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Magnesium: a new probe of the thermosphere / exosphere region
V. Bourrier, A. Lecavelier des Etangs, A. Vidal-Madjar
Institut d'Astrophysique de Paris/CNRS, Paris, France

Atmospheric escape from HD209458b
Transit observations in the Lyman- line of neutral Oxygen, carbon and silicium were also found in its Schlawin et al. 2010), carried away to high altitudes heavy species at high altitude: neutral magnesium spectra. hydrogen led to the first detection of atmospheric escape from HD209458b (Vidal-Madjar et al. 2003). extended atmosphere (Vidal-Madjar et al. 2004; Linsky et al. 2010; Ben-Jaffel & Sona Hosseini 2010; with the flow of hydrogen. This atmospheric blow-off is further confirmed by the detection of another (Vidal-Madjar et al. 2013). No signature of ionized magnesium was detected in the HST/STIS transit

Abstract
Observations of HD209458b in the UV with HST/STIS reveal signatures of neutral magnesium escaping the planet's upper atmosphere, while no absorption is found in the line of singly ionized magnesium. We compare these observations with theoretical profiles generated by a 3D numerical model of atmospheric escape. The observed velocities of the planetescaping magnesium are explained by radiation pressure acceleration, provided UV-photoionization is compensated for by electron recombination. We constrain the escape rate of neutral magnesium, the exobase properties and the exospheric electron density. While hydrogen can be used to study the exosphere of an evaporating planet, magnesium is a probe of the transition region between the thermosphere and the exosphere.

MgI line (Mg0)

Absorption signature : 8.8% ± 2.1% [-60 ; - 19] km s -1

MgII line (Mg+)

No excess of transit depth (< 1 %)

Hydrogen and magnesium
Lyman- absorption signature (Vidal-Madjar et al. 2008) : 16.3% ± 3.5% in [-130 ; - 40] km s-1 Hydrogen atoms must be present above 3.3 Rp at very high velocity, in the EXOSPHERE MgI line absorption signature : 8.8% ± 2.1% in [-60; - 19] km s-1 Neutral magnesium atoms must be present above 2.4 Rp at high velocity, in the EXOSPHERE/THERMOSPHERE How can neutral magnesium have a short UV-photoionization lifetime of 0.6 h and be found at high altitude? We explored the possibility that there are enough electrons at high altitudes for ionized magnesium to recombine efficiently into neutral magnesium.

3D model of magnesium escape
Bourrier et al. 2014a, accepted arxiv 1404.2120
We adjust the general model described in Bourrier & Lecavelier 2013 to the case of magnesium. The deep hydrodynamic atmosphere is described with an analytical model, characterized by its mean temperature (7000K, Koskinen et al. 2012b) and a spherically symmetric density profile. To model the blow-off mechanism we assume the different gases in the atmosphere arrive at the exobase mixed in a global radial planetary wind. The wind velocity and the altitude of the exobase are free parameters of the model, as is the escape rate of neutral magnesium at the exobase. We use particle simulations to compute the dynamics of the neutral and ionized magnesium populations in the upper collisionless atmosphere above the exobase. Neutral magnesium is photo-ionized by the stellar UV radiation (Vidal-Madjar et al. 2013) and ionized magnesium can recombine with electrons. The density of electrons depends on altitude and the electronic density profile is fixed by its value at 3Rp

Planetary wind

Model parameters are displayed in green

Comparison with the observations
Theoretical absorption profiles generated by our model (black lines) are directly compared to the observations (blue lines) in the MgI and MgII lines. The plots show the best-fit profiles (2 of 802.4 for 1067 degrees of freedom).

Structure of the magnesium cloud
The velocities of the observed magnesium atoms are naturally explained by radiation pressure, which shapes the escaping cloud into a cometary tail. Because of the shadow of the planet and the self-shielding of the lower atmosphere, there are no particles accelerated in the center of the tail.

Mg0

Mg+

MgI line

MgII line

Orbital plane of HD209458b. The star is toward the top of the plot

Density of neutral magnesium

Density of ionized magnesium HD209458b atmospheric properties Escape rate of neutral magnesium: Mg0 Electron density at 3Rp: ne(3Rp) Exobase altitude: Rexo Planetary wind velocity at the exobase: Vpl-w Best-fit value 1 sigma error bars

2.9x107 g s-1 6.4x1010 cm-3 3R
p

[ 2.0x107 ; 3.4x107 ] g s-1 [ 2.7x1010 ; 1011 ] cm-3 [ 2.1 ; 4.3 ] R
p

25 km s-1

[ 14 ; 42 ] km s-1

Remarkably the exobase is found close to the Roche lobe (i.e. the limit of the planet gravitational influence at 2.8 Rp). Simulations show that the planetary wind velocity which best reproduces the observations increases with decreasing exobase altitudes below the Roche lobe because the planetary gravity limits the expansion of the atmosphere. Above the Roche lobe the wind velocity remains constant at 25 km s-1, in the order of theoretical models estimations (e.g., Yelle 2004)

The recombination rate of ionized magnesium into neutral magnesium depends on the electron density, and thus decreases with altitude. At low altitudes there are so many electrons that all ionized particles quicky recombine and the cloud is entirely neutral up to about 4Rp (blue zone). Ionization becomes dominant above ~13Rp (red zone), and so magnesium atoms remain neutral long enough to be accelerated by radiation pressure to the observed velocities.

Survey of evaporating exoplanets (Bourrier et al. 2014b, submitted)
Escaping atmospheres have been detected in a small number of exoplanets, mainly through observations in the HI Lyman- line. Using our 3D model of atmospheric escape, we show that fourteen planets (red circles) are expected to produce atmospheric signatures in the MgI line that would be easily detected with UV facilities such as HST. The detectability of these signatures depends mainly on the magnesium escape rate, and the brightness and radiation pressure strength of the star. MgI line observations of this sample, which covers a wide range of planetary and stellar properties, would allow to draw comparisons between exoplanets' upper atmospheres and provide an unprecedented vision of the blow-off mechanism.

The electron density is higher than estimations from theoretical models (e.g., Guo 2013) because recombination must be efficient to explain both the detection of neutral magnesium far above the planet and the non-detection of ionized magnesium. It would be overestimated if temperatures or recombination rates are higher than the ones we used, or if no MgII line absorption signature was found because the ionized magnesium cloud is extending far away from the planet to occult the stellar disk at all observed orbital phases (as in WASP-12b; Haswell et al. 2012). If the electron density is correct, it could mean that electrons are more abundant than ions in this collisionless part of the upper atmosphere.

Conclusions
First detection of neutral magnesium in an extended atmosphere (HD209458b) Direct comparison between theoretical and observed spectra in the MgI and MgII lines shows: Escape rate of neutral hydrogen of 3x1010 g s-1 consistent with standard values First observational constraint on the exospheric electron density Exobase close to the Roche lobe and planetary wind velocity of 25km s-1 Mean temperature of the thermosphere > 6100 K Hydrogen and magnesium are probes of different regions of the upper atmosphere: the exosphere and its transition with the thermosphere Transit observations in the MgI line have the potential to reveal and characterize the extended upper atmospheres of new evaporating exoplanets
References
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