A Gemini ground-based transmission spectrum of WASP-29b: a featureless spectrum from 515 to 720nm

TL;DR

This study presents a detailed transmission spectrum of exoplanet WASP-29b from 515 to 720nm, revealing a featureless spectrum that suggests the absence of a clear Na-rich atmosphere, likely due to clouds or hazes.

Contribution

First ground-based transmission spectrum of WASP-29b using Gemini-South GMOS, demonstrating effective noise modeling and constraining atmospheric composition.

Findings

No spectral features detected in the transmission spectrum.

Na is likely absent or masked by clouds/hazes.

High-precision ground-based spectrophotometry achievable.

Abstract

We report Gemini-South GMOS observations of the exoplanet system WASP-29 during primary transit as a test case for differential spectrophotometry. We use the multi-object spectrograph to observe the target star and a comparison star simultaneously to produce multiple light curves at varying wavelengths. The 'white' light curve and fifteen 'spectral' light curves are analysed to refine the system parameters and produce a transmission spectrum from 515 to 720nm. All light curves exhibit time-correlated noise, which we model using a variety of techniques. These include a simple noise rescaling, a Gaussian process model, and a wavelet based method. These methods all produce consistent results, although with different uncertainties. The precision of the transmission spectrum is improved by subtracting a common signal from all the spectral light curves, reaching a typical precision of ~1x10^-4 in transit depth. The transmission spectrum is free of spectral features, and given the non-detection of a pressure broadened Na feature, we can rule out the presence of a Na rich atmosphere free of clouds or hazes, although we cannot rule out a narrow Na core. This indicates that Na is not present in the atmosphere, and/or that clouds/hazes play a significant role in the atmosphere and mask the broad wings of the Na feature, although the former is a more likely explanation given WASP-29b's equilibrium temperature of ~970 K, at which Na can form various compounds. We also briefly discuss the use of Gaussian process and wavelet methods to account for time correlated noise in transit light curves.