寻找HD209458b的远紫外极光/日辉发射
Searching for Far-Ultraviolet Auroral/Dayglow Emission from HD209458b
摘要：
我们提出了最近的观测HST宇宙起源光谱仪，旨在表征极光发射的太阳系外行星HD209458 b。我们在相位0.25和相位0.75的正交位置获得了中等分辨率（R~18- 20，000）的远紫外（1150- 1700 A）光谱，以及在二次日食时的恒星基线测量。分析包括恒星发射谱线目录和行星的恒星扣除光谱。我们提出了一个行星H2的排放模型，并比较这个模型的行星光谱。没有明确可识别的原子或分子特征的检测，和上限极光/日光线强度。轨道速度互相关分析发现，在行星的静止坐标系中，在1582 A的+15（+/- 20）km/s处有一个统计上显著的（3.8 sigma）特征。这一特征与H2 B-X（2-9）P（4）（lambda_{rest} = 1581.11 A）的发射一致，然而激发这一跃迁所需的物理机制尚不清楚。我们比较了在系外行星光谱中看到的相对线强度的限制与紫外荧光模型，以约束恒星和行星表面之间的中性氢的大气柱密度。这些结果支持了具有弱磁场和扩展原子大气的短周期太阳系外巨行星模型。
英文摘要：
We present recent observations from the HST-Cosmic Origins Spectrograph aimed at characterizing the auroral emission from the extrasolar planet HD209458b. We obtained medium-resolution (R~18-20,000) far-ultraviolet (1150-1700A) spectra at both the Phase 0.25 and Phase 0.75 quadrature positions as well as a stellar baseline measurement at secondary eclipse. This analysis includes a catalog of stellar emission lines and a star-subtracted spectrum of the planet. We present an emission model for planetary H2 emission, and compare this model to the planetary spectrum. No unambiguously identifiable atomic or molecular features are detected, and upper limits are presented for auroral/dayglow line strengths. An orbital velocity cross-correlation analysis finds a statistically significant (3.8 sigma) feature at +15 (+/- 20) km/s in the rest frame of the planet, at 1582 A. This feature is consistent with emission from H2 B-X (2-9) P(4) (lambda_{rest} = 1581.11 A), however the physical mechanism required to excite this transition is unclear. We compare limits on relative line strengths seen in the exoplanet spectrum with models of ultraviolet fluorescence to constrain the atmospheric column density of neutral hydrogen between the star and the planetary surface. These results support models of short period extrasolar giant planets with weak magnetic fields and extended atomic atmospheres.
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