[论文解读] The CARMENES search for exoplanets around M dwarfs - Photospheric parameters of target stars from high-resolution spectroscopy

V. M. Passegger, A. Reiners+15 more|arXiv (Cornell University)|Feb 8, 2018

Stellar, planetary, and galactic studies参考文献 46被引用 45

一句话总结

本研究利用来自 CARMENES 及其他分光仪的高分辨率光谱，针对 300 个 M 型矮星目标推导基本的光球层参数（Teff、log g、[Fe/H]），并验证新的 PHOENIX-ACES 模型。

ABSTRACT

The new CARMENES instrument comprises two high-resolution and high-stability spectrographs that are used to search for habitable planets around M dwarfs in the visible and near-infrared regime via the Doppler technique. Characterising our target sample is important for constraining the physical properties of any planetary systems that are detected. The aim of this paper is to determine the fundamental stellar parameters of the CARMENES M-dwarf target sample from high-resolution spectra observed with CARMENES. We also include several M-dwarf spectra observed with other high-resolution spectrographs, that is CAFE, FEROS, and HRS, for completeness. We used a {chi}^2 method to derive the stellar parameters effective temperature T_eff, surface gravity log g, and metallicity [Fe/H] of the target stars by fitting the most recent version of the PHOENIX-ACES models to high-resolution spectroscopic data. These stellar atmosphere models incorporate a new equation of state to describe spectral features of low-temperature stellar atmospheres. Since T_eff, log g, and [Fe/H] show degeneracies, the surface gravity is determined independently using stellar evolutionary models. We derive the stellar parameters for a total of 300 stars. The fits achieve very good agreement between the PHOENIX models and observed spectra. We estimate that our method provides parameters with uncertainties of {sigma} T_eff = 51 K, {sigma} log g = 0.07, and {sigma} [Fe/H] = 0.16, and show that atmosphere models for low-mass stars have significantly improved in the last years. Our work also provides an independent test of the new PHOENIX-ACES models, and a comparison for other methods using low-resolution spectra. In particular, our effective temperatures agree well with literature values, while metallicities determined with our method exhibit a larger spread when compared to literature results.

研究动机与目标

• 表征目标 M 型矮星样本，以便准确解释行星系统。
• 利用高分辨率光谱拟合，为每颗恒星提供精确的 Teff、log g 和 [Fe/H]。
• 评估 PHOENIX-ACES 大气模型在冷恒星中的性能与局限。

提出的方法

• 使用卡方最小化方法，将高分辨率光谱拟合到 PHOENIX-ACES 模型大气参数。
• 采用两步网格搜索（粗略然后精细）来确定 Teff、log g 和 [Fe/H]。
• 从独立测量中固定 v sin i，以考虑线宽化。
• 从演化 Baraffe 模型独立推定 log g，以破解 Teff–log g–[Fe/H] 的简并性。
• 排除模型无法很好再现的谱线，并对鲁棒特征（如 gamma-TiO、Mg I）应用加权拟合。
• 通过对合成光谱进行泊松噪声模拟来估计不确定性。

实验结果

研究问题

• RQ1 能否使用 PHOENIX-ACES 模型的高分辨率光谱在大样本的 M 型矮星中一致地恢复 Teff、log g 和 [Fe/H]？
• RQ2 推导的参数与文献值以及独立的质量–温度关系相比如何？
• RQ3 在拟合冷恒星光谱时存在哪些限制和简并性，独立的 log g 估计如何提高鲁棒性？

主要发现

• 为 300 颗恒星获得的参数在 Teff 约为 51 K、log g 约为 0.07 dex、[Fe/H] 约为 0.16 dex 的不确定性。
• 拟合显示 PHOENIX-ACES 模型与观测的高分辨率光谱之间有非常好的一致性。
• Teff 值通常与文献一致，而 [Fe/H] 在文献比较中呈现更大的离散。
• 大多数恒星在年龄为 5 Gyr、金属丰度为太阳的情况下，位于 Baraffe et al. (1998) 的质量–Teff 关系附近；讨论了轻微的偏移。
• 金属丰度测定更具挑战性，可能显示出与模型线深度和固定太阳组分相关的系统偏差。
• 讨论了 8 个离群值，可能与活动性、丰度变化或年龄效应影响拟合。