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arXiv 2412.04438astro-ph.EPastro-ph.SR
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从错位的亚土星到对齐的褐矮星：最高 $M_{\rm p}/M{_*}$ 系统表现出低倾斜角，即使在热恒星周围也是如此

From Misaligned Sub-Saturns to Aligned Brown Dwarfs: The Highest $M_{\rm p}/M{_*}$ Systems Exhibit Low Obliquities, Even around Hot Stars

Jace Rusznak, Xian-Yu Wang, Malena Rice, Songhu Wang

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15 pages, 2 figures, 2 tables, Submitted to Apjl
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AI总结：

本研究通过恒星倾斜角测量发现，高行星-恒星质量比系统（如超级木星和褐矮星）即使在热恒星周围也趋于对齐，与低质量比系统的错位形成3.7σ偏差，表明这种对齐是原生的，并提出了经验质量比边界2×10⁻³。

摘要：

我们展示了单恒星系统中恒星倾斜角测量中出现的一种模式：具有高行星-恒星质量比（$M_{\rm p}/M{_*}$$>$ $2\times10^{-3}$）的行星——例如超级木星、褐矮星伴星以及拥有类木行星的M矮星——往往是对齐的，即使在热恒星周围也是如此。这种对齐代表了与在较低质量比（$M_{\rm p}/M{_*}$$<$ $2\times10^{-3}$）系统中观测到的倾斜角分布的3.7$\sigma$偏差，后者主要包括木星和亚土星。唯一已知的离群系统XO-3表现出错位，这一点通过我们新收集的Rossiter-McLaughlin效应测量（$\lambda=41.8^{+2.1}_{-2.0}$度）得到确认。然而，XO-3相对较大的$\textit{Gaia}$重整单位权误差（RUWE）表明它可能隐藏着一个未被探测到的双星伴星，这可能对其错位有所贡献。鉴于潮汐重新对齐机制对热恒星较弱，高$M_{\rm p}/M{_*}$系统中观测到的对齐很可能是$\textit{原生的}$，而不是潮汐相互作用的结果。一种可能的解释是，只有动力学上孤立的行星才能继续吸积气体并演化为超级木星，同时保持其原生对齐。相反，在紧凑构型中形成的行星可能无法增长超过间隙开启质量，我们的工作为此提出了一个经验边界$M_{\rm p}/M{_*}$$=$ $2\times10^{-3}$，该边界在对齐的高$M_{\rm p}/M{_*}$系统和错位的低$M_{\rm p}/M{_*}$系统之间被识别，其中动力学不稳定性对后者观测到的多样化自旋-轨道错位有所贡献。

英文摘要：

We present a pattern emerging from stellar obliquity measurements in single-star systems: planets with high planet-to-star mass ratios ($M_{\rm p}/M{_*}$$>$ $2\times10^{-3}$) -- such as super-Jupiters, brown dwarf companions, and M-dwarfs hosting Jupiter-like planets -- tend to be aligned, even around hot stars. This alignment represents a 3.7$σ$ deviation from the obliquity distribution observed in systems with lower mass ratios ($M_{\rm p}/M{_*}$$<$ $2\times10^{-3}$), which predominantly include Jupiters and sub-Saturns. The only known outlier system, XO-3, exhibits misalignment confirmed via our newly collected Rossiter-McLaughlin effect measurement ($λ=41.8^{+2.1}_{-2.0}$ degrees). However, the relatively large $\textit{Gaia}$ Renormalized Unit Weight Error (RUWE) of XO-3 suggests that it may harbor an undetected binary companion, potentially contributing to its misalignment. Given that tidal realignment mechanisms are weak for hot stars, the observed alignment in high $M_{\rm p}/M{_*}$ systems is likely $\textit{primordial}$ rather than resulting from tidal interactions. One possible explanation is that only dynamically isolated planets can continue accreting gas and evolve into super-Jupiters while maintaining their primordial alignment. Conversely, planets formed in compact configurations may be unable to grow beyond the gap-opening mass, for which our work suggests an empirical boundary $M_{\rm p}/M{_*}$$=$ $2\times10^{-3}$, identified between aligned high $M_{\rm p}/M{_*}$ systems and misaligned low $M_{\rm p}/M{_*}$ systems, with dynamical instabilities contributing to the diverse spin-orbit misalignments observed in the latter.

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