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arXiv 1702.02150astro-ph.EP
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VLT/FORS2比较透射光谱II:确认WASP-31 b中的云甲板和瑞利散射,但没有钾?

VLT/FORS2 comparative transmission spectroscopy II: confirmation of a cloud-deck and Rayleigh scattering in WASP-31b, but no potassium?

Neale P. Gibson, Nikolay Nikolov, David K. Sing, Joanna K. Barstow, Thomas M. Evans, Tiffany Kataria, Paul A. Wilson

摘要:

我们目前的热木星WASP-31 b的透射光谱使用FORS 2的VLT在两个主要的过境。观测覆盖了$\approx$ 400- 840 nm的波长范围。光变曲线被显着的系统学破坏,但这些都是一阶不变的波长,并可以使用来自白光曲线的共模校正删除。我们在15 nm箱中达到了$\approx$ 140 ppm的渡越深度精度,尽管精度在波长范围内变化很大。我们的FORS 2观测证实了以前使用HST/STIS推断的云甲板。我们还使用高斯过程模型重新分析了HST/STIS数据,发现与早期测量结果非常一致。我们重现了短波长的瑞利散射特征($\lesssim$ 5300 $Å$)和较长波长的云层。然而,我们的FORS 2观测似乎排除了先前使用STIS检测到的大钾特征,但它从HST/STIS数据中恢复,尽管幅度和显著性降低($\approx$ 2.5 $σ$)。我们的结果和早期的STIS钾探测($\approx$ 4.3 $σ$)之间的差异是地面观测的大地污染的结果,或者是低估了HST/STIS中窄带特征的不确定性,当使用线性基模型来解释系统学。我们的研究结果进一步证明了使用地面多目标光谱仪研究系外行星大气,并强调了我们在解释热彗星低分辨率光谱中的窄带特征时需要谨慎。

英文摘要:

We present transmission spectroscopy of the hot-Jupiter WASP-31b using FORS2 on the VLT during two primary transits. The observations cover a wavelength range of $\approx$400-840nm. The light curves are corrupted by significant systematics, but these were to first order invariant with wavelength and could be removed using a common-mode correction derived from the white light curves. We reach a precision in the transit depth of $\approx$140 ppm in 15 nm bins, although the precision varies significantly over the wavelength range. Our FORS2 observations confirm the cloud-deck previously inferred using HST/STIS. We also re-analyse the HST/STIS data using a Gaussian process model, finding excellent agreement with earlier measurements. We reproduce the Rayleigh scattering signature at short wavelengths ($\lesssim$5300 $Å$) and the cloud-deck at longer wavelengths. However, our FORS2 observations appear to rule out the large potassium feature previously detected using STIS, yet it is recovered from the HST/STIS data, although with reduced amplitude and significance ($\approx$2.5$σ$). The discrepancy between our results and the earlier STIS detection of potassium ($\approx$4.3$σ$) is either a result of telluric contamination of the ground-based observations, or an underestimate of the uncertainties for narrow-band features in HST/STIS when using linear basis models to account for the systematics. Our results further demonstrate the use of ground-based multi-object spectrographs for the study of exoplanet atmospheres, and highlight the need for caution in our interpretation of narrow-band features in low-resolution spectra of hot-Jupiters.

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