LRG-BEASTS: Ground-based Detection of Sodium and a Steep Optical Slope in the Atmosphere of the Highly Inflated Hot-Saturn WASP-21b

TL;DR

This study presents the optical transmission spectrum of exoplanet WASP-21b, detecting sodium absorption and a steep scattering slope likely caused by aerosols, demonstrating ground-based capabilities for atmospheric characterization.

Contribution

First ground-based detection of sodium and a steep optical slope in WASP-21b's atmosphere using transmission spectroscopy.

Findings

Na I absorption detected at >4σ confidence

Steep scattering slope inconsistent with Rayleigh scattering from H2

No evidence of potassium absorption

Abstract

We present the optical transmission spectrum of the highly inflated Saturn-mass exoplanet WASP-21b, using three transits obtained with the ACAM instrument on the William Herschel Telescope through the LRG-BEASTS survey (Low Resolution Ground-Based Exoplanet Atmosphere Survey using Transmission Spectroscopy). Our transmission spectrum covers a wavelength range of 4635-9000 Angstrom, achieving an average transit depth precision of 197ppm compared to one atmospheric scale height at 246ppm. We detect Na I absorption in a bin width of 30 Angstrom, at >4$\sigma$ confidence, which extends over 100 Angstrom. We see no evidence of absorption from K I. Atmospheric retrieval analysis of the scattering slope indicates it is too steep for Rayleigh scattering from H$_2$, but is very similar to that of HD 189733b. The features observed in our transmission spectrum cannot be caused by stellar activity alone, with photometric monitoring of WASP-21 showing it to be an inactive star. We therefore conclude that aerosols in the atmosphere of WASP-21b are giving rise to the steep slope that we observe, and that WASP-21b is an excellent target for infra-red observations to constrain its atmospheric metallicity.