3D model of hydrogen atmospheric escape from HD209458b and HD189733b: radiative blow-out and stellar wind interactions

Transit observations in Ly-alpha of HD209458b and HD189733b revealed signatures of neutral hydrogen escaping the planets. We present a 3D particle model of the dynamics of the escaping atoms, and calculate theoretical Ly-alpha absorption line profiles, which can be directly compared with the absorption observed in the blue wing of the line. For HD209458b the observed velocities of the escaping atoms up to -130km/s are naturally explained by radiation-pressure acceleration. The observations are well-fitted with an ionizing flux of about 3-4 times solar and a hydrogen escape rate in the range 10^9-10^11g/s, in agreement with theoretical predictions. For HD189733b absorption by neutral hydrogen was observed in 2011 in the velocity range -230 to -140km/s. These velocities are higher than for HD209458b and require an additional acceleration mechanism for the escaping hydrogen atoms, which could be interactions with stellar wind protons. We constrain the stellar wind (temperature ~3x10^4K, velocity 200+-20km/s and density in the range 10^3-10^7/cm3) as well as the escape rate (4x10^8-10^11g/s) and ionizing flux (6-23 times solar). We also reveal the existence of an 'escape-limited' saturation regime in which most of the escaping gas interacts with the stellar protons. In this regime, which occurs at proton densities above ~3x10^5/cm3, the amplitude of the absorption signature is limited by the escape rate and does not depend on the wind density. The non-detection of escaping hydrogen in earlier observations in 2010 can be explained by the suppression of the stellar wind at that time, or an escape rate of about an order of magnitude lower than in 2011. For both planets, best-fit simulations show that the escaping atmosphere has the shape of a cometary tail.Comment: 21 pages, 26 figures, accepted for publication in A&

The orbit of Beta Pic b as a transiting planet

In 1981, Beta Pictoris showed strong and rapid photometric variations possibly due to a transiting giant planet. Later, a planetary mass companion to the star, Beta Pic b, was identified using imagery. Observations at different epochs (2003 and 2009-2015) detected the planet at a projected distance of 6 to 9 AU from the star and showed that the planet is on an edge-on orbit. The observed motion is consistent with an inferior conjunction in 1981, and Beta Pic b can be the transiting planet proposed to explain the photometric event observed at that time. Assuming that the 1981 event is related to the transit or the inferior conjunction of Beta Pic b on an edge-on orbit, we search for the planetary orbit in agreement with all the measurements of the planet position published so far. We find two different orbits that are compatible with all these constraints: (i) an orbit with a period of 17.97$\pm$0.08 years along with an eccentricity of around 0.12 and (ii) an orbit with a period of 36.38$\pm$0.13 years and a larger eccentricity of about 0.32. In the near future, new imaging observations should allow us to discriminate between these two different orbits. We also estimate the possible dates for the next transits, which could take place as early as 2017 or 2018, even for a long-period orbit.Comment: Accepted for publication in A&

Radiative braking in the extended exosphere of GJ436b

The recent detection of a giant exosphere surrounding the warm Neptune GJ436 b has shed new light on the evaporation of close-in planets, revealing that moderately irradiated, low-mass exoplanets could make exceptional targets for studying this mechanism and its impact on the exoplanet population. Three HST/STIS observations were performed in the Lyman-$\alpha$ line of GJ436 at different epochs, showing repeatable transits with large depths and extended durations. Here, we study the role played by stellar radiation pressure on the structure of the exosphere and its transmission spectrum. We found that the neutral hydrogen atoms in the exosphere of GJ436 b are not swept away by radiation pressure as shown to be the case for evaporating hot Jupiters. Instead, the low radiation pressure from the M-dwarf host star only brakes the gravitational fall of the escaping hydrogen toward the star and allows its dispersion within a large volume around the planet, yielding radial velocities up to about -120 km s$^{-1}$ that match the observations. We performed numerical simulations with the EVaporating Exoplanets code (EVE) to study the influence of the escape rate, the planetary wind velocity, and the stellar photoionization. While these parameters are instrumental in shaping the exosphere and yield simulation results in general agreement with the observations, the spectra observed at the different epochs show specific, time-variable features that require additional physics.Comment: 10 pages, 5 figure