在HI莱曼α观测到的行星HD189733b的蒸发
Evaporation of the planet HD189733b observed in HI Lyman-alpha
摘要：
我们在HI Lyman-alpha和HST/ACS的紫外线中观察到太阳系外行星HD 189733 b的三次凌日，以寻找大气特征。我们在莱曼-阿尔法光变曲线中检测到一个过境签名，过境深度为5.05 +/-0.75%。这个深度超过了行星盘单独产生的3.5-sigma水平的掩星深度（统计学）。其他恒星的发射谱线亮度较低，单独来看，它们没有显示出凌日信号，而莱曼-α谱线的整个光谱向红方向有足够的光子显示出凌日信号，这与行星盘单独的吸收一致。在发射谱线中，OI在1305 A处的渡越深度上限为11.1%，CII在1335 A处的渡越深度上限为5.5%。HD 189733 b周围存在一个扩展的氢原子散逸层，产生5%的未解析莱曼α线通量的吸收，表明这颗行星正在失去气体。莱曼-α光变曲线通过逃逸氢的数值模拟得到很好的拟合，其中行星原子被恒星辐射压推动。我们将原子氢的逃逸速率限制在10 ^9和10^{11} g/s之间，电离极紫外线通量限制在2到40倍太阳值（1-sigma）之间，较大的逃逸速率对应于较大的EUV通量。当dM/dt=10^{10} g/s和EUV通量F_{EUV} =20倍太阳值时，获得最佳拟合。HD 189733 b是第二颗被探测到大气蒸发的太阳系外行星。
英文摘要：
We observed three transits of the extrasolar planet HD189733b in HI Lyman-alpha and in a few other lines in the ultraviolet with HST/ACS, in the search for atmospheric signatures. We detect a transit signature in the Lyman-alpha light curve with a transit depth of 5.05 +/- 0.75 %. This depth exceeds the occultation depth produced by the planetary disk alone at the 3.5-sigma level (statistical). Other stellar emission lines are less bright, and, taken individually, they do not show the transit signature, while the whole spectra redward of the Lyman-alpha line has enough photons to show a transit signature consistent with the absorption by the planetary disk alone. The transit depth's upper limits in the emission lines are 11.1% for OI at 1305A and 5.5% for CII at 1335A. The presence of an extended exosphere of atomic hydrogen around HD189733b producing 5% absorption of the full unresolved Lyman-alpha line flux shows that the planet is losing gas. The Lyman-alpha light curve is well-fitted by a numerical simulation of escaping hydrogen in which the planetary atoms are pushed by the stellar radiation pressure. We constrain the escape rate of atomic hydrogen to be between 10^9 and 10^{11} g/s and the ionizing extreme UV flux between 2 and 40 times the solar value (1-sigma), with larger escape rates corresponding to larger EUV flux. The best fit is obtained for dM/dt=10^{10} g/s and an EUV flux F_{EUV}=20 times the solar value. HD189733b is the second extrasolar planet for which atmospheric evaporation has been detected.
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