We present the optical transmission spectrum of the highly inflated Saturn-mass exoplanet WASP-21b, using three transits obtained with the ACAM instrument on the William Herschel Telescope through the LRG-BEASTS survey (Low Resolution Ground-Based Exoplanet Atmosphere Survey using Transmission Spectroscopy). Our transmission spectrum covers a wavelength range of 4635-9000 Angstrom, achieving an average transit depth precision of 197ppm compared to one atmospheric scale height at 246ppm. We detect Na I absorption in a bin width of 30 Angstrom, at >4$\sigma$ confidence, which extends over 100 Angstrom. We see no evidence of absorption from K I. Atmospheric retrieval analysis of the scattering slope indicates it is too steep for Rayleigh scattering from H$_2$, but is very similar to that of HD 189733b. The features observed in our transmission spectrum cannot be caused by stellar activity alone, with photometric monitoring of WASP-21 showing it to be an inactive star. We therefore conclude that aerosols in the atmosphere of WASP-21b are giving rise to the steep slope that we observe, and that WASP-21b is an excellent target for infra-red observations to constrain its atmospheric metallicity.

中文翻译：

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LRG-BEASTS：对高度膨胀的热土星 WASP-21b 大气中钠和陡峭光学斜率的地面探测

我们展示了高度膨胀的土星质量系外行星 WASP-21b 的光学透射光谱，使用威廉赫歇尔望远镜上的 ACAM 仪器通过 LRG-BEASTS 调查（使用透射光谱的低分辨率地面系外行星大气调查）获得的三个凌日. 我们的透射光谱涵盖 4635-9000 埃的波长范围，与大气标度高度 246ppm 相比，平均传输深度精度为 197ppm。我们在 30 埃的 bin 宽度内检测到 Na I 吸收，置信度大于 4$\sigma$，超过 100 埃。我们没有看到 K I 吸收的证据。散射斜率的大气反演分析表明它对于来自 H$_2$ 的瑞利散射来说太陡峭，但与 HD 189733b 的非常相似。在我们的透射光谱中观察到的特征不能单独由恒星活动引起，WASP-21 的光度监测表明它是一颗不活动的恒星。因此，我们得出结论，WASP-21b 大气中的气溶胶正在引起我们观察到的陡坡，并且 WASP-21b 是红外观测以限制其大气金属丰度的极好目标。