ACCESS: Confirmation of no potassium in the atmosphere of WASP-31b

TL;DR

This study presents a detailed optical transmission spectrum of WASP-31b, confirming the absence of potassium in its atmosphere and highlighting the role of clouds and hazes in muting spectral features.

Contribution

It provides the first combined optical spectrum from multiple transits and clarifies previous conflicting results regarding potassium detection in WASP-31b's atmosphere.

Findings

No potassium detected in the atmosphere.

High-altitude clouds likely mute spectral features.

Optical data combined with IR observations constrains atmospheric properties.

Abstract

We present a new optical (400-950nm) transmission spectrum of the hot Jupiter WASP-31b (M=0.48 MJ; R= 1.54 RJ; P=3.41 days), obtained by combining four transits observations. These transits were observed with IMACS on the Magellan Baade Telescope at Las Campanas Observatory as part of the ACCESS project. We investigate the presence of clouds/hazes in the upper atmosphere of this planet as well as the contribution of stellar activity on the observed features. In addition, we search for absorption features of the alkali elements Na I and K I, with particular focus on K I, for which there have been two previously published disagreeing results. Observations with HST/STIS detected K I, whereas ground-based low- and high-resolution observations did not. We use equilibrium and non-equilibrium chemistry retrievals to explore the planetary and stellar parameter space of the system with our optical data combined with existing near-IR observations. Our best-fit model is that with a scattering slope consistent with a Rayleigh slope (alpha=5.3+2.9-3.1), high-altitude clouds at a log cloud top pressure of -3.6+2.7-2.1 bars, and possible muted H2O features. We find that our observations support other ground-based claims of no K I. Clouds are likely why signals like H2O are extremely muted and Na or K cannot be detected. We then juxtapose our Magellan/IMACS transmission spectrum with existing VLT/FORS2, HST/WFC3, HST/STIS, and Spitzer observations to further constrain the optical-to-infrared atmospheric features of the planet. We find that a steeper scattering slope (alpha = 8.3+/-1.5) is anchored by STIS wavelengths blueward of 400 nm and only the original STIS observations show significant potassium signal.