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

Abstract

Observations of transits of the hot giant exoplanet HD 189733b in the unresolved H i Lyman-α line show signs of hydrogen escaping the upper atmosphere of the planet. New resolved Lyman-α 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-α 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 (Si iii at 1206.5 Å, N v at 1240 Å). Variations in these lines could be explained either by early occultation by a bow-shock rich in highly ionized species, or by stellar variations.

3D model of hydrogen atmospheric escape from HD 209458b and HD 189733b: radiative blow-out and stellar wind interactions

“…However, as for HD 209458b, this feature is not confirmed, and the probability that it is a false signal caused by statistical noise in the spectrum has been estimated to be 25% (Bourrier et al 2013). …”

“…with t ion the characteristic lifetime of a hydrogen atom subject to photoionization, a pl the semi-major axis of the planet orbit, F ion the ionizing flux, and t ≈ 150 days the lifetime at 1 AU assuming an ionization flux corresponding to a solar minimum (Bzowski 2008). Hydrogen atoms are removed from the simulation when they become ionized, because they are not visible in Lyman-α anymore.…”

“…The ensuing depletion of high-velocity atoms created by these interactions can be compensated for, up to a certain extent, by increasing the density of either the stellar or the planetary wind (i.e., the escape rate). Note that the ionizing flux has to be high enough for atoms escaping the atmosphere without Bourrier et al 2013). Two absorption profiles are calculated for best-fit parameters (ionizing flux F ion = 10 F ) in the saturation regime (solid line: escape rateṀ = 10 9 g s −1 and proton density n wind = 10 6 cm −3 ) and out of this regime (dashed line: escape ratė M = 8 × 10 10 g s −1 and proton density n wind = 2 × 10 3 cm −3 ).…”

“…It may be possible to probe the structure of the hydrogen cloud by studying the evolution of the velocity structure of the absorption profile as a function of time during and after the planet transit (see Bourrier et al 2013). To distinguish between the "geometric" variations related to variations of the occulting area and opacity of the cloud in front of the stellar disk (Sect.…”

“…Closer to the Earth than HD 209458b and orbiting a brighter star, the very hot-Jupiter HD 189733b came as a particularly good candidate to study atmospheric evaporation (Bouchy et al 2005). Transits have been observed in the Lyman-α line with the Advanced Camera for Surveys (HST/ACS) in −2008(Lecavelier des Etangs et al 2010 and with HST/STIS in September 2011 (e.g., Bourrier et al 2013). This revealed the evaporation of neutral hydrogen from the planet's upper atmosphere.…”