We observed the ultrahot Jupiter WASP-33b with the SpectroPolarimètre InfraRouge on the Canada–France–Hawaii Telescope. Previous observations of the dayside of WASP-33b show evidence of CO and Fe emission indicative of a thermal inversion. We observed its nightside over five Earth nights to search for spectral signatures of CO in the planet’s thermal emission. Our three pretransit observations and two posttransit observations are sensitive to regions near the morning or evening terminators, respectively. From spectral retrievals, we detect CO molecular absorption in the planet’s emission spectrum after transit at ∼6.6σ. This is the strongest ground-based detection of nightside thermal emission from an exoplanet and only the third ever. CO appearing in absorption suggests that the nightside near the evening terminator does not have a temperature inversion; this makes sense if the dayside inversion is driven by absorption of stellar radiation. On the contrary, we do not detect CO from the morning terminator. This may be consistent with heat advection by an eastward jet. Phase-resolved high-resolution spectroscopy offers an economical alternative to space-based full-orbit spectroscopic phase curves for studying the vertical and horizontal atmospheric temperature profiles of short-period exoplanets.

中文翻译：

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走出黑暗：WASP-33b 夜间 CO 吸收的高分辨率检测


我们在加拿大-法国-夏威夷望远镜上用 SpectroPolarimètre InfraRouge 观测了超热的木星 WASP-33b。先前对 WASP-33b 日边的观测表明，CO 和 Fe 发射表明热反转。我们在地球的五个夜晚观察了它的夜侧，以寻找行星热发射中 CO 的光谱特征。我们的 3 次凌日前观测值和 2 次凌日后观测值分别对早晨或傍晚终止符附近的区域敏感。通过光谱检索，我们在 ∼6.6σ 凌日后检测到行星发射光谱中的 CO 分子吸收。这是对系外行星夜侧热发射的最强地面探测，也是有史以来的第三次。吸收中出现的一氧化碳表明，靠近傍晚终结器的夜侧没有逆温;如果日边逆温是由恒星辐射的吸收驱动的，这是有道理的。相反，我们没有从早晨终止器中检测到 CO。这可能与向东急流的热平流一致。相位分辨高分辨率光谱为研究短周期系外行星的垂直和水平大气温度曲线提供了一种经济的替代方案，可以替代天基全轨道光谱相位曲线。