Revisiting the potassium feature of WASP-31b at high-resolution

TL;DR

High-resolution UVES observations of WASP-31b do not confirm previous strong potassium detections from HST, highlighting the importance of addressing instrumental systematics in exoplanet atmospheric studies.

Contribution

This study provides the first high-resolution spectroscopic analysis of WASP-31b's potassium feature, resolving discrepancies between prior low- and high-resolution observations.

Findings

No potassium absorption detected at previously reported levels.

Set an upper limit on the potassium core line depth at 0.007 (3σ).

Demonstrated limitations of current instrumental systematics in exoplanet spectroscopy.

Abstract

The analysis and interpretation of exoplanet spectra from time-series observations remains a significant challenge to our current understanding of exoplanet atmospheres, due to the complexities in understanding instrumental systematics. Previous observations of the hot Jupiter WASP-31b using transmission spectroscopy at low-resolution have presented conflicting results. Hubble Space Telescope (HST) observations detected a strong potassium feature at high significance (4.2\sigma), which subsequent ground-based spectro-photometry with the Very Large Telescope (VLT) failed to reproduce. Here, we present high-resolution observations (R>80,000) of WASP-31b with the UVES spectrograph, in an effort to resolve this discrepancy. We perform a comprehensive search for potassium using differential transit light curves, and integration over the planet's radial velocity. Our observations do not detect K absorption at the level previously reported with HST, consistent with the VLT observations. We measure a differential light curve depth $\Delta F = 0.00031 \pm 0.00036$ using 40\AA\ bins centred on the planet's K feature, and set an upper limit on the core line depth of $\Delta F \leq 0.007$ (3\sigma) at a few times the resolution limit ($\approx0.24\AA$). These results demonstrate that there are still significant limitations to our understanding of instrumental systematics even with our most stable space-based instrumentation, and that care must be taken when extracting narrow band signatures from low-resolution data. Confirming exoplanet features using alternative instruments and methodologies should be a priority, and confronting the limitations of systematics is essential to our future understanding of exoplanet atmospheres.