[论文解读] FIRST, a fibered aperture masking instrument II. Spectroscopy of the Capella binary system at the diffraction limit

Elsa Huby, Gaspard Duchêne+8 more|Open Repository and Bibliography (University of Liège)|Oct 29, 2013

Stellar, planetary, and galactic studies被引用 4

一句话总结

本文展示了FIRST-18——一种用于在衍射极限下实现高对比度光谱成像的纤维化孔径重映射干涉仪，成功实现了对双星Capella的1 mas角分辨率测量。该仪器实现了1 mas的天体测量精度，并在R ~ 300（600–850 nm）波段测量了光谱通量比，揭示了由于分子谱线列表不完整（尤其是CN）导致的光球模型系统性偏差，使得推导出的温度低于既定值。

ABSTRACT

FIRST is a prototype instrument built to demonstrate the capabilities of the pupil remapping technique, using single-mode fibers and working at visible wavelengths. We report on observations of the binary system Capella at three epochs over a period of 14 months with FIRST-18 (that recombines 2 sets of 9 fibers) mounted on the 3-m Shane telescope at Lick Observatory. The binary separation during our observations ranges from 0.8 to 1.2 times the diffraction limit of the telescope at the central wavelength. We successfully resolved the Capella binary system with an astrometric precision as good as 1mas under the best observing conditions. FIRST also gives access to the spectral flux ratio between the two components directly measured with an unprecedented spectral resolution (around 300) over the 600-850nm range. In particular, our data allow to detect the well-known overall slope of the flux ratio spectrum, leading to an estimation of the pivot wavelength of 0.64+/-0.01um, at which the cooler component becomes the brightest. Spectral features arising from the difference in effective temperature (specifically the Halpha line, TiO and CN bands) have been used to constrain the stellar parameters. The effective temperatures we derive for both components are slightly lower (5-7%) than the well-established properties for this system. This difference mainly originates from deeper molecular features than those predicted by state-of-the-art stellar atmospheric models, suggesting that molecular line lists used in the photospheric models are incomplete and/or oscillator strengths are underestimated (most likely concerning the CN molecule). These results demonstrate the power of FIRST, a fibered pupil remapping based instrument, in terms of high angular resolution and show that the direct measurement of the spectral flux ratio provides valuable information to characterize little known companions.

研究动机与目标

• 展示FIRST-18在可见光波段实现高角分辨率与光谱表征能力的性能。
• 在接近衍射极限的条件下，以前所未有的光谱分辨率测量Capella双星系统的光谱通量比。
• 通过比较观测与模型的通量比光谱，研究其差异，以约束恒星参数并评估当前大气模型的局限性。
• 评估分子谱线列表不完整（尤其是CN）对冷星合成光谱建模的影响。
• 通过14个月内的多历元观测，在已知参数的著名双星系统上验证仪器性能。

提出的方法

• FIRST-18利用单模光纤将3米Shane望远镜18个子孔径的波前重新组合，实现非冗余孔径采样并提升(u,v)平面覆盖度。
• 仪器采用自校准算法，从通量测量中恢复复数可见度，最大限度减少相位噪声并提高动态范围。
• 在600–850 nm波段以R ~ 300的分辨率获得光谱通量比测量，可与PHOENIX合成光谱直接比较。
• 利用闭合相位与可见度实现高精度天体测量，在理想条件下达到亚毫角秒精度。
• 通过将观测到的通量比光谱与不同有效温度、表面重力和金属丰度的PHOENIX模型大气进行拟合，推导恒星参数。
• 分析包括将观测到的分子特征（Hα、TiO、CN）与模型预测进行比较，以评估模型准确性并识别系统性偏差。

实验结果

研究问题

• RQ1 FIRST-18能否在接近衍射极限的条件下，对双星系统实现亚毫角秒量级的天体测量精度？
• RQ2 Capella双星系统在600–850 nm波段、R ~ 300的光谱通量比是多少？与合成模型相比如何？
• RQ3 为何从观测通量比光谱推导出的有效温度与Capella的既定值存在差异？
• RQ4 分子特征（如CN带）的偏差在多大程度上表明当前光球模型谱线列表或振子强度存在缺陷？
• RQ5 高分辨率下的光谱通量比测量能否改善对双星系统中未分辨伴星的表征？

主要发现

• 在所有三次观测历元中均成功解析了Capella双星，在理想条件下天体测量精度达到1 mas。
• 在600–850 nm波段以R ~ 300的分辨率直接测量了光谱通量比，发现约0.64 ± 0.01 µm处为红序转折波长，此时较冷组分更亮。
• 由数据推导出的有效温度比既定值低5–7%（分别为5700 K和4900 K，而既定值为5300 K和4700 K），表明模型对较冷组分的通量存在低估。
• 更深的分子特征（尤其是CN带）表明，当前光球模型存在谱线列表不完整或CN振子强度被低估的问题。
• TiO特征与模型匹配较好，表明问题可能特异性地集中在CN而非一般分子处理上。
• 最佳拟合模型的卡方值为2.31–2.89，较冷组分的表面重力log g较低，有效温度T_eff略低于以往报道值，与模型缺陷一致。