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

T. Stolker, Gabriel-Dominique Marleau+5 more|arXiv (Cornell University)|Sep 9, 2020

Stellar, planetary, and galactic studies参考文献 119被引用 8

一句话总结

本研究利用VLT/NACO首次在4–5 µm波段直接探测到PDS 70 b，通过黑体拟合得出其大气层温度为1193 ± 20 K，半径为3.0 ± 0.2 RJ。其巨大半径与较低的总辐射 luminosity（log(L/L⊙) = −3.79 ± 0.02）表明该行星被富含尘埃的扩展包层所包裹，因此在不依赖年龄或形成后熵值的前提下，通过吸积 luminosity 效应推导出其质量与半径的约束范围分别为Mp ≈ 0.5–1.5 MJ和Rp ≈ 1–2.5 RJ。

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$\alpha$ and $M'$ filters with VLT/NACO, a tentative detection of PDS 70 c in Br$\alpha$, 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$\alpha$ 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

• 利用中红外测光约束直接成像的吸积型系外行星PDS 70 b的质量与半径。
• 判断4–5 µm波段观测到的谱能分布（SED）是否可由单一黑体描述，或是否需要额外组分（如原行星盘）来解释。
• 评估吸积 luminosity 对行星半径与质量估计的影响，且不依赖于不确定的冷却时间或形成后熵值。
• 探究其大气层半径显著大于行星半径的物理成因，推测存在尘埃遮蔽环境。

提出的方法

• 获取了新的VLT/NACO在Brα与M′波段的观测数据，以在4–5 µm波段探测PDS 70 b。
• 重新分析了档案中的NACO L′与SPHERE H23/K12数据，构建从1至5 µm的完整SED。
• 采用单一黑体模型拟合SED，以推导大气层温度与半径。
• 利用孤立气态巨行星的标准结构模型推导质量-半径解，结合Hashimoto等人（2020）估算的吸积 luminosity。
• 通过引入第二个黑体组分建模潜在的原行星盘辐射，并利用贝叶斯因子评估模型偏好度。
• 将推导出的大气层半径与希尔球半径进行比较，以评估来自原恒星盘的尘埃持续补给。

实验结果

研究问题

• RQ1 PDS 70 b的大气层温度与半径在4–5 µm测光约束下分别为多少？
• RQ2 观测到的SED最符合单一黑体模型，还是需要引入如原行星盘等额外组分？
• RQ3 在不依赖年龄或熵值的前提下，考虑吸积 luminosity 后，PDS 70 b的质量与半径约束为何？
• RQ4 为何大气层半径显著大于行星半径？这对其环境意味着什么？
• RQ5 观测辐射在多大程度上受尘埃消光或散射影响？这又如何影响 luminosity 估计？

主要发现

• PDS 70 b的大气层温度被约束为1193 ± 20 K，大气层半径为3.0 ± 0.2 RJ，与黑体谱一致。
• 总辐射 luminosity 测得为log(L/L⊙) = −3.79 ± 0.02，对于其半径而言相对较低。
• 当考虑吸积 luminosity 时，仅质量不超过1.5 MJ的行星可重现观测到的 luminosity，因此质量约束为Mp ≈ 0.5–1.5 MJ。
• 行星半径被约束为Rp ≈ 1–2.5 RJ，与群体合成模型及间隙宽度估算一致。
• 大气层半径（3.0 RJ）与行星半径（1–2.5 RJ）之间的差异表明PDS 70 b被富含尘埃的扩展包层所包裹，该包层重新处理辐射。
• 大气层半径位于希尔球内部，表明尘埃通过气体吸积流持续从原恒星盘获得补给。