[论文解读] Searching for faint companions with VLTI/PIONIER. I. Method and first results

Olivier Absil, J.-B. Le Bouquin+11 more|CaltechAUTHORS (California Institute of Technology)|Oct 6, 2011

Stellar, planetary, and galactic studies参考文献 30被引用 38

一句话总结

该论文提出一种新颖的干涉测量方法，利用VLTI/PIONIER通过闭合相位分析检测微弱恒星伴星，深积分下动态范围可达1:500。结果证实，织田星和鲸鱼座υ星周围的近红外过量源自扩展的尘埃盘，而非点状伴星，并首次直接探测到40 mas处已知伴星delta Aquarii，其流量比为2.05×10⁻²。

ABSTRACT

Context. A new four-telescope interferometric instrument called PIONIER has recently been installed at VLTI. It provides improved imaging capabilities together with high precision. Aims. We search for low-mass companions around a few bright stars using different strategies, and determine the dynamic range currently reachable with PIONIER. Methods. Our method is based on the closure phase, which is the most robust interferometric quantity when searching for faint companions. We computed the chi^2 goodness of fit for a series of binary star models at different positions and with various flux ratios. The resulting chi^2 cube was used to identify the best-fit binary model and evaluate its significance, or to determine upper limits on the companion flux in case of non detections. Results. No companion is found around Fomalhaut, tau Cet and Regulus. The median upper limits at 3 sigma on the companion flux ratio are respectively of 2.3e-3 (in 4 h), 3.5e-3 (in 3 h) and 5.4e-3 (in 1.5 h) on the search region extending from 5 to 100 mas. Our observations confirm that the previously detected near-infrared excess emissions around Fomalhaut and tau Cet are not related to a low-mass companion, and instead come from an extended source such as an exozodiacal disk. In the case of del Aqr, in 30 min of observation, we obtain the first direct detection of a previously known companion, at an angular distance of about 40 mas and with a flux ratio of 2.05e-2 \pm 0.16e-2. Due to the limited u,v plane coverage, its position can, however, not be unambiguously determined. Conclusions. After only a few months of operation, PIONIER has already achieved one of the best dynamic ranges world-wide for multi-aperture interferometers. A dynamic range up to about 1:500 is demonstrated, but significant improvements are still required to reach the ultimate goal of directly detecting hot giant extrasolar planets.

研究动机与目标

• 开发一种利用多孔径干涉测量实现高动态范围检测明亮未resolved恒星周围微弱伴星的稳健方法。
• 确定VLTI上新型PIONIER仪器当前的动态范围性能。
• 检验先前在织田星、鲸鱼座υ星和室女座α星周围探测到的近红外过量是否源于未分辨的低质量伴星或扩展发射源。
• 通过注入虚假伴星的双盲测试验证该方法。
• 展示在亚角秒分离距离下直接探测已知伴星的可行性，并提升天体测量精度。

提出的方法

• 该方法依赖闭合相位分析，对大气和仪器相位误差不敏感，因此在检测微弱伴星方面具有鲁棒性。
• 在伴星位置与流量比的三维参数空间中执行χ²拟合优度检验，以识别最佳拟合双星模型。
• 利用所得χ²立方体评估检测显著性，或在未检测到时设定伴星流量的3σ上限。
• 通过至少四至五次在不同时角的校准观测（OBs），采用多种基线配置（u,v平面覆盖）进行观测，以最小化系统误差。
• 使用具有已知闭合相位的参考星进行校准，并使用多个校准星以减少系统漂移。
• 通过多次观测平均及使用稳定校准星，考虑潜在系统效应（如条纹漂移和大气变化）的影响。

实验结果

研究问题

• RQ1 PIONIER在检测明亮未resolved恒星周围微弱伴星时，可实现的最大动态范围是多少？
• RQ2 先前在织田星和鲸鱼座υ星周围探测到的近红外过量是否源于低质量伴星，还是扩展发射源？
• RQ3 PIONIER能否在高角分辨率下实现对已知、此前约束较差的伴星的直接探测？
• RQ4 校准观测次数如何影响伴星检测的可靠性与天体测量精度？
• RQ5 当前PIONIER配置中，限制动态范围的主要系统误差是什么？

主要发现

• 在5–100 mas的搜索区域内，Fomalhaut（4小时积分）的中位3σ伴星流量比上限为2.3×10⁻³，tau Cet（3小时）为3.5×10⁻³，Regulus（1.5小时）为5.4×10⁻³。
• 在Fomalhaut和tau Cet中未检测到伴星，证实其近红外过量源于扩展源（如系外尘埃盘），而非点状伴星。
• 在40 mas分离处成功实现对delta Aquarii伴星的直接探测，流量比为2.05×10⁻² ± 0.16×10⁻²，但因u,v平面覆盖有限，其位置仍存在歧义。
• 通过深度积分（数小时）实现了1:500的动态范围，而三组高质量观测（约1小时）通常可实现1:200的动态范围。
• 在90%完整度下，动态范围分别为Fomalhaut的1:360、tau Cet的1:220和Regulus的1:130，表明在实际探测场景中灵敏度有所提升。
• 未来PIONIER的升级（包括更快的条纹扫描和改进的校准）有望将动态范围推进至1:1000，但实现1:5000的热木星探测仍不确定。