HubbleSpace Telescope detection of oxygen in the atmosphere of exoplanet HD 189733b

Abstract

Detecting heavy atoms in the inflated atmospheres of giant exoplanets that orbit close to their parent stars is a key factor for understanding their bulk composition, their evolution, and the processes that drive their expansion and interaction with the impinging stellar wind. Unfortunately, very few detections have been made thus far. Here, we use archive data obtained with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope to report an absorption of ∼6.4%±1.8% by neutral oxygen during the HD 189733b transit. Using published results from a simple hydrodynamic model of HD 189733b, and assuming a mean temperature of ∼(8−12) × 10 3 K for the upper atmosphere of the exoplanet, a mean vertical integrated O I density column of ∼8 × 10 15 cm −2 produces only a 3.5% attenuation transit. Much like the case of the hot-Jupiter HD 209458b, super-solar abundances and/or super-thermal broadening of the absorption lines are required to fit the deep transit drop-off observed in most far-ultraviolet lines. We also report evidence of short-time variability in the measured stellar flux, a variability that we analyze using time series derived from the time-tagged exposures, which we then compare to solar flaring activity. In that frame, we find that non-statistical uncertainties in the measured fluxes are not negligible, which calls for caution when reporting transit absorptions. Despite cumulative uncertainties that originate from variability in the stellar and sky background signals and in the instrument response, we also show a possible detection for both a transit and early-ingress absorption in the ion C II 133.5 nm lines. If confirmed, this would be the second exoplanet for which an early ingress absorption is reported. In contrast, such an early ingress signature is not detected for neutral O I. Assuming the HD 189733b magnetosphere to be at the origin of the early absorption, we use the Parker model for the stellar wind and a particle-in-cell code for the magnetosphere to show that its orientation should be deflected ∼10−30• from the planet-star line, while its nose's position should be at least ∼16.7 R p upstream of the exoplanet in order to fit the C II transit light curve. The derived stand-off distance is consistent with a surface magnetic field strength of ∼5.3 Gauss for the exoplanet, and a supersonic stellar wind impinging at ∼250 km s −1 , with a temperature of 1.2 × 10 5 K and a density ∼6.3 × 10 6 cm −3 at the planetary orbit, yet the fit is not unique.

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

“…The observations can be well reproduced for proton densities between 10 3 and 3 × 10 5 cm −3 and escape rates above 5 × 10 8 g s −1 . This is consistent with the upper limit on the proton density estimated by Ben-Jaffel & Ballester (2013) to be 5 × 10 7 cm −3 . A solar-like wind would correspond to a proton density of about 7×10 3 cm −3 at the orbit of HD 189733b (Quémerais et al 2007).…”

“…Together with the non-detection of atmospheric escape in April 2010, these observations led to the conclusion that there are significant temporal variations in the evaporating atmosphere of HD 189733b (Lecavelier des Etangs et al 2012). Recently, Ben-Jaffel & Ballester (2013) also detected neutral oxygen, and possibly ionized carbon, in the extended atmosphere of HD 189733b.…”

Ground-based near-UV observations of 15 transiting exoplanets: constraints on their atmospheres and no evidence for asymmetrical transits

“…Previous observations of an early ingress on WASP-12b and HD 189733b observe a flux drop difference of about ∼1 per cent and a timing difference of ∼>30 min between the near-UV and optical light curves (Fossati et al 2010;Haswell et al 2012;Ben-Jaffel & Ballester 2013;Nichols et al 2015). Both these properties are well within reach for ground-based metre-sized telescopes (e.g.…”

An evaporating planet in the wind: stellar wind interactions with the radiatively braked exosphere of GJ 436 b

“…The intense stellar X-ray and extreme ultraviolet energy input at the base of a hydrogen-rich thermosphere has been shown to be responsible for the expansion of the upper atmospheric layers (e.g., Lammer et al 2003;Lecavelier des Etangs et al 2004;Koskinen et al 2013a,b). Heavier species can be carried to high altitudes through collisions with the expanding flow of hydrogen, and several metals and ions were detected around these planets , 2013Linsky et al 2010;Ballester & Ben-Jaffel 2015;Ben-Jaffel & Ballester 2013;Fossati et al 2010;Haswell et al 2012), confirming that their atmospheres are in a state of hydrodynamic blow-off.…”

“…Transit observations at high resolution in the UV have also been used to probe the structure of these extended exospheres, revealing that they are shaped by interactions with the host star such as photoionization, radiation pressure, and stellar wind interactions (e.g., Holmström et al 2008;Ekenbäck et al 2010;Ben-Jaffel & Ballester 2013;Bourrier et al 2014;Kislyakova et al 2014a;Guo & Ben-Jaffel 2016;Schneiter et al 2016). Theoretical studies based on hydrodynamical simulations have also studied the processes that can affect the planetary outflow, such as charge-exchange reactions (e.g., Tremblin & Chiang 2013;Christie et al 2016) or interactions with the planetary magnetic field (e.g., Khodachenko et al 2015).…”