5.7\times 10^{7}\) g s \({}^{-1}\) (high activity level). For the planet L 98-59 b, the an analogue of Venus, we got the following estimates: \(1.3\times 10^{7}\) and \(2.0\times 10^{7}\) g s \({}^{-1}\) (for the intermediate and high activity levels, respectively). It can be assumed that the relatively modest outflow of matter from their atmospheres \(\dot{M}\) , despite the proximity of the planets to their star, is due to a small flux of XUV photons from the star due to the low level of its activity.

中文翻译：

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M矮星L 98-59的活动及其系外行星的大气逃逸

摘要

据报道，对一颗明亮的 M 矮星 L 98-59 的活动进行了研究，该矮星距离地球 10.6 pc。该恒星拥有多行星系统，其中一颗行星L 98-59 b是一颗岩石行星，质量不超过金星的质量。结果发现，L 98-59 的色球活动水平不超过具有相似颜色指数值的其他冷矮星的平均值特征\((BV)\)。该水平低于太阳光，L 98-59 的平均\(\log R{{}^{\prime}}_{\textrm{HK}}\)参数等于\(-\) 5.297 dex。有关母星及其活动的数据被用来估计五颗行星（包括候选行星05）的大气逃逸程度。为了计算行星大气中的物质损失（无需对“恒星-行星”系统中的过程进行详细建模），我们应用了与能量限制大气损失模型相对应的近似公式。XUV光子通量的估计——中高活动水平的极紫外（EUV）和X射线光子，以及潮汐参数\(K_{\textrm{tide}}(\xi)\)的估计被发现。考虑到恒星可能的活动水平变化，已为 L 98-59 系统的五颗行星指定了\ (\dot{M}\)参数的变化。行星 c–e 的 \(\dot{M}\) 参数变化范围为\ ( 7.6\times 10^{6}\) gs \({}^{-1}\)到\(3.8\times 10 ^{7}\) gs \({}^{-1}\)（平均活动水平）和\(1.2\) – \(5.7\times 10^{7}\) gs \({}^{- 1}\)（高活动水平）。对于行星 L 98-59 b（金星的类似物），我们得到以下估计：\(1.3\times 10^{7}\)和\(2.0\times 10^{7}\) gs \({ }^{-1}\)（分别用于中级和高级活动水平）。可以假设，尽管行星与其恒星很接近，但物质从其大气层\(\dot{M}\)的流出量相对较小，这是由于恒星的 XUV 光子通量较小，这是由于恒星的 XUV 光子通量较低。其活动水平。