The GAPS Programme with HARPS-N at TNG. III: The retrograde orbit of HAT-P-18b
The GAPS Programme with HARPS-N at TNG. III: The retrograde orbit of HAT-P-18b
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TNG 与 HARPS-N 的 GAPS 计划。
DOI：
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发表时间：
2014
期刊：
影响因子：
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通讯作者：
R. Smareglia
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作者：
M. Esposito;E. Covino;L. Mancini;A. Harutyunyan;J. Southworth;K. Biazzo;D. Gandolfi;Antonino Francesco Lanza;M. Barbieri;A. Bonomo;F. Borsa;R. Claudi;R. Cosentino;S. Desidera;R. Gratton;I. Pagano;A. Sozzetti;C. Boccato;A. Maggio;G. Micela;E. Molinari;V. Nascimbeni;G. Piotto;E. Poretti;R. Smareglia
对凌日系外行星的Rossiter-McLaughlin效应的测量限制了轨道轴相对于恒星自转轴的取向，这可以揭示行星系统轨道构型的形成机制。在这里，我们提出了土星质量行星HAT-P-18 b的有趣案例，它围绕着迄今为止测量到RossiterMcLaughlin效应的最冷的恒星之一运行。我们获得了光谱时间序列，跨越整个过境，与HARPS-N光谱仪安装在TNG望远镜。HARPS-N管道提供的非常精确的径向速度测量用于测量Rossiter McLaughlin效应。还分析了另一次完整凌日的补充新光度观测，以独立确定星星和行星参数。我们发现HAT-P-18 b位于一个反旋转轨道上，恒星自转轴与行星轨道轴的天空投影角为λ = 132± 15 °。通过对径向速度和测光数据的联合建模，我们获得了星星（M = 0.770± 0.027 M; R = 0.717± 0.026 R; V sin I = 1.58± 0.18 km s −1）和行星（Mp = 0.196± 0.008 MJ; Rp = 0.947± 0.044 RJ）参数的新测定。我们的光谱提供了主星星的有效温度Teff = 4870± 50 K，表面重力logg ψ = 4.57± 0.07 cm s −2，铁丰度[Fe/H] = 0.10± 0.06。HAT-P-18 b是已知少数在逆行轨道上经过Teff 0.6250 K的星星的行星之一。像HAT-P-18 b这样的天体（低行星质量和/或相对较长的轨道周期）很可能与它们的母星有较弱的潮汐耦合，因此它们的轨道保持着任何原始的错位。因此，它们是研究冷主序星轨道演化原因的理想目标。
The measurement of the Rossiter-McLaughlin effect for transiting exoplanets places constraints on the ori entation of the orbital axis with respect to the stellar spin axis, which can shed light on the mechanisms shaping the orbital configuration of planeta ry systems. Here we present the interesting case of the Saturn-mass planet HAT-P-18b, which orbits one of the coolest stars for which the RossiterMcLaughlin effect has been measured so far. We acquired a spectroscopic time-series, spanning a full transit, with the HARPS-N spectrograph mounted at the TNG telescope. The very precise radial velocity measurements delivered by the HARPS-N pipeline were used to measure the Rossiter-McLaughlin effect. Complementary new photometric observations of another full transit were also analysed to obtain an independent determination of the star and planet parameters. We find that HAT-P-18b lies on a counte r-rotating orbit, the sky-projected angle between the stellar spin axi s and the planet orbital axis beingλ = 132± 15 deg. By joint modelling of the radial velocity and photometric data we obtain new determinations of the star ( M⋆ = 0.770± 0.027 M⊙; R⋆ = 0.717± 0.026 R⊙; V sin I⋆ = 1.58± 0.18 km s −1 ) and planet (Mp = 0.196± 0.008 MJ; Rp = 0.947± 0.044 RJ) parameters. Our spectra provide for the host star an effective temperature Teff = 4870± 50 K, a surface gravity of logg⋆ = 4.57± 0.07 cm s −2 , and an iron abundance of [Fe/H] = 0.10± 0.06. HAT-P-18b is one of the few planets known to transit a star with Teff .6250 K on a retrograde orbit. Objects such as HAT-P-18b (low planet mass and/or relatively long orbital period) most likely have a weak ti dal coupling with their parent stars, therefore their orbits preserve any original misali gnment. As such, they are ideal targets to study the causes of orbital evolution in cool main-sequence stars.