ACCESS: Confirmation of a Clear Atmosphere for WASP-96b and a Comparison of Light Curve Detrending Techniques

One of the strongest Na I features was observed in WASP-96b. To confirm this novel detection, we provide a new 475-825 nm transmission spectrum obtained with Magellan/IMACS, which indeed confirms the presence of a broad sodium absorption feature. We find the same result when reanalyzing the 400-825 nm VLT/FORS2 data. We also utilize synthetic data to test the effectiveness of two common detrending techniques: (1) a Gaussian processes (GP) routine, and (2) common-mode correction followed by polynomial correction (CMC+Poly). We find that both methods poorly reproduce the absolute transit depths but maintain their true spectral shape. This emphasizes the importance of fitting for offsets when combining spectra from different sources or epochs. Additionally, we find that, for our data sets, both methods give consistent results, but CMC+Poly is more accurate and precise. We combine the Magellan/IMACS and VLT/FORS2 spectra with literature 800-1644 nm HST/ WFC3 spectra, yielding a global spectrum from 400 to 1644 nm. We used the PLATON and Exoretrievals retrieval codes to interpret this spectrum, and find that both yield relatively deeper pressures where the atmosphere is optically thick at log-pressures between 1.3(-1.1)(+1.0) and 0.29(-)(2.02)(+1.86) bars, respectively. Exoretrievals finds solar to supersolar Na I and H2O log-mixing ratios of -5.4(-1.9)(+2.0) and -4.5(-2.0)(+2.0), respectively, while PLATON finds an overall metallicity of log(10) (Z/Z(circle dot)) = -0.49(-0.37)(+1.0) dex. Therefore, our findings are in agreement with the literature and support the inference that the terminator of WASP-96b has few aerosols obscuring prominent features in the optical to near-infrared (near-IR) spectrum.

Automated summary

The study confirms the presence of strong Na I features in WASP-96b using data from multiple sources and analytical techniques, highlighting the importance of fitting offsets when combining spectra. The analysis shows that both Gaussian processes and common-mode correction followed by polynomial correction methods can reproduce the spectral shape accurately, with CMC+Poly method being more precise. The combination of various spectra yields a global spectrum from 400 to 1644 nm, supporting the hypothesis that the terminator of WASP-96b has few aerosols obscuring prominent features in the optical to near-infrared spectrum.