around HD 209458b. The planet itself shields only. 1.5% of stellar radiation in the visible range (Ballester. Suprathermal Hydrogen Produced by the dissociation.
ISSN 00380946, Solar System Research, 2010, Vol. 44, No. 2, pp. 96–103. © Pleiades Publishing, Inc., 2010. Original Russian Text © V.I. Shematovich, 2010, published in Astronomicheskii Vestnik, 2010, Vol. 44, No. 2, pp. 108–116.
Suprathermal Hydrogen Produced by the dissociation of Molecular Hydrogen in the Extended Atmosphere of Exoplanet HD 209458b V. I. Shematovich Institute of Astronomy, Russian Academy of Science, Moscow, Russia Received September 29, 2009
Abstract—The ionization and dissociation of molecular hydrogen by the ultraviolet (UV) radiation of the parent star lead to the formation of hydrogen atoms with an excess of kinetic energy and, thus, are an important source of suprathermal hydrogen atoms in the upper atmosphere of exoplanet HD 209458b. Contemporary aeronomical models did not investigate these processes because they assumed the fast local thermalization of the hot atoms of hydrogen by elastic collisions. However, the kinetics and transfer of these atoms were not calculated in detail, because they require the solving of the Boltzmann equation for a nonthermal atom population. This work estimates the effect of the UV radiation of the parent star and the accompanying photocleacton flux on the production of the suprathermal fraction of atomic hydrogen in the H2 H transition region. We also consider the formation of the escaping flux of Hatoms created by this effect in the upper atmosphere of HD 209458b. We calculate the produc tion rate and energy spectrum of the hydrogen atoms with excess kinetic energy during the dissociation of H2. Using the numerical stochastic model created by Shematovich (2004) for a hot planetary corona, we investigate the molecularscale kinetics and transfer of suprathermal hydrogen atoms in the upper atmosphere and the emergent flux of atoms evaporating from the atmosphere. The latter is estimated as 3.4 × 1012 cm–2 s–1 for a moderate stellar activity level of UV radiation, which leads to a planetary atmosphere evaporation rate of 3.4 × 109 g s–1 due to the process of the dissociation of H2. This estimate is close to the observational value of ~1010 g s–1 for the rate of atmo spheric loss of HD 209458b. DOI: 10.1134/S0038094610020024
technique, the structure and chemical composition of the upper atmospheres of transiting exoplanets were studied. In the work by Charbonneau et al. (2002), the first results on the atmospheres of transiting exoplan ets were reported: the absorption of a ~10–4 resonance Dline Na (589 nm) was detected in the atmosphere of HD 209458b. Further, Tinetti et al. (2007) observed the transit of HD 189733b and came to the conclusion that the observed effective planetary radius depen dence on the wavelength may be explained by water absorption bands in the infrared region at 3.6, 5.8, and 8 μm. The most interesting results were obtained in the UV range by VidalMadjar et al. (2003). They observed the sufficiently strong weakening of the stel lar emission of HD 209458: in the Lyman α line of atomic hydrogen H I 121.6 nm from 5 to 15% depend ing on the spectral resolution (VidalMadjar et al. (2003, 2004, 2008), BenJaffel (2007); Ehrenreich et al. (2008)), and in the lines of atomic oxygen at 130.5 nm and the ionized carbon at 133.5 nm of 13 and 7%, respectively (VidalMadjar et al., 2004). In the UV resonance lines, the planetary atmosphere was weak ened substantially more than the planet itself, which indicates the presence of an extended atmosphere around HD 209458b. The planet itself shields only 1.5% of stellar radiation in the visible range (Ballester
INTRODUCTION More than 350 exoplanets are known today, and nearly 40% of them have a semimajor axis smaller than 0.1 AU (Schneider, 2006; http://exoplanets.eu). Most of the recently discovered exoplanets have relatively large masses and show similarities to Jupiter and Sat urn in our Solar System. They are therefore called giant exoplanets (GEPs or hot Jupiters). Proximity to a star leads to the exposure of a GEP to intense radia tion fields and plasma flows from the star. Energy absorption by the upper atmospheres of the GEPs sig nificantly affects their thermal balance and chemistry. The high temperatures of the outermost atmospheric layers may produce mass outflows that are sufficiently high for the subsequent evolution of the planet. Direct observations of exoplanets are scarce because a planetary signal is difficult to isolate from the much stronger signal from the star. Nonetheless, the orbits of some GEPs lie on a line of sight with the Earth, which allows for the study of the absorption spectrum of the atmosphere of the planet transiting in front of the star. The first observations (Charbonneau et al., 2000; Henry et al., 2000) of the transit of HD 209458b showed a 1% stellar flux attenuation, which confirmed the gas nature of the giant planet. Using this 96
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et al., 2007), which means that the planet is sur rounded by a neutral hydrogen cloud as large as about 3.3 planetary radii (Rp). The Hill sphere radius for this exoplanet, determining the area of the dominance of the gravitational attraction of the planet over the gravita tional attraction of the central star, is equal to 4.08 Rp. The observed size of the hydrogen atmosphere is compara ble to this value. The atoms and molecules reaching the Hill sphere can leave the atmosphere; therefore, strong outflows may form. In their observations, VidalMadjar et al. (2003) also detected absorption in the extended wings of Ly α, corresponding to velocities of about 100 km/s both towards the star and towards the observer. The exist ence of hydrogen atoms with velocities substantially exceeding the escape velocity of the planetary atmo sphere was interpreted as the direct detection of parti cles leaving the atmosphere, and the atmosphere itself as evaporating (VidalMadjar et al. (2003, 2008); Lecavelier des Etangs et al. (2004)). Ballester et al. (2007) reported the results of observations of HD 209458b with the STIS imaging spectrograph onboard the Hubble Space Telescope (HST) in a broad UVto optical spectral range. They detected a new detail in the planetary atmosphere absorption spectrum at 356–390 nm that they interpreted as Balmercontin uum absorption by hot (suprathermal) neutral hydro gen atoms in the upper atmosphere of the planet. To interpret the observational results cited above, aeronomical models for the physical and chemical processes in the upper atmospheres of hot Jupiters were developed (Yelle (2004, 2006); Muñoz (2007); Penz et al. (2008); Yelle et al. (2008)). Based on the results of the observational and aeronomical models, the following different interpretations were proposed for the physical and evolutionary state of the upper atmosphere of HD 209458b: Evaporating upper atmosphere. The close proxim ity of the planet to its parent star (0.045 AU) leads to the strong insolation of its upper atmosphere by UV solar radiation. This strong energy input leads to the heating of the upper atmosphere to about 104 K, and the atmosphere expands and fills its Hill sphere (or Roche lobe). The expanding atmosphere produces strong outflows, i.e., the atmosphere starts to evapo rate (VidalMadjar et al. (2003, 2008); Lammer et al. (2003); Lecavelier des Etangs et al. (2004)). The cor responding large spatial size of the extended atmo sphere leads to its large optical depth in the Lyα line. The possible atmospheric massloss rate is estimated at about 1010 g/s (Ehrenreich et al. (2008)). Chargeexchange reactions between the stellar wind and neutral planetary corona. The observed Lyα pro file may also be explained by the interaction of stellar wind protons with the neutral atoms of the planetary atmosphere. Then, the protons of the solar wind become the source of the observed highvelocity neu tral hydrogen atoms (Holmström et al., 2008; Eken back et al., 2009). SOLAR SYSTEM RESEARCH
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Currently, both of these explanations together with several other possible hypotheses regarding the forma tion and evolutionary status of the extended upper atmosphere of HD 209458b are being actively dis cussed, but full clarity has not yet been achieved. Fur ther observations of the transiting planet are needed. One of the important sources of suprathermal hydrogen atoms in the upper atmosphere of HD 209458b is the processes of the dissociation and ionization of molecular hydrogen by the extreme UV radiation of the star that produces hydrogen atoms with an excess of kinetic energy. The models men tioned above did not consider these processes in detail, because the rapid thermalization of hot hydrogen atoms by elastic collisions with the ambient gas parti cles was assumed. No detailed studies of suprathermal hydrogen atom kinetics and transfer were performed at the molecular level (Yelle et al., 2008), because this requires solving the kinetic Boltzmann equation (She matovich, 2007; Johnson et al., 2008). Accordingly, in this study, we consider the effect of molecular hydro gen dissociation by UV radiation and the subsequent flux of photoelectrons on the suprathermal atomic hydrogen production in the Н2 H transition region and the formation of the corresponding outflow in the upper atmosphere of HD 209458b. For this, we calcu late the formation rate and the energy spectrum of hydrogen atoms forming with an excess of kinetic energy during the dissociation of Н2. Then, we use the stochastic hot planetary corona model by Shematov ich (2004) to study the kinetics and transfer of the suprathermal hydrogen atoms in the expanded upper atmosphere and estimate the atmospheric massloss rate. MOLECULAR HYDROGEN DISSOCIATION IN THE UPPER ATMOSPHERE OF HD 209458b The thermal regime and escape rate in the atmo spheres of GEPs significantly depend on the chemical composition of the atmosphere. In contrast to the planets of the Solar System that have atmospheric compositions stable over geological timescales, the composition of giant exoplanets may change rapidly as a consequence of the influence of the intense radiation field of the parent star. Moreover, the thermal regime and the composition of the atmosphere are tightly connected through heating and cooling (Yelle, 2004; Munoz, 2007). Aeronomical models such as that of Yelle (2004) generally assume that reaching tempera tures of several thousand Kelvin at particle densities on the order of ~1010 cm–3 leads to the thermal disso ciation of molecular hydrogen, (1) H 2 + M → H + H + M, and the formation of the Н2 H transition region in the inner thermosphere of the exoplanet. In the upper thermospheric layers, the photoionization of atomic hydrogen starts to play the dominant role. Corre
spondingly, the makeup of the upper atmosphere Н, Н+, which is an changes with altitude as Н2 additional factor forming the extended atmosphere, since such a change in the composition is accompa nied by the growth of the characteristic scale height. Figure 1 shows the composition and temperature change with height in the Н2 H transition region of the upper atmosphere of HD 209458b for a moderate level of stellar activity according to the model by Yelle (2004). The moleculartoatomic transition occurs at altitudes of about (5–15) × 103 km, and the tempera ture of the gas attains values of about a few 103 K. The temperature and density values in the transition region are consistent with the observations (Ballester et al., 2007). The model of the upper neutral atmosphere shown in Fig. 1 consists primarily of H2, H, and He; we will use it to trace the kinetic properties of suprath ermal hydrogen atoms. Munoz (2007) showed that the concentrations of atomic oxygen and ionized carbon observed in the vicinity of the exoplanet (VidalMad jar et al., 2004) are more than an order of magnitude lower than those in the upper atmosphere of HD 209458b. The dissociation processes such as photodissocia tion, collisional dissociation, dissociative ionization, etc., are the primary sources of thermal and suprather mal fragments of molecules in electronically excited states in the upper planetary atmospheres (Wayne, 1991). Although a hydrogen molecule is simplistic, its UV and/or electron collisional dissociation may occur in several ways:
H 2 + hν ( eν) → H*2 → H(1 s ) + H(1 s ,2s ,2p ,...) + ( eν) + ΔE dis.
(2)
If as a result of the absorption of radiation, elec trons are excited to unbound or antibound orbits and their excitation energy exceeds the binding energy of the molecule, the molecule may dissociate. This mechanism allows for the photodissociation of mole cules excited either to a continuum bound state or immediately to an unbound (repulsive) state. The photodissociation cross sections for these processes are usually smooth functions dependent on the wave length; therefore, low (0.05–0.1 nm) spectral resolu tion data on the incident flux and cross sections are sufficient to estimate the dissociation rate (Fox et al., 2008). Another important mechanism is predissocia tion, when an absorbed photon excites the molecule into a bound state of electron excitation, from which the subsequent nonradiative transition into an unbound state is possible. The dissociation rate depends on the absorption rates at the wavelengths of the selected line transitions and the predissociation probabilities (Fox et al., 2008). The Н2 dissociation energy is 4.48 eV (which corre sponds to 276.9 nm), but the photoabsorption cross sections at wavelengths longer than 111.6 nm are neg
Altitude, km
SHEMATOVICH H+ He H2 H
10000
106 Altitude, km
98
107
108 109 1010 1011 Concentration, сm–3
1012
10000
Temperature, eV
1
Fig. 1. Chemical composition (upper panel) and tempera ture (lower panel) in the H2 H transition region in the upper atmosphere of HD209458b according to the aero nomical model by Yelle (2004).
ligibly small. In planetary atmospheres, Н2 photodis sociation by UV photons in the spectral range of 84.5– 111.6 nm occurs mainly through dipole transitions from the ground state X 1Σ +g( v) into excited bound states B1 Σ u+(v' ), C1 Π u(v' ), B ' 1Σu+(v' ), and D1 Π u(v' ) . From these states, Н2 molecules may either transfer to the discrete levels of the ground state, radiating away the excess energy, or to the ground state continuum with the subsequent dissociation into two groundstate hydrogen atoms. The predissociation probabilities for these levels are in the range of 0.1–0.15 (Abgrall et al., 1997). At shorter wavelengths of 84.5 nm, direct absorption into B1 Σ u+(v'), C1 Π u(v' ), B ' 1Σu+(v' ), and D1 Πu(v' ) electron excitation continua dominate the photodissociation processes. These processes have sufficiently high cross sections and as a result of the dissociation lead to the formation of a hydrogen atom in the ground state H(1s) and a hydrogen atom in the excited H(2s,2p) state (GlassMaujean, 1986). The excess kinetic energy Δ E dis of the atoms formed through the photodissociation of the hydrogen molecule (2) was calculated as the difference between the energy of the absorbed UV photon, the energy of electronically excited state, and the dissociation energy. In the case of the dissociation of Н2 by photo electrons, the excess energy was found through the distributions calculated by Ajello et al. (1995). The distributions calculated for several electron beam energies show the populations of relatively slow ther mal energy (0–1 eV) and fast highenergy (1–10 eV, with a main at about 4 eV) atoms. For dissociative ion ization by photoelectrons,
H 2 + ev → H+ + e + H(1s ,2s,2p,... ) + (ev) + ΔE disi (3) SOLAR SYSTEM RESEARCH
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excess kinetic energy was obtained through the distri butions calculated by van Zyl–Stephen (1994). Suprathermal hydrogen atoms created through pro cesses (2) and (3) lose their energy in elastic collisions with the basic neutral components of the surrounding atmospheric gas:
velocity distribution for the fresh particles formed with an excess of kinetic energy. The collision integrals at the righthand sides of the kinetic equations describe the gasstate changes due to chemical reactions and equal
H h( E) + H th, He th, H 2 th (4) → H( E′ < E) + H th, He th, H 2 th. It should be noted that for suprathermal energies of hydrogen atoms, the energy transfer efficiency between hot and thermal atoms through elastic scat tering is largely determined by the phase functions– distributions by the scattering angle. Experimental and numerical simulation data (Hodges–Breig, 1991; Krstic–Schulz, 1999a,b) show that these distributions peak at low scattering angles, although the total cross sections are relatively high. The effectiveness of the energy transfer is thus strongly dependent on the col lision energy. These properties of elastic collisions with thermal H2, He, and H to a high degree determine of the parameters of the fraction of suprathermal hydro gen atoms in the upper photosphere of HD 209458b.
J mH( FH, Fm) = g ijd σ md c j[ FH( c 'i)Fm( c 'l) − FH( c i) Fm( c j)], (7)
KINETICS OF SUPRATHERMAL HYDROGEN ATOMS As hydrogen atoms appear in the process of disso ciation with an excess of kinetic energy, their distribu tion in the Н2 H transition region of the upper atmosphere of HD 209458b is determined by the Bolt zmann equation with a photochemical source func tion
∂FH ∂F ∂F +r H+ Γ H = ∂t ∂r mH ∂c
∑Q + ∑ J H s
s
H m ( FH, Fm),
(5)
where gij is the relative velocity and dσm is the cross section of the elastic scattering of suprathermal hydro gen atoms elastically colliding with hydrogen and helium. The scattering cross sections were taken from the works by Hodges–Breig (1991) and Krstic–Schutz (1999a,b). For thermal components, we used Max wellian distributions with the local temperature and density values calculated by the aeronomical model (see Fig. 1). To estimate the source function (6) for the suprath ermal hydrogen atoms, it is necessary to calculate the dissociation and ionization rates of atmospheric gas caused by UV stellar radiation, the photoelectron pro duction rate, and the molecular hydrogen dissociation and dissociative ionization rates caused by electron collisions. The extreme UV radiation of the star is absorbed by atmospheric gas and leads to the excita tion, dissociation, and ionization of different compo nents of the atmosphere and produces photoelectrons with energies sufficient for the subsequent ionization and excitation of atomic and molecular hydrogen. The energy of the ionizing quanta by definition exceeds the ionization potential, and its excess produces electrons with an excess of kinetic energy and ions in excited states. The photoelectron production rate at a given altitude z in the upper atmosphere is defined by the following expression: λi
m
together with the initial and boundary conditions for the atmospheric gas in the volume V influenced by the gravitation Γ of the planet and the physical assump tions such as the low density of the gas and a finite interaction of the radii during collisions (Shemato vich, 2004, 2007, 2008). We use a microscopic descrip tion of the suprathermal hydrogen atom population by the distribution function FH(t , r, c) = nH(t , r)f H(t , r, c ), where nH(t , r) is the number density of suprathermal particles, and f H(t , r, c) is a normalized oneparticle velocity distribution function. Source functions QsH(t , r, c) set the formation rates for the suprathermal atoms in photochemical reactions (2) and (3) and are usually written as follows:
QsH(t , r,c ) = q sH(E) f sH(t , r,c ).
(6)
Here, q sH( E) = c i − c j σ s(E) is the differential suprathermal hydrogen atom production rate by a cer tain photochemical source s at the particle collision energy E; the function f sH(t , r, c) gives the normalized SOLAR SYSTEM RESEARCH
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qe(E, z ) =
∑∑ n (z)∫ dλI (λ ) ∞
k
k
l
0
(8)
× exp(−τ(λ , z ))σ ik pk(λ , E k,l), where the optical thickness τ is given by τ( λ , z ) =
∑ k
∞
∫
σ k( λ ) nk( z ') d z ', a
z
and nk is the neutral component k number density; σ ik( λ ) and σ ak( λ ) are the ionization and absorption cross sections, respectively, dependent on the wave length λ. In expression (8), we use the relative yields pk(λ, Ek,l) and the potential of ionization Ek,l for the electronically excited states of the ion. The energy of the forming photoelectron is E = Eλ – Ek,l,, where Eλ is the energy of the photon and λk is the wavelength cor responding to the ionization potential of the kth neu tral component. I∞(λ) is the incident stellar radiation flux at the wavelength λ. In the calculations below, we used a flux of solar radiation in the wavelength range of
SHEMATOVICH
Altitude, km
100
10000 UVphotons Photoelectrons
Q, сm–3 s–1 eV–1
101 106 105 104 103 102 101 100 0.1
102 103 104 105 Formation rate, сm–3 s–1
106
8000 km 5000 km 1.0 Energy, eV
10.0
Fig. 2. Upper panel: suprathermal hydrogen atom produc tion rate via dissociation by UV photons (solid line) and photoelectrons (dashed line). Lower panel: energy spec trum at altitudes of 5000 km (solid line) and 8000 km (dashed) for the suprathermal atoms formed by the H2 H dissociation processes.
1–115 nm for the moderateactivity solar spectrum model from Huebner et al. (1992) scaled for the dis tance of 0.045 AU equal to the semimajor axis of the HD 209458b exoplanet. The relative yields for excited ionic states, absorption, and ionization cross sections are taken from the same work for the main atmo spheric components Н2, H, and He. It is necessary to note that the suprathermal hydrogen atom production rates qsH(E, z) through the dissociation and dissociative ionization of Н2 by extreme UV radiation were calcu lated using formula (8) with photodissociation and dissociative photoionization cross sections, respec tively. The Monte Carlo method developed by Shematov ich et al. (2008) adapted for hydrogen atmospheres was used to calculate the collisional kinetics and the transport of photoelectrons in the atmosphere of HD 209458b. In the daytime atmosphere of HD 209458b, highenergy electrons are produced through the pho toionization of the main atmospheric components by the extreme UV and soft Xray radiation of the star. The appearing electrons propagate in the atmosphere, where they lose their energy in elastic, inelastic, and ionizing collisions with the main components of the atmospheric gas: ⎧e (E ') + X ⎪ , e (E) + X → ⎨e (E ') + X* ⎪e (E ') + X + + e (E ) ⎩ s
where E and E' ( 0)
10–1 10–2
8000 km Hhot (>1 eV)
10–3 Hhot (>2 eV)
10–4 10–6 10–1 10–2
Altitude, km
10–5
F, (vr > 0)
101
10 Energy, eV 16000 km
H2th
10000
Hth
10–3 10–4 10–5 10–6
106
10 Energy, eV Fig. 3. Oneparticle distribution function for upwardmoving suprathermal hydrogen atoms at altitudes of 8000 km (upper panel) and 16000 km (lower panel). The dashed curves show the local equilibrium distributions of atomic hydro gen corresponding to the parameters of the model by Yelle (2004). Vertical dot–dashed lines mark the escape energies of hydrogen atoms at the given altitudes.
producing hydrogen atoms with energies in the range of 1–10 eV. The given calculated formation rates and spectra were used as source functions (6) in the kinetic Boltz mann equation (5). The solution to the kinetic equa tion was done numerically with our stochastic model (Shematovich, 2004), which considered the kinetics and transport of suprathermal hydrogen atoms on the molecular level as a function of the distribution of suprathermal hydrogen atoms in the transfer region of the upper atmosphere. The calculations were carried out for stationary conditions in the daytime upper atmosphere in the direction of the planet facing the star. In Fig. 3, oneparticle distribution functions are given for suprathermal hydrogen atoms moving up at altitudes of 8000 km (upper panel) and 16000 km (lower panel). The dashed curves in Fig. 3 correspond to the local equilibrium of the oneparticle distribu tions of atomic hydrogen calculated based on the tem perature profile from the model of Yelle (2004). The vertical dot–dashed lines show the escape energies of hydrogen atoms at the given altitudes. As the aim of the current work is the estimation of the escape rates of hydrogen atoms due to the process of the dissociation of Н2, in Fig. 3 the distribution function is presented only in the suprathermal energy range above 2 eV. Simulations show that the distribution functions sig nificantly deviate from the local equilibrium distribu tions. At a height of 8000 km, close to the region of the maximal production of hydrogen atoms as a result of the dissociation of Н2, a significant fraction of hydro SOLAR SYSTEM RESEARCH
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1012
Fig. 4. Thermal and suprathermal hydrogen distribution with altitude for the H2 H transition region of the upper atmosphere of HD 209458b.
gen atoms with energies high enough to escape the gravitational field (about 6.9 eV) is formed, in contrast to the local equilibrium distribution. At an altitude of H 16000 km close to the upper boundary of the Н2 transition region, the value of the distribution function is higher than the value of the equilibrium distribution; at that, the value of the number density at energies above the local escape energy (about 5.9 eV) of suprathermal hydrogen atoms is higher than the number density cal culated from the local equilibrium distribution by one order of magnitude. The distribution functions shown in Fig. 3 allow for an estimation of the number density of suprathermal hydrogen atoms forming as a result of the dissociation of H2, as well as a comparison of it with the height dis tribution of the warm neutral components of the atmosphere. The altitude profile of suprathermal (with energies above 2 eV) hydrogen atoms is shown in Fig. 4. For comparison, we also show the densities of thermal atomic and molecular hydrogen corresponding to the model shown in Fig. 1. The calculations show that a stationary fraction of suprathermal hydrogen atoms with energies higher than 2 eV is formed only in the outermost parts of the transition region, where most collisions of suprathermal hydrogen occur with neu tral hydrogen atoms while the temperature of the atmosphere itself is sufficiently hot (see Fig. 1, lower panel). As follows from the analysis of the distribution functions presented in Fig. 3, the concentration of suprathermal hydrogen atoms, due to the process of the dissociation of H2, is here several times higher than the concentration of atmospheric hydrogen in the area of energy above 2 eV.
102
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Fe, сm–2 s–1 eV–1
1013
1012
Energy, eV
10
Fig. 5. Energy spectrum of the atoms escaping the upper atmosphere of HD 209458b formed by dissociation pro cesses.
The calculations of the distribution functions pre sented in Fig. 3 show that dissociation processes (2) and (3) of molecular hydrogen are accompanied by the formation and transfer in the uppermost layers of the Н2 H transition region in the upper atmosphere of the exoplanet of suprathermal hydrogen atoms with energies exceeding the local escape energy moving upwards. In Fig. 5, we show the energy spectrum of the stream of hydrogen atoms leaving the atmosphere of HD 209458b through the upper boundary of the trans fer region at an altitude of ~2 × 104 km as a result of the dissociation processes. For moderate stellar activity in the considered range of UV radiation, the flux of escaping particles is 3.4 × 1012 cm–2 s–1. Averaged over the upper atmosphere, this flux corresponds to an atmospheric massloss rate of about 3.4 × 109 g/s due to the dissociation of H2, which is somewhat lower than the observational estimate of about 1010 g/s (Ehrenreich et al., 2008). Our atmospheric massloss estimate may be considered as a lower estimate because our simulations were run for a moderate stel lar activity level of UV radiation, and for the probabil ities of the predissociation, we took the minimal value of 0.1. Naturally, for higher UV fluxes, the Н2 dissoci ation processes due to the action of extreme UV stellar radiation and the subsequent flux of photoelectrons will create a significantly higher contribution to the formation of hydrogen atoms leaving the atmosphere. CONCLUSIONS The Н2 dissociation processes by the UV radiation of the parent star and the subsequently formed photo electrons are important sources of suprathermal
hydrogen atoms in the upper atmosphere of the exoplanet HD 209458b, which lead to the production of a stable fraction of suprathermal hydrogen atoms. One of the important consequences is the formation of an out flowing atom flux that we estimate as 3.4 × 1012 cm–2 s–1 for a moderate stellar activity level in the UVradiation range, which leads to an overall atmospheric loss rate of about 3.4 × 109 g/s due to the process of H2 dissoci ation. This estimate differs from the observed quantity by about a factor of 2 (VidalMadjar et al., 2003; 2004; 2008; Ehrenreich et al., 2008). More precise estimates on the atmospheric loss rate may be made when better data on the UV flux from the parent star or better data on the planetary atmosphere are available. Dissocia tion processes are an important source of suprather mal hydrogen atoms and should therefore be included into future aeronomical models of the physical and chemical processes of the upper atmospheres of extra solar planets. We are planning the further development of our stochastic model for HD 209458b by including the influence of the coronal stellar wind plasma on the planetary corona. This influence will be accompanied by chargeexchange reactions between hot ions and the atoms of the neutral corona (Holmström et al., 2008), as well as the atmospheric sputtering of the outer atmosphere. Atmospheric sputtering is also an important source of suprathermal particles and leads to the further additional loss of the neutral corona of the planet (Johnson et al., 2008). ACKNOWLEDGMENTS The work was supported by Russian Foundation for Basic Research, project no. 080200263, and the Grant for Leading Scientific Schools no. NSh4354.2008.2. REFERENCES Abgrall, H., Roueff, E., Liu, X., and Shemansky, D.E. The Emission Continuum of ElectronExcited Molecular Hydrogen, Astrophys. J, 1997. vol. 481. pp. 557–566. Ajello, J.M., Kanik, I., Ahmed, S.M., and Clarke, J.T. Line Profile of H Lyman α from Dissociative Excitation of H2 with Application to Jupiter, J. Geophys. Res., 1995, vol. 100, pp. 26411–26420. Ballester, G., Sing, D.K., and Herbert, F. The Signature of Hot Hydrogen in the Atmosphere of the Extrasolar Planet HD 209458b, Nature, 2007, vol. 445. pp. 511– 514. BenJaffel L. Exoplanet HD 209458b: Inflated Hydrogen Atmosphere But no Sign of Evaporation, Astrophys. J. 2007. vol. 671. pp. L61–L64. Charbonneau, D., Brown, T.M., Latham, D.W., and Mayor, M. Detection of Planetary Transits across a SunLike Star, Astrophys. J., 2000, vol. 529, vol 1, pp. L45–L48. Charbonneau, D., Brown, T.M., Noyes, R.W., and Gilli land, R.E. Detection of an Extrasolar Planet Atmo sphere, Astrophys. J., 2002, vol. 568, pp. 377–384. SOLAR SYSTEM RESEARCH
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