Transmission spectroscopy of the inflated exoplanet WASP-52b, and evidence for a bright region on the stellar surface

TL;DR

This study presents the transmission spectrum of the hot Jupiter WASP-52b, revealing cloud-dominated opacity and evidence for a bright stellar surface region, achieved through precise ground-based photometry.

Contribution

First ground-based measurement showing that clouds dominate WASP-52b's atmosphere and identifying a bright stellar surface feature as the cause of in-transit anomalies.

Findings

Rayleigh scattering is not dominant in the atmosphere.

Transmission spectrum suggests cloud opacity.

Bright stellar region explains in-transit anomaly.

Abstract

We have measured the transmission spectrum of the extremely inflated hot Jupiter WASP-52b using simultaneous photometric observations in SDSS u', g' and a filter centred on the sodium doublet (NaI) with the ULTRACAM instrument mounted on the 4.2m William Herschel Telescope. We find that Rayleigh scattering is not the dominant source of opacity within the planetary atmosphere and find a transmission spectrum more consistent with wavelength-independent opacity such as from clouds. We detect an in-transit anomaly that we attribute to the presence of stellar activity and find that this feature can be more simply modelled as a bright region on the stellar surface akin to Solar faculae rather than spots. A spot model requires a significantly larger planet/star radius ratio than that found in previous studies. Our results highlight the precision that can be achieved by ground-based photometry with errors in the scaled planetary radii of less than one atmospheric scale height, comparable to HST observations.