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arXiv 0903.3405astro-ph.EP
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在凌日系外行星HD189733b大气中寻找一氧化碳

Search for Carbon Monoxide in the atmosphere of the Transiting Exoplanet HD189733b

Jean-Michel Desert, Alain Lecavelier des Etangs, Guillaume Hebrard, David K. Sing, David Ehrenreich, Roger Ferlet, Alfred Vidal-Madjar

摘要:

水、甲烷和一氧化碳预计是近距离EGPs热大气中除氢分子外最丰富的分子。在中红外波段的凌日观测可以确定凌日行星的大气成分。我们提出了新的主要过境观测热木星HD 189733 b,同时获得在4.5和8微米与IRAC仪器机载斯皮策。再加上一个新的精细分析以前的观测在3.6和5.8微米使用相同的仪器,我们能够得到的系统参数,包括行星-恒星半径比,碰撞参量,系统的规模,并从适合的过境光变曲线在这四个波长的过境中心时间。在3.6、4.5、5.8和8微米处,我们测量了四个行星与恒星的半径比,分别为(R_p/R_*)= 0.1545 +/-0.0003、0.1557 +/-0.0003、0.1547 +/-0.0005、0.1544 +/- 0.0004。测量的高精度允许大气分子吸收剂的搜索。与以前对同一数据集的分析相反,我们的研究对系统学是稳健的,并揭示了在该光度数据集中未检测到5.8微米处的水蒸气吸收。此外,在以4.5微米为中心的带中,我们发现了过量吸收的暗示,其表观行星半径Delta(R_p/R_*)= 0.00128 +/- 0.00056(2.3 sigma)比在8微米处同时测量的大。该值比大气中水蒸气是近红外中唯一吸收物质的预期值高4 σ。这表明,大气中可能存在吸收约4.5微米的其他物质。CO在该波长处是强吸收剂是可能的候选者,并且这可能表明在5和60之间的大CO/H2O比。

英文摘要:

Water, methane and carbon-monoxide are expected to be among the most abundant molecules besides molecular hydrogen in the hot atmosphere of close-in EGPs. Transit observations in the mid-IR allow the atmospheric content of transiting planets to be determined. We present new primary transit observations of the hot-jupiter HD189733b, obtained simultaneously at 4.5 and 8 micron with IRAC instrument onboard Spitzer. Together with a new refined analysis of previous observations at 3.6 and 5.8 micron using the same instrument, we are able to derive the system parameters, including planet-to-star radius ratio, impact parameter, scale of the system, and central time of the transit from fits of the transit light curves at these four wavelengths. We measure the four planet-to-star radius ratios, to be (R_p/R_*)= 0.1545 +/- 0.0003, 0.1557 +/- 0.0003, 0.1547 +/- 0.0005, 0.1544 +/- 0.0004 at 3.6, 4.5, 5.8, and 8 micron respectively. The high accuracy of the measurement allows the search for atmospheric molecular absorbers. Contrary to a previous analysis of the same dataset, our study is robust against systematics and reveals that water vapor absorption at 5.8 micron is not detected in this photometric dataset. Furthermore, in the band centered around 4.5 micron we find a hint of excess absorption with an apparent planetary radius Delta(R_p/R_*) = 0.00128 +/- 0.00056 larger (2.3 sigma) than the one measured simultaneously at 8 micron. This value is 4 sigma above what would be expected for an atmosphere where water vapor is the only absorbing species in the near infrared. This shows that an additional species absorbing around 4.5 micron could be present in the atmosphere. CO being a strong absorber at this wavelength is a possible candidate and this may suggest a large CO/H2O ratio between 5 and 60.

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