Exoplanet Transit Variability: Bow Shocks and Winds Around HD 189733b

TL;DR

This paper models how stellar wind variations around HD 189733b cause changes in bow shock geometry, leading to observable differences in transit light curves, which can reveal stellar wind properties.

Contribution

It introduces a method to predict variable bow shock shapes and transit features based on magnetic maps and stellar wind models, linking stellar wind conditions to transit variability.

Findings

Transit duration and ingress time vary with stellar wind conditions.

Near-UV transit light curves can reveal stellar wind structure and evolution.

Bow shock geometry changes cause observable transit asymmetries.

Abstract

By analogy with the solar system, it is believed that stellar winds will form bow shocks around exoplanets. For hot Jupiters the bow shock will not form directly between the planet and the star, causing an asymmetric distribution of mass around the exoplanet and hence an asymmetric transit. As the planet orbits thorough varying wind conditions, the strength and geometry of its bow shock will change, thus producing transits of varying shape. We model this process using magnetic maps of HD 189733 taken one year apart, coupled with a 3D stellar wind model, to determine the local stellar wind conditions throughout the orbital path of the planet. We predict the time-varying geometry and density of the bow shock that forms around the magnetosphere of the planet and simulate transit light curves. Depending on the nature of the stellar magnetic field, and hence its wind, we find that both the transit duration and ingress time can vary when compared to optical light curves. We conclude that consecutive near-UV transit light curves may vary significantly and can therefore provide an insight into the structure and evolution of the stellar wind.