Characterisation of TOI-406 as showcase of the THIRSTEE program: A 2-planet system straddling the M-dwarf density gap

G. Lacedelli, E. Pallè, R. Luque, C. Cadieux, J. M. Akana Murphy, F. Murgas, M. R. Zapatero Osorio, H. M. Tabernero, K. A. Collins, C. N. Watkins, A. L'Heureux, R. Doyon, D. Jankowski, G. Nowak, È. Artigau, N. M. Batalha, J. L. Bean, F. Bouchy, M. Brady, B. L. Canto Martins, I. Carleo, M. Cointepas, D. M. Conti, N. J. Cook, I. J. M. Crossfield, J. I. Gonzàlez Hernàndez, P. Lewin, N. Nari, L. D. Nielsen, J. Orell-Miquel, L. Parc, R. P. Schwarz, G. Srdoc, V. Van Eylen
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Abstract

The exoplanet sub-Neptune population currently poses a conundrum. Are small-size planets volatile-rich cores without atmosphere, or are they rocky cores surrounded by 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 necessary 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 the concept and strategy of THIRSTEE, a project which aims at shedding 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 2-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 $R_b = 1.32 \pm 0.12 R_{\oplus}$, $M_b = 2.08_{-0.22}^{+0.23} M_{\oplus}$ and $R_c = 2.08_{-0.15}^{+0.16} R_{\oplus}$, $M_c = 6.57_{-0.90}^{+1.00} M_{\oplus}$, and periods of $3.3$ and $13.2$ days, respectively. Planet b is consistent with an Earth-like composition, while planet c 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 equilibrium temperature of only 368 K, TOI-406 c stands up as a particularly interesting target for atmospheric characterisation with JWST in the low-temperature regime.
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TOI-406的特征描述,作为THIRSTEE计划的展示:横跨M矮星密度鸿沟的双行星系统
海王星以下的系外行星群目前构成了一个难题。小尺寸行星是没有大气层的富挥发性内核,还是被H-He包层包围的岩石内核?要从观测的角度检验不同的假说,首先需要对小尺寸行星进行大量取样,并对其质量和半径进行精确测量。除此之外,可能还需要更多的信息,包括大气特性分析和从人口学角度分析行星的整体特性。我们介绍了 THIRSTEE 项目的概念和战略,该项目旨在通过增加亚海王星的数量和提高体积密度测量的精度,以及研究它们的大气层和进行统计人口分析,来揭示各恒星类型中亚海王星群的构成。我们报告了该计划的首批成果,描述了M矮星TOI-406周围的2个行星系统的特征。我们分析了 TESS 和地基光度计,以及 ESPRESSO 和 NIRPS/HARPS RVs,从而推导出轨道参数,并研究了环绕 TOI-406 的 2 颗行星的内部组成,这 2 颗行星的半径和质量分别为:$R_b = 1.32 \pm 0.12 R_{\oplus}$、$M_b =2.08_{-0.22}^{+0.23}M_{\oplus}$ 和 $R_c = 2.08_{-0.15}^{+0.16}R_{\oplus}$, $M_c = 6.57_{-0.90}^{+1.00} M_{\oplus}$,周期分别为3.3$天和13.2$天。行星b与类地行星的成分相一致,而行星c则与多种内部成分模型相一致,包括不含H/He大气的富挥发性行星。这两颗行星位于质量密度图中两个不同的区域,支持了M矮星周围小系外行星之间存在密度差距的观点。TOI-406 c的平衡温度仅为368 K,是利用JWST进行低温状态下大气特征描述的一个特别有趣的目标。
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