GJ ~ 436 b巨散逸层的长出口
The long egress of GJ~436b's giant exosphere
摘要：
M矮星GJ 436拥有一个过境的温暖海王星，已知经历大气逃逸。先前的观测揭示了一个巨大的氢散逸层的存在，该散逸层经过恒星超过5小时，并吸收了恒星莱曼-α中性氢线（H i Lyα）蓝翼中高达56%的通量。这颗彗星般的散逸层出乎意料的大小阻止了观察它的尾巴的完整过境。在这封信中，我们提出了新的Lyα观测GJ 436获得的空间望远镜成像光谱仪（STIS）仪器上的哈勃太空望远镜。Lyα线在六年中的稳定性使我们能够将这些新的观测结果与档案数据集结合起来，大大扩展了外逸层过境的覆盖范围。在行星凌日后，外逸层云尾部的氢原子持续遮蔽恒星10-25小时，显著证实了之前基于蒸发系外行星代码（EVE）的3D数值模拟的预测。这一结果加强了GJ 436 b的外逸层是由辐射制动和与恒星风的电荷交换形成的解释。我们进一步报告了Lyα线红翼和电离硅（Si iii）线的通量分别下降了15 +/- 2%和47 +/- 10%。尽管有一些可能与恒星活动有关的时间变化，这两个信号发生在外逸层过境期间，可能是行星起源。需要进行后续观测，以评估红移的Lyα和Si iii吸收特征是由外逸层流和恒星磁场之间的相互作用引起的可能性。
英文摘要：
The M dwarf GJ 436 hosts a transiting warm Neptune known to experience atmospheric escape. Previous observations revealed the presence of a giant hydrogen exosphere transiting the star for more than 5 h, and absorbing up to 56% of the flux in the blue wing of the stellar Lyman-α line of neutral hydrogen (H i Lyα). The unexpected size of this comet-like exosphere prevented observing the full transit of its tail. In this Letter, we present new Lyα observations of GJ 436 obtained with the Space Telescope Imaging Spectrograph (STIS) instrument onboard the Hubble Space Telescope. The stability of the Lyα line over six years allowed us to combine these new observations with archival data sets, substantially expanding the coverage of the exospheric transit. Hydrogen atoms in the tail of the exospheric cloud keep occulting the star for 10-25 h after the transit of the planet, remarkably confirming a previous prediction based on 3D numerical simulations with the EVaporating Exoplanet code (EVE). This result strengthens the interpretation that the exosphere of GJ 436b is shaped by both radiative braking and charge exchanges with the stellar wind. We further report flux decreases of 15 +/- 2% and 47 +/- 10% in the red wing of the Lyα line and in the line of ionised silicon (Si iii). Despite some temporal variability possibly linked with stellar activity, these two signals occur during the exospheric transit and could be of planetary origin. Follow-up observations will be required to assess the possibility that the redshifted Lyα and Si iii absorption signatures arise from interactions between the exospheric flow and the magnetic field of the star.
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