Atmospheric escape from HD 189733b observed in HI Lyman-alpha: detailed analysis of HST/STIS September 2011 observations

TL;DR

This study confirms hydrogen atmospheric escape from HD 189733b using HST/STIS Lyman-alpha observations, revealing temporal variability and additional ionized species, advancing understanding of exoplanet atmospheric evaporation.

Contribution

The paper provides a detailed analysis of September 2011 HST observations, introducing methods to detect and characterize hydrogen escape signatures and assess false positives.

Findings

Confirmed hydrogen escape with 3.6% false-positive probability.

Detected temporal and spectro-temporal variability in absorption features.

Observed ionized species indicating complex atmospheric interactions.

Abstract

Observations of transits of the hot giant exoplanet HD 189733b in the unresolved HI Lyman-alpha line show signs of hydrogen escaping the upper atmosphere of the planet. New resolved Lyman-alpha observations obtained with the STIS spectrograph onboard the Hubble Space Telescope in April 2010 and September 2011 confirmed that the planet is evaporating, and furthermore discovered significant temporal variations in the physical conditions of its evaporating atmosphere. Here we present a detailed analysis of the September 2011 observations of HD 189733b, when an atmospheric signature was detected. We present specific methods to find and characterize this absorption signature of escaping hydrogen in the Lyman-alpha line, and to calculate its false-positive probability, found to be 3.6%. Taking advantage of the spectral resolution and high sensitivity of the STIS spectrograph, we also present new results on temporal and spectro-temporal variability of this absorption feature. We also report the observation of HD 189733b in other lines (SiIII at 1206.5A, NV at 1240A). Variations in these lines could be explained either by early occultation by a bow-shock rich in highly ionized species, or by stellar variations.