We report ground-based transmission spectroscopy of the highly irradiated and ultra-short period hot-Jupiter WASP-103b covering the wavelength range ≈ 400 – 600 nm using the FORS2 instrument on the Very Large Telescope. The light curves show significant time-correlated noise which is mainly invariant in wavelength and which we model using a Gaussian process. The precision of our transmission spectrum is improved by applying a common-mode correction derived from the white light curve, reaching typical uncertainties in transit depth of ≈ 2 × 10−4 in wavelength bins of 15 nm. After correction for flux contamination from a blended companion star, our observations reveal a featureless spectrum across the full range of the FORS2 observations and we are unable to confirm the Na absorption previously inferred using Gemini/GMOS or the strong Rayleigh scattering observed using broad-band light curves. We performed a Bayesian atmospheric retrieval on the full optical-infrared transmission spectrum using the additional data from Gemini/GMOS, HST/WFC3 and Spitzer observations and recover evidence for H2O absorption at the 4.0 σ level. However, our observations are not able to completely rule out the presence of Na, which is found at 2.0 σ in our retrievals. This may in part be explained by patchy/inhomogeneous clouds or hazes damping any absorption features in our FORS2 spectrum, but an inherently small scale height also makes this feature challenging to probe from the ground. Our results nonetheless demonstrate the continuing potential of ground-based observations for investigating exoplanet atmospheres and emphasise the need for the application of consistent and robust statistical techniques to low-resolution spectra in the presence of instrumental systematics.

中文翻译：

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使用 FORS2 的地基透射光谱：超热木星 WASP-103b 的无特征光学透射光谱和 H2O 检测

我们使用甚大望远镜上的 FORS2 仪器报告了高辐射和超短周期热木星 WASP-103b 的地面透射光谱，覆盖波长范围 ≈ 400 – 600 nm。光变曲线显示出显着的时间相关噪声，其主要是波长不变的，我们使用高斯过程对其进行建模。通过应用源自白光曲线的共模校正，我们的透射光谱的精度得到了提高，在 15 nm 的波长范围内达到了典型的 ≈ 2 × 10−4 传输深度不确定性。在校正来自混合伴星的通量污染后，我们的观察结果揭示了整个 FORS2 观察范围内的无特征光谱，我们无法确认先前使用 Gemini/GMOS 推断的 Na 吸收或使用宽带光曲线观察到的强瑞利散射。我们使用来自 Gemini/GMOS、HST/WFC3 和 Spitzer 观测的附加数据对全光学红外传输光谱进行了贝叶斯大气反演，并恢复了 4.0 σ 水平的 H2O 吸收证据。然而，我们的观察并不能完全排除 Na 的存在，在我们的检索中发现它在 2.0 σ 处。这可能部分是由于片状/不均匀的云或雾阻碍了我们的 FORS2 光谱中的任何吸收特征，但固有的小比例高度也使得从地面探测该特征具有挑战性。