[论文解读] Searching for faint companions with VLTI/PIONIER. II. 92 main sequence stars from the Exozodi survey

Lindsay Marion, Olivier Absil+7 more|arXiv (Cornell University)|Sep 22, 2014

Stellar, planetary, and galactic studies参考文献 35被引用 23

一句话总结

本研究利用VLTI/PIONIER干涉仪数据，在Exozodi巡天的92颗主序星中搜索微弱的恒星伴星，发现了五颗此前未被探测到的伴星——其中四颗围绕A型星，从而消除了该巡天中系外行星盘统计的偏差，并表明A型星中视觉双星的比例比以往认知高出约13%。该研究证明，结合闭合相位与平方可见度可最大化检测完整性，尤其在仅使用闭合相位时灵敏度较低的区域效果显著。

ABSTRACT

The Exozodi survey aims to determine the occurrence rate of bright exozodiacal discs around nearby main sequence stars using infrared interferometry. Although the Exozodi survey targets have been carefully selected to avoid the presence of binary stars, the results of this survey can still be biased by the presence of unidentified stellar companions. Using the PIONIER data set collected within the Exozodi survey, we aim to search for the signature of point-like companions around the Exozodi target stars. We use both the closure phases and squared visibilities collected by PIONIER to search for companions within the ~100 mas interferometric field of view. The presence of a companion is assessed by computing the goodness of fit to the data for a series of binary models with various separations and contrasts. Five stellar companions are resolved for the first time around five A-type stars: HD 4150, HD 16555, HD 29388, HD 202730, and HD 224392 (although the companion to HD 16555 was independently resolved by speckle interferometry while we were carrying out the survey). In the most likely case of main sequence companions, their spectral types range from A5V to K4V. Three of these stars were already suspected to be binaries from Hipparcos astrometric measurements, although no information was available on the companions themselves so far. In addition to debiasing the statistics of the Exozodi survey, these results can also be used to revise the fraction of visual binaries among A-type stars, suggesting that an extra ~13% A-type stars are visual binaries in addition to the ones detected in previous direct imaging surveys. We estimate that about half the population of nearby A-type stars could be resolved as visual binaries using a combination of state-of-the-art interferometry and single-aperture imaging, and we suggest that a significant fraction of these binaries remains undetected to date.

研究动机与目标

• 识别Exozodi巡天中92颗主序星（这些恒星被选为单星，以避免对系外行星盘探测造成污染）中此前未被探测到的恒星伴星。
• 校正因未探测到的近距离或微弱伴星而可能对Exozodi巡天中系外行星盘发生率统计造成的偏差。
• 评估PIONIER在检测微弱伴星时，结合闭合相位与平方可见度的灵敏度与完整性。
• 通过识别先前天体测量与成像巡天遗漏的伴星，重新评估A型星中视觉双星的比例。
• 评估闭合相位与平方可见度在干涉仪视场内检测非轴向伴星时的相对性能。

提出的方法

• 利用ESO项目089.C-0365与090.C-0526获取的92颗恒星的PIONIER数据，测量近红外波段的闭合相位与平方可见度。
• 通过计算不同分离度与流量对比度的双星模型拟合优度，进行模型拟合分析，以探测点源伴星。
• 定义闭合相位的放大因子 $ m_{\text{CP}} = \sin(\alpha_{12}) + \sin(\alpha_{23}) - \sin(\alpha_{12}+\alpha_{23}) $，量化视场内几何灵敏度的变化。
• 定义平方可见度的互补放大因子 $ m_{V^2} = \sum_{ij}(1 - V^2_{ij}) $，评估基于可见度的灵敏度与可探测性。
• 利用甚大望远镜干涉仪（VLTI）的三望远镜构型，绘制干涉仪视场内的灵敏度分布，识别出伴星无法被探测到的盲区。
• 结合闭合相位与平方可见度信息，提升检测完整性，尤其在仅使用闭合相位时灵敏度较低的区域。

实验结果

研究问题

• RQ1 考虑到许多A型星因探测极限而未被确认为双星，其真实视觉双星频率是多少？
• RQ2 Exozodi巡天中未解析的伴星在多大程度上导致了系外行星盘发生率测量的偏差？
• RQ3 在干涉仪数据中，闭合相位与平方可见度在检测微弱、非轴向伴星时的灵敏度与完整性如何比较？
• RQ4 基于高分辨率干涉测量，先前被归类为单星的A型星中，有多少比例实际上是视觉双星？
• RQ5 与单独使用任一方法相比，结合闭合相位与平方可见度是否能显著提升对微弱伴星的检测完整性？

主要发现

• 首次在A型星HD 4150、HD 16555、HD 29388、HD 202730与HD 224392周围解析出五颗新的恒星伴星，其光谱型从A5V至K4V不等。
• 五颗伴星中的三颗通过斑点干涉测量（HD 16555）独立确认，或此前由Hipparcos天体测量数据推测存在（HD 4150、HD 224392），验证了检测方法的有效性。
• 在A型星中探测到伴星，使视觉双星比例估计值从约34%提升至约47%，表明A型星中视觉双星的实际比例比以往认知高出约13%。
• PIONIER巡天模式的中位灵敏度（每颗恒星3个观测块）达到约1%的对比度，闭合相位在90%完整性下对应约2.7%的对比度，平方可见度则为约1.6%的对比度。
• 由于闭合相位具有振荡特性（可为负或零），其盲区（灵敏度低的区域）显著多于平方可见度，而平方可见度的放大因子始终为正。
• 结合闭合相位与平方可见度对最大化检测完整性至关重要，因为两种方法的盲区与灵敏度分布具有互补性。