[论文解读] The Penn State-Torun Centre for Astronomy Planet Search stars. I. Spectroscopic analysis of 348 red giants

Paweł Zieliński, A. Niedzielski+3 more|arXiv (Cornell University)|Jun 27, 2012

Stellar, planetary, and galactic studies参考文献 102被引用 27

一句话总结

本研究对宾夕法尼亚州立大学-托伦天文中心系外行星搜索项目中的348颗红巨星进行了光谱分析，利用高分辨率HET光谱数据推导出大气参数（Teff, log g, vt, [Fe/H]）、光度、质量、半径和绝对视向速度。其主要贡献在于提供了一个校准良好、均匀一致的演化星样本，对解释低质量系外行星伴星以及研究中等质量恒星周围行星系统的演化具有重要意义。

ABSTRACT

We present basic atmospheric parameters (Teff, logg, vt and [Fe/H]) as well as luminosities, masses, radii and absolute radial velocities for 348 stars, presumably giants, from the ~1000 star sample observed within the Penn State-Torun Centre for Astronomy Planet Search with the High Resolution Spectrograph of the 9.2m Hobby-Eberly Telescope. The stellar parameters are key ingredients in proper interpretation of newly discovered low-mass companions while a systematic study of the complete sample will create a basis for future statistical considerations concerning low-mass companions appearance around evolved low and intermediate-mass stars. The atmospheric parameters were derived using a strictly spectroscopic method based on the LTE analysis of equivalent widths of FeI and FeII lines. With existing photometric data and the Hipparcos parallaxes we estimated stellar masses and ages via evolutionary tracks fitting. The stellar radii were calculated from either estimated masses and the spectroscopic logg or from the spectroscopic Teff and estimated luminosities. The absolute radial velocities were obtained by cross-correlating spectra with a numerical template. We completed the spectroscopic analysis for 332 stars of which 327 were found to be giants. For the remaining 16 stars with incomplete data a simplified analysis was applied. The results show that our sample is composed of stars with Teff = 4055-6239 K, logg = 1.39-4.78 (5 dwarfs were identified), logL/Lo = -1.0-3, M = 0.6-3.4 Mo, R = 0.6-52 Ro. The stars in our sample are generally less metal abundant than the Sun with median [Fe/H] = -0.15. The estimated uncertainties in the atmospheric parameters were found to be comparable to those reached in other studies. However, due to lack of precise parallaxes the stellar luminosities and, in turn, the masses are far less precise, within 0.2 Mo in best cases, and 0.3 Mo on average.

研究动机与目标

• 确定宾夕法尼亚州立大学-托伦天文中心系外行星搜索项目中348颗演化恒星的精确大气参数（Teff, log g, vt, [Fe/H]）。
• 结合光谱参数与演化轨迹拟合，估算恒星的光度、质量、半径和年龄。
• 通过与数值模板互相关的方法，提供绝对视向速度以探测行星伴星。
• 创建一个均匀、高精度的数据集，用于统计研究演化低质量与中等质量恒星周围低质量伴星的特性。
• 评估所推导参数的可靠性与不确定性，尤其关注视差精度有限对质量与光度估计的影响。

提出的方法

• 对Fe I和Fe II谱线等效宽度进行LTE分析，以推导大气参数（Teff, log g, vt, [Fe/H]）。
• 将光谱参数与Hipparcos视差及测光数据结合，通过演化轨迹拟合估算恒星光度与质量。
• 利用光谱log g与推导出的质量，或光谱Teff与估算的光度，计算恒星半径。
• 通过CCF（互相关函数）方法将观测光谱与数值模板互相关，计算绝对视向速度。
• 对16颗数据不完整的恒星采用简化分析，而对332颗恒星完成完整分析。
• 评估参数的不确定性与系统误差，特别关注视差精度有限对质量与光度估计的影响。

实验结果

研究问题

• RQ1 在PTPS调查中，348颗红巨星样本的大气与基本恒星参数（Teff, log g, vt, [Fe/H], L, M, R, age）是什么？
• RQ2 考虑到样本中视差精度有限，所推导的恒星质量与光度的精确度如何？
• RQ3 所推导的参数（尤其是[Fe/H]）在多大程度上反映了太阳邻近区演化恒星的金属量分布？
• RQ4 CCF方法在精度与可靠性方面，对演化恒星的视向速度测量表现如何？
• RQ5 样本中质量大于2 M☉的恒星所占比例是多少？这与低质量行星伴星存在的可能性有何关联？

主要发现

• 样本中包含327颗确认的红巨星，有效温度范围为4055 K至6239 K，表面重力为1.39至4.78 dex。
• 恒星质量范围为0.6至3.4 M☉，仅有63颗恒星质量超过2 M☉，表明高质质量演化恒星的比例较低。
• 半径范围为0.6至52 R☉，大多数恒星半径在9至11 R☉之间，与红巨星演化特征一致。
• 中位金属量[Fe/H]为−0.15，表明这些恒星通常比太阳更贫金属。
• 质量不确定度在最佳情况下约为0.2 M☉，平均为0.3 M☉，主要受限于视差精度。
• 大气参数的估计不确定度与其它高精度光谱研究相当，表明该方法在数据受限条件下仍具可靠性。