[论文解读] MIRACLES: atmospheric characterization of directly imaged planets and substellar companions at 4-5 $μ$m. II. Constraints on the mass and radius of the enshrouded planet PDS 70 b

T. Stolker, Gabriel-Dominique Marleau+5 more|UvA-DARE (University of Amsterdam)|Sep 9, 2020

Stellar, planetary, and galactic studies参考文献 117被引用 7

一句话总结

本研究利用甚大望远镜/NACO在Br𝛼和M′滤镜下首次直接探测到PDS 70 b，测得其大气层温度为1193 ± 20 K，半径为3.0 ± 0.2 R_J。通过将光谱能量分布（SED）的黑体拟合与吸积光度估计相结合，作者将该行星的质量约束在0.5–1.5 M_J之间，半径为1–2.5 R_J，与正在进行的形成过程及尘埃包层一致。

ABSTRACT

The circumstellar disk of PDS 70 hosts two forming planets, which are actively accreting gas from their environment. In this work, we report the first detection of PDS 70 b in the Br$α$ and $M'$ filters with VLT/NACO, a tentative detection of PDS 70 c in Br$α$, and a reanalysis of archival NACO $L'$ and SPHERE $H23$ and $K12$ imaging data. The near side of the disk is also resolved with the Br$α$ and $M'$ filters, indicating that scattered light is non-negligible at these wavelengths. The spectral energy distribution of PDS 70 b is well described by blackbody emission, for which we constrain the photospheric temperature and photospheric radius to $T_\mathrm{eff}=1193 \pm 20$ K and $R=3.0 \pm 0.2$ $R_\mathrm{J}$. The relatively low bolometric luminosity, $\log(L/L_\odot) = -3.79 \pm 0.02$, in combination with the large radius, is not compatible with standard structure models of fully convective objects. With predictions from such models, and adopting a recent estimate of the accretion rate, we derive a planetary mass and radius in the range of $M_\mathrm{p}\approx 0.5-1.5$ $M_\mathrm{J}$ and $R_\mathrm{p}\approx 1-2.5$ $R_\mathrm{J}$, independently of the age and post-formation entropy of the planet. The blackbody emission, large photospheric radius, and the discrepancy between the photospheric and planetary radius suggests that infrared observations probe an extended, dusty environment around the planet, which obscures the view on its molecular composition. Finally, we derive a rough upper limit on the temperature and radius of potential excess emission from a circumplanetary disk, $T_\mathrm{eff}\lesssim256$ K and $R\lesssim245$ $R_\mathrm{J}$, but we do find weak evidence that the current data favors a model with a single blackbody component.

研究动机与目标

• 确定PDS 70 b的物理特性，该行星为位于原恒星盘中、直接成像的吸积原行星。
• 通过分析4–5 μm波段的测光与光谱，解决其光谱外观的模糊性。
• 利用光度测量与吸积模型，独立于不确定的冷却时间，约束行星的质量与半径。
• 通过M′波段潜在的超量辐射，评估原行星盘（CPD）的存在及其特性。

提出的方法

• 利用甚大望远镜/NACO在Br𝛼和M′滤镜下获取高对比度成像数据，并重新分析了档案中的SPHERE与NACO数据。
• 从1至5 μm构建PDS 70 b的光谱能量分布（SED），并用单温度黑体模型进行拟合。
• 通过SED拟合计算得到总光度为log(L/L⊙) = −3.79 ± 0.02。
• 应用孤立气态巨行星的标准模型，推导质量–半径解，结合Hashimoto等人（2020）提供的吸积光度。
• 通过在M′波段建模潜在的超量辐射，评估原行星盘（CPD）的存在。
• 使用贝叶斯模型比较（贝叶斯因子）检验两组分模型（黑体 + CPD）是否比单黑体模型更优。

实验结果

研究问题

• RQ1 基于其4–5 μm测光，PDS 70 b的大气层温度与半径是多少？
• RQ2 将吸积光度纳入考虑后，对PDS 70 b的质量与半径的推断有何影响？
• RQ3 M′波段是否存在超量辐射，可作为原行星盘（CPD）存在的证据？
• RQ4 其大气层半径与行星半径之间的显著差异在多大程度上表明存在尘埃包覆、延伸的包层？
• RQ5 所推导的光度与半径与形成行星的群体合成模型预测相比如何？

主要发现

• PDS 70 b的大气层温度被约束为T_eff = 1193 ± 20 K，大气层半径为R = 3.0 ± 0.2 R_J。
• 总光度为log(L/L⊙) = −3.79 ± 0.02，与SED拟合的黑体模型一致。
• 当纳入吸积光度（基于Hashimoto等人，2020）时，仅质量不超过1.5 M_J的解与观测光度一致。
• PDS 70 b的质量被约束在0.5–1.5 M_J之间，物理半径为1–2.5 R_J，与形成模型及间隙宽度估计一致。
• 大气层半径显著小于希尔半径，表明存在一个由气态吸积流持续补充的尘埃包层。
• M′波段可能存在第二个黑体组分，暗示原行星盘的温度上限为T_eff ≲ 256 K，半径上限为R ≲ 245 R_J，但贝叶斯因子更支持单黑体模型。