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Source of Atomic Hydrogen in
the Atmosphere of HD 209458b
Mao-Chang Liang
Caltech
Related publications
1. Liang et al. 2003, ApJ Letters, in press
2. Liang et al. 2003, manuscript in preparation
Outline

Motivation of this Study
 Observation: Properties of HD 209458b
 Simulation: One-dimensional Model
 Results
 Summary
Motivation
to scale
not
It is
a Jupiter-size
star
planet outside
our solar system
intensity
– relate to our solar system
planet
orbit
– how it formed/how it evolves

Roche lobe
(Hill sphere)
HD 209458b is close-in, and is the best-studied
transitprocesses?
duration
– chemical

To be more specific, source of atomic hydrogen?
– fuel hydrodynamic loss?
– evolution of the atmosphere
Observation of HD 209458 system

The central star is a G0 solar-type dwarf star
 One giant planet found, HD 209458b
 It is nearly edge-on, ~85 inclination
– facilitates detection of the atmosphere

Physical parameters: 1.54 RJ and 0.68 MJ (gravity
~800 cm s-2 < gearth)
 Orbital parameters: ~0.05 AU and 3.5 days period
–
–
–
–
probably tidally locked
permanent day/night
high UV flux/stellar irradiance: 104 of Jupiter
hot : > 1000 K
1-D KINETICS model to
simulate the chemical processes
Model description
generating model atmosphere

Model atmosphere calculated according to
Seager et al. (2000)
– Heating from stellar irradiance is uniformly distributed
to the whole planet
– Cloud-free and high temperature condensation-free
– Temperature-Pressure-Altitude profile: radiative
equilibrium + hydrostatic equilibrium
– Chemical abundances: thermochemical equilibrium,
using solar abundances (elements; reference model A)

Eddy diffusion  n-,  = 0.6-0.7
Model atmosphere
Simulation setup

253 chemical reactions involving C, H, and O
 Continuity of mass
 Solve for steady-state solution
• <ni/t>  0
Results
H Production
high H/H2 ratio
H2O + h  H + OH
OH + H2  H2O + H
UV-flux limited
H2O Production
CO + h  C + O
O + H2  OH + H
OH + H2  H2O + H
H
CO2
CO
CH4
important for
water-poor
atmosphere
H2O
Summary

OH and O radicals drive most of chemical reactions
 H2O plays as a catalyst in producing H
 H production is insensitive to the exact abundances of
H2O, CO, and CH4, as well as the eddy diffusion
– H is 1000 times more than that of Jupiter
– H formation is UV-flux limit

H production timescale ~ 1 day ~ circulation time scale
– importance of global circulation

H mixing ratio > 1% at the top of atmosphere
– fuel hydrodynamical loss? if escape parameter
esc( gravitational energy / thermal energy) < 10
End
0.46 MJ, 0.05 AU, e ~ 0.013, G2
Goukenleuque et al. 2000
Generating model atmosphere
 Temperature-Pressure-Altitude profile:
radiative transfer + radiative equilibrium +
hydrostatic equilibrium
 Chemical abundances: thermochemical
equilibrium, using solar abundances
 Iteration until the model is converged
Generating model atmosphere
 A table
that contains T, P, and chemical
abundances
– minimizing Gibbs free energy

Starting model atmosphere code
– initial guess for T and P as a function of z

Get chemical abundance from the table
 Calculate T and P as function of z
 Model converged
 New chemical abundances obtained
 Iteration until T, P, and chemical abundances
converged
1-D model technical detail

mass continuity
 ni/t + i/z = Pi  Li
 I = -Di[ni/z + ni/Hi + n(1+i)/T T/z]
-K[ni/z + ni/Ha + n/T T/z]
 Hi and Ha are scale heights for species i and atmosphere

boundary conditions
– lower boundary: initial abundances in the seep
atmosphere, derived from thermochemical equilibrium
– Upper boundary: zero flux for all species

steady-state condition: time evolves until <ni/t>  0
Eddy diffusion

determined from He distribution
 density-dependence ( n-) calculated from
the upward-propagating gravity wave
generated in the troposphere
– from the constancy of energy density (e.g.,
n*u2=const)
– constant below tropopause
– exponential decay above tropopause
Timescales

Radiative relaxation timescale of the
atmosphere (cp/Teff3)
– 1 day (~10 days on Earth, ~1000 days on Jupiter)

Eddy diffusion transport timescale
– greater than 106 sec at the bottom
– less than 1000 sec at the top
Hydrodynamic loss

Escape parameter: esc  (GMpm/r)/(kT)
Future Prospect

Tidally locked
– high wind speed, a few km/s  importance of global
circulation  redistribute the produced species

Temperature-pressure profiles
– cloud distribution and high temperature condensation

Haze/aerosol/hydrocarbon formation (in preparation)
– affect optical spectra/albedo

Observationally constrain the atmospheric
abundance
 Effect of stellar wind
 Evolution of the produced H and planet itself
 Set constraints to see if planetary features can be
detected in near future
Survey of extrasolar planets

Debra Fishcer 2003
Techniques
–
–
–
–



radial velocity
pulsar timing
eclipse/transit
astrometry
First extrasolar planet, 51 Peg b, in 1995
First atmospheric detection, HD 209468b, in 2002
111 planets found so far (July of 2003)
–
–
–
–
Jupiter size
high eccentricity
close in
correlation of iron abundance with planetary formation
California & Carnegie Planet Search website
http://exoplanets.org/
Determination of planet’s orbital
and physical properties
HD 209548
Mazeh et al. 2000
Charbonneau et al. 2000
amplitude + period  Msin i + Torbit
duration + obscuration  R + i
Atmospheric features
Sodium line

Na D lines detected,
~4 sigma detection
(2.320.57)10-4
Charbonneau et al. 2002
Atmospheric features
Atomic hydrogen

hydrogen in the atmosphere,
– 15 4% detection

larger than Roche lobe (?),
3.6 RJ -> 10% maximum
over exaggerated
planet
Vidal-Madjar et al. 2003
Results
CO2 Production
OH + CO  CO2 + H
CH4 Production
CO + h  C + O
C + H2 + M  3CH2 + M
2 3CH2  C2H2 + 2H
C2H2 + H + M  C2H3 + M
C2H3 + H2  C2H4 + H
C2H4 + H + M  C2H5 + M
C2H5 + H  2CH3
CH3 + H + M  CH4 + M
source of
hydrocarbons
H Production
high H/H2 ratio
H2O + h  H + OH
OH + H2  H2O + H
UV-flux limited
H2O Production
CO + h  C + O
O + H2  OH + H
OH + H2  H2O + H
important for
water-poor
atmosphere
Barman et al. (2002) T-P profiles
Fortney et al. (2003) T-P profiles
Barman et al. (2002) T-P profiles
Fortney et al. (2003) T-P profiles
this work
cross section (cm-2)
wavelength (angstrom)