TOI-406 的特征刻画：THIRSTEE 计划的展示案例——一个跨越 M 矮星密度间隙的双行星系统
Characterisation of TOI-406 as showcase of the THIRSTEE program: A 2-planet system straddling the M-dwarf density gap
AI总结：
本文介绍 THIRSTEE 项目并刻画 M 矮星 TOI-406 的双行星系统，通过测光和视向速度数据精确测量质量与半径，发现两行星位于密度间隙两侧，其中 TOI-406 b 是 JWST 低温大气观测的理想目标。
摘要：
系外行星中的亚海王星族群目前构成一个难题，即小尺寸行星究竟是富含挥发物的无大气核心，还是被氢氦包层包围的岩石核心。为了从观测角度检验不同的假说，获取大量具有精确质量和半径测量值的小尺寸行星样本是第一步。除此之外，可能还需要更多信息，包括大气特征描述及其整体属性的统计学视角。我们在此介绍 THIRSTEE 项目的概念与策略，该项目旨在通过增加亚海王星族群的数量、提高其体积密度测量的精度，并研究其大气及进行统计与人口学分析，来阐明不同恒星类型下亚海王星族群的组成。我们报告了该计划的首批结果，刻画了围绕 M 矮星 TOI-406 的一个新的双行星系统。我们分析了 TESS 和地面测光数据，以及 ESPRESSO 和 NIRPS/HARPS 的视向速度数据，以推导轨道参数并研究围绕 TOI-406 运行的两颗行星的内部组成，它们的半径和质量分别为 $R_c = 1.32 \pm 0.12 R_{\oplus}$，$M_c = 2.08_{-0.22}^{+0.23} M_{\oplus}$ 和 $R_b = 2.08_{-0.15}^{+0.16} R_{\oplus}$，$M_b = 6.57_{-0.90}^{+1.00} M_{\oplus}$，周期分别为 $3.3$ 和 $13.2$ 天。行星 c 与类地组成一致，而行星 b 则与多种内部组成模型兼容，包括无 H/He 大气但富含挥发物的行星。这两颗行星位于质量-密度图中的两个不同区域，支持了 M 矮星周围小系外行星中存在密度间隙的观点。TOI-406 b 的平衡温度仅为 368 K，使其成为在低温区间利用 JWST 进行大气特征描述的特别有趣的目标。
英文摘要：
The exoplanet sub-Neptune population currently poses a conundrum, as to whether small-size planets are volatile-rich cores without an atmosphere, or rocky cores surrounded by a H-He envelope. To test the different hypotheses from an observational point of view, a large sample of small-size planets with precise mass and radius measurements is the first step. On top of that, much more information will likely be needed, including atmospheric characterisation and a demographic perspective on their bulk properties. We present here the concept and strategy of the THIRSTEE project, which aims to shed light on the composition of the sub-Neptune population across stellar types by increasing their number and improving the accuracy of bulk density measurements, as well as investigating their atmospheres and performing statistical, demographic analysis. We report the first results of the program, characterising a new two-planet system around the M-dwarf TOI-406. We analyse TESS and ground-based photometry, together with ESPRESSO and NIRPS/HARPS RVs to derive the orbital parameters and investigate the internal composition of the 2 planets orbiting TOI-406, which have radii and masses of $R_c = 1.32 \pm 0.12 R_{\oplus}$, $M_c = 2.08_{-0.22}^{+0.23} M_{\oplus}$ and $R_b = 2.08_{-0.15}^{+0.16} R_{\oplus}$, $M_b = 6.57_{-0.90}^{+1.00} M_{\oplus}$, and periods of $3.3$ and $13.2$ days, respectively. Planet c is consistent with an Earth-like composition, while planet b is compatible with multiple internal composition models, including volatile-rich planets without H/He atmospheres. The 2 planets are located in 2 distinct regions in the mass-density diagram, supporting the existence of a density gap among small exoplanets around M dwarfs. With an T$_{\rm eq}$ of only 368 K, TOI-406 b stands up as a particularly interesting target for atmospheric characterisation with JWST in the low-temperature regime.
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