[论文解读] First light of the VLT planet finder SPHERE. IV. Physical and chemical properties of the planets around HR8799

M. Bonnefoy, A. Zurlo+64 more|UvA-DARE (University of Amsterdam)|Nov 12, 2015

Stellar, planetary, and galactic studies参考文献 112被引用 73

一句话总结

本研究分析SPHERE仪器数据，以确定HR8799恒星周围四颗气态巨行星的物理与化学特性。通过将其光谱测光数据与场域棕矮星对比，并利用大气模型进行建模，发现HR8799d和e与有尘埃的L6–L8型棕矮星相符，而b和c则需要由亚微米级硅酸盐、刚玉或铁颗粒造成的红化；只有Exo-REM模型中厚云层的设定能将它们的SED在2σ范围内重现，表明强烈的尘埃消光效应。

ABSTRACT

The system of four planets around HR8799 offers a unique opportunity to probe the physics and chemistry at play in the atmospheres of self-luminous young (~30 Myr) planets. We recently obtained new photometry of the four planets and low-resolution (R~30) spectra of HR8799 d and e with the SPHERE instrument (paper III). In this paper (paper IV), we compare the available spectra and photometry of the planets to known objects and atmospheric models (BT-SETTL14, Cloud-AE60, Exo-REM) to characterize the atmospheric properties of the planets. We find that HR8799d and e properties are well reproduced by those of L6-L8 dusty dwarfs discovered in the field, among which some are candidate members of young nearby associations. No known object reproduces well the properties of planets b and c. Nevertheless, we find that the spectra and WISE photometry of peculiar and/or young early-T dwarfs reddened by submicron grains made of corundum, iron, enstatite, or forsterite successfully reproduce the SED of these two planets. Our analysis confirms that only the Exo-REM models with thick clouds fit (within 2σ) the whole set of spectrophotometric datapoints available for HR8799 d and e for Teff = 1200 K, log g in the range 3.0-4.5, and M/H=+0.5. The models still fail to reproduce the SED of HR8799c and b. The determination of the metallicity, log g, and cloud thickness are degenerate. We conclude that an enhanced content in dust and decreased CIA of H2 is certainly responsible for the deviation of the properties of the planet with respect to field dwarfs. The analysis suggests in addition that HR8799c and b have later spectral types than the two other planets, and therefore could both have lower masses.

研究动机与目标

• 利用新的SPHERE光谱测光数据，确定围绕HR8799恒星的四颗直接成像行星的物理与化学特性。
• 评估这些行星大气层与场域棕矮星及年轻伴星的差异。
• 评估亚微米颗粒（刚玉、硅酸盐、铁）的大气消光在塑造其光谱能量分布（SED）中的作用。
• 测试大气模型（BT-SETTL14、Cloud-AE60、Exo-REM）对观测SED的重现能力，并约束有效温度（T_eff）、表面重力（log g）和金属丰度（M/H）。
• 探究非平衡化学或分块云结构是否对解释数据是必要的。

提出的方法

• 利用甚大望远镜（VLT）上的SPHERE仪器，获取HR8799 d和e的低分辨率（R ~ 30）光谱及多波段测光数据。
• 将行星的SED与场域L型和T型棕矮星、年轻伴星以及已知有尘埃或红化的棕矮星的SED进行比较。
• 应用亚微米颗粒（刚玉、顽辉石、橄榄石、铁）的消光定律，模拟其对早期T型棕矮星光谱的红化效应。
• 使用三种大气模型（BT-SETTL14、Cloud-AE60、Exo-REM）拟合观测SED，变化有效温度（T_eff）、表面重力（log g）和金属丰度（M/H）。
• 使用卡方统计量（在1–5 μm波段内2σ范围内）评估模型拟合质量。
• 通过比较模型表现，评估非平衡化学与分块云结构的必要性。

实验结果

研究问题

• RQ1 HR8799 b、c、d、e的物理与化学特性与场域棕矮星及年轻伴星相比有何异同？
• RQ2 HR8799 b和c的异常SED在多大程度上可通过亚微米尘埃颗粒（如硅酸盐、刚玉或铁）红化已知的早期T型棕矮星来重现？
• RQ3 哪种大气模型最能重现HR8799 d和e的完整光谱测光数据？其推导出的大气参数为何？
• RQ4 是否必须引入非平衡化学或分块云结构来解释观测到的SED，还是仅厚云消光已足够？
• RQ5 增强的尘埃消光与减弱的H2碰撞诱导吸收（CIA）在解释行星偏离场域棕矮星行为方面起何作用？

主要发现

• HR8799d和e可被有尘埃的L6–L8型场域棕矮星良好拟合，有效温度为1100–1300 K，表面重力（log g）在3.0至4.5之间。
• Exo-REM模型中厚云层设定可将HR8799d和e的SED在1–5 μm波段内以2σ范围拟合，得出T_eff ≈ 1200 K，log g = 3.0–4.5，M/H = +0.5。
• 尚无已知场域天体能重现HR8799b和c的特性，但其SED可通过亚微米硅酸盐、刚玉或铁颗粒红化的早期T型棕矮星成功匹配。
• 分析证实，增强的尘埃消光与减弱的H2 CIA是导致行星光谱偏离场域棕矮星行为的关键因素。
• 模型无法重现HR8799b和c的SED，表明当前建模中云层厚度、金属丰度与重力仍存在退化关系。
• 对于HR8799d和e的SED，非平衡化学与分块云结构并非关键必要条件，因为在Exo-REM框架下仅厚云层已足够。