[论文解读] Chemical enrichment and star formation in the Milky Way disk; I. Sample description and chromospheric age-metallicity relation

H. J. Rocha–Pinto, W. J. Maciel+2 more|arXiv (Cornell University)|Jan 21, 2000

Stellar, planetary, and galactic studies参考文献 2被引用 15

一句话总结

本研究利用552颗晚型矮星的样本，结合色球层活动指数估算年龄与uvby测光法测定金属量，重新研究了太阳邻域的年龄-金属量关系（AMR）。研究发现，150亿年间金属量至少上升0.56 dex，呈平滑、稳定的增长，内在离散度为0.13 dex，挑战了以往光谱研究中报告的高离散度，表明选择偏差——尤其是Edv93样本中的偏差——可能夸大了观测到的离散度。

ABSTRACT

The age-metallicity relation of the solar neighbourhood is studied using a sample of 552 late-type dwarfs. This sample was built from the intersection of photometric catalogues with chromospheric activity surveys of the Mount Wilson group. For these stars, metallicities were estimated from uvby data, and ages were calculated from their chromospheric emission levels using a new metallicity-dependent chromospheric activity-age relation developed by Rocha-Pinto & Maciel (1998). A careful estimate of the errors in the chromospheric age is made. The errors in the chromospheric indices are shown to include partially the effects of the stellar magnetic cycles, although a detailed treatment of this error is still beyond our knowledge. It is shown that the results are not affected by the presence of unresolved binaries in the sample. We derive an age-metallicity relation which confirms the mean trend found by previous workers. The mean metallicity shows a slow, steady increase with time, amounting at least 0.56 dex in 15 Gyr. The initial metallicity of the disk is around -0.70 dex, in agreement with the G dwarf metallicity distribution. According to our data, the intrinsic cosmic dispersion in metal abundances is around 0.13 dex, a factor of two smaller than that found by Edvardsson et al. (1993). We show that chromospheric ages are compatible with isochrone ages, within the expected errors, so that the difference in the scatter cannot be caused by the accuracy of our ages and metallicities. This reinforces some suggestions that the Edvarsson et al.'s sample is not suitable to the determination of the age-metallicity relation.

研究动机与目标

• 利用独立、大样本的晚型矮星，重新表达太阳邻域的年龄-金属量关系（AMR）
• 检验Edv93光谱样本中报告的高金属量离散度是否为银河系盘的真实特征，或仅为选择偏差所致
• 评估色球层活动作为恒星年龄指标的可靠性，并检验其与等龄线年龄的一致性
• 调查Carraro等人提出的3–5 Gyr间金属量丰富的恒星缺失现象是否真实存在，或仅为采样偏差所致
• 确定银河系盘的初始金属量，并评估金属丰度的内在离散度

提出的方法

• 通过将测光星表与威尔逊山天文台色球层活动调查结果交叉比对，构建了552颗晚型矮星的样本
• 利用校准至光谱值的uvby测光法估算金属量
• 基于Rocha-Pinto & Maciel (1998)提出的新金属量依赖的活动-年龄关系，利用色球层发射指数计算恒星年龄
• 进行了详细的误差分析，考虑了恒星磁周期和未分辨双星的影响，发现对年龄估计无显著影响
• 将色球层年龄与等龄线年龄进行比较以验证方法，发现在预期不确定度范围内具有良好的一致性
• 评估了选择效应——尤其是距离偏差和对明亮演化星的优先采样——对AMR离散度的影响

实验结果

研究问题

• RQ1 Edv93光谱样本中观测到的高金属量离散度是银河系盘的真实特征，还是样本选择偏差所致？
• RQ2 色球层活动-年龄关系是否能为晚型矮星提供可靠且一致的年龄估计，特别是与基于等龄线的年龄相比？
• RQ3 年龄-金属量关系的内在离散度是多少？与先前估计相比如何？
• RQ4 3–5 Gyr年龄区间金属量丰富的恒星缺失现象是否真实存在，还是观测偏差所致？
• RQ5 色球层年龄能否解决AMR中的矛盾，特别是关于金属量丰富恒星年龄的问题？

主要发现

• 年龄-金属量关系显示金属量呈平滑、稳定的增长，在太阳邻域150亿年间总上升至少0.56 dex
• 内在金属量离散度估计为0.13 dex，显著低于Edv93报告的0.25 dex，表明后者的高离散度可能源于样本偏差
• 银河系盘的初始金属量估计约为-0.70 dex，表明在薄盘形成之前已有金属丰度的预先富集
• 色球层年龄与等龄线年龄在预期误差范围内一致，支持色球层方法在年龄估计中的可靠性
• 本研究识别出Edv93样本中潜在的选择偏差，包括距离偏差和对明亮演化星的优先选择，可能解释了观测到的高离散度以及3–5 Gyr间金属量丰富恒星的缺失现象
• 结果支持AMR作为化学演化模型的稳健约束，表明Edv93样本可能因系统性偏差而不适于推导真实的AMR