Magnesium: a new probe of the thermosphere / exosphere region
Atmospheric escape from HD209458b
V. Bourrier, A. Lecavelier des Etangs,
A. Vidal-Madjar
Transit observations in the Lyman-α line of neutral hydrogen led to the first detection of atmospheric escape from HD209458b (Vidal-Madjar et al. 2003).
Oxygen, carbon and silicium were also found in its extended atmosphere (Vidal-Madjar et al. 2004; Linsky et al. 2010; Ben-Jaffel & Sona Hosseini 2010;
Schlawin et al. 2010), carried away to high altitudes with the flow of hydrogen. This atmospheric blow-off is further confirmed by the detection of another
heavy species at high altitude: neutral magnesium (Vidal-Madjar et al. 2013). No signature of ionized magnesium was detected in the HST/STIS transit
spectra.
Institut d’Astrophysique de Paris/CNRS, Paris, France
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 %)
3D model of magnesium escape
Hydrogen and magnesium
Bourrier et al. 2014a, accepted
arxiv 1404.2120
Lyman-α 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.
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 (χ2 of 802.4 for 1067 degrees of freedom).
MgI line
MgII line
Structure of the
magnesium cloud
Mg0
Mg+
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.
Orbital plane of HD209458b. The star is toward the top of the plot
Density of neutral magnesium
Density of ionized magnesium
HD209458b atmospheric
properties
Best-fit value
1 sigma error bars
Escape rate of neutral
magnesium: 𝑴Mg0
2.9x107 g s−1
[ 2.0x107 ; 3.4x107 ] g s−1
Electron density
at 3Rp: ne(3Rp)
6.4x1010 cm−3
[ 2.7x1010 ; 1011 ] cm−3
Exobase altitude: Rexo
3 Rp
[ 2.1 ; 4.3 ] Rp
Planetary wind velocity at
the exobase: Vpl-w
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 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
Ben-Jaffel, L. & Sona Hosseini, S. 2010, ApJ, 709, 1284
Bourrier, V. & Lecavelier des Etangs, A. 2013, A&A, 557, A124
Guo, J. H. 2013, ApJ, 766, 102
Haswell, C. A., Fossati, L., et al. 2012, ApJ, 760, 79
Linsky, J. L., Yang, H., et al. 2010, ApJ, 717, 1291
Schlawin, E., Agol, E., Walkowicz, L. M., 2010, ApJ, 722, L75
Vidal-Madjar, A., Lecavelier des Etangs, et al. 2003, Nature, 422, 143
Vidal-Madjar, A., Désert, J.-M., et al. 2004, ApJ, 604, L69
Vidal-Madjar, A., Lecavelier des Etangs, A, et al. 2008, ApJ, 676, L57
Vidal-Madjar, A., Huitson, C. M., et al. 2013, A&A, 560, A54
Yelle, R. V. 2004, Icarus, 170, 167
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-α 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.