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arXiv 1302.4443astro-ph.SRastro-ph.EP
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紧凑多行星系统中的低恒星质量

Low stellar obliquities in compact multiplanet systems

Simon Albrecht, Joshua N. Winn, Geoffrey W. Marcy, Andrew W. Howard, Howard Isaacson, John A. Johnson

备注：
Accepted version ApJ; this version includes referee comments, one additional figure (Fig. 4), and additional Kepler photometry for KOI-94 now including the epoch of the spectroscopic transit observations
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摘要：

利用Rossiter-McLaughlin效应测量了多次凌日行星系统KOI-94和Kepler-25的天空投影恒星质量（λ）。在这两种情况下，主星都与行星的轨道平面对齐。对于KOI-94，我们发现λ=-11+-11 deg，证实了Hirano及其同事最近的结果。开普勒-25是一个更具挑战性的案例，因为过境深度异常小（0.13%）。为了获得这种一致性，有必要使用恒星投影旋转速率的先验知识，并对光谱的独立波长区域应用两种不同的分析方法。这两种方法给出了一致的结果，λ=7 ± 8度和-0.5 ± 5.7度。现在，对于多个凌日行星系统的主恒星，总共有五个测量结果，所有这些都与自旋轨道对齐一致。这种排列不太可能是潮汐相互作用的结果，因为这些系统的轨道距离相对较大，行星质量较低。在这方面，多行星的主星与热木星的主星不同，后者通常在潮汐相互作用较弱时具有较大的自旋轨道偏差。特别是热木星宿主的弱潮汐子集具有符合各向同性分布（p=0.6）的密度，但多行星宿主与这样的分布不相容（p~10^-6）。这意味着高木星质量仅限于热木星系统，并提供了进一步的证据表明热木星的形成涉及使行星轨道倾斜的过程。

英文摘要：

We measure the sky-projected stellar obliquities (λ) in the multiple-transiting planetary systems KOI-94 and Kepler-25, using the Rossiter-McLaughlin effect. In both cases the host stars are well-aligned with the orbital planes of the planets. For KOI-94 we find λ=-11+-11 deg, confirming a recent result by Hirano and coworkers. Kepler-25 was a more challenging case because the transit depth is unusually small (0.13 %). To obtain the obliquity it was necessary to use prior knowledge of the star's projected rotation rate, and apply two different analysis methods to independent wavelength regions of the spectra. The two methods gave consistent results, λ=7+-8 deg and -0.5+-5.7 deg. There are now a total of five obliquity measurements for host stars of systems of multiple transiting planets, all of which are consistent with spin-orbit alignment. This alignment is unlikely to be the result of tidal interactions, because of the relatively large orbital distances and low planetary masses in the systems. In this respect the multiplanet host stars differ from hot-Jupiter host stars, which commonly have large spin-orbit misalignments whenever tidal interactions are weak. In particular the weak-tide subset of hot-Jupiter hosts have obliquities consistent with an isotropic distribution (p=0.6), but the multiplanet hosts are incompatible with such a distribution (p~10^-6). This suggests that high obliquities are confined to hot-Jupiter systems, and provides further evidence that hot Jupiter formation involves processes that tilt the planetary orbit.

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