Source of Atomic Hydrogen in Source of Atomic Hydrogen in the Atmosphere of HD 209458bthe Atmosphere of HD 209458b

Mao-Chang LiangCaltech

Related publications1. Liang et al. 2003, ApJ Letters, in press2. Liang et al. 2003, manuscript in preparation

OutlineOutline

Motivation of this StudyObservation: Properties of HD 209458bSimulation: One-dimensional ModelResultsSummary

MotivationMotivation

It is a Jupiter-size planet outside our solar system– relate to our solar system– how it formed/how it evolves

HD 209458b is close-in, and is the best-studied– chemical processes?

To be more specific, source of atomic hydrogen?– fuel hydrodynamic loss?– evolution of the atmosphere

transit duration

inte

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star

planet

Roche lobe(Hill sphere)

orbit

not to scale

Observation of HD 209458 systemObservation of HD 209458 systemThe central star is a G0 solar-type dwarf starOne giant planet found, HD 209458bIt 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 1-D KINETICS model to simulate the chemical processessimulate the chemical processes

Model description Model description generating model atmospheregenerating 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 atmosphereModel atmosphere

Simulation setupSimulation setup

253 chemical reactions involving C, H, and OContinuity of massSolve for steady-state solution

• <ni/t> 0

ResultsResultsH Productionhigh H/H2 ratio

H2O + h H + OH OH + H2 H2O + H

H2O Production

CO + h C + OO + H2 OH + HOH + H2 H2O + H

UV-flux limited

important for water-poor atmosphere

CO

H2O

H

CO2

CH4

SummarySummary

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

Goukenleuque et al. 2000

0.46 MJ, 0.05 AU, e ~ 0.013, G2

Generating model atmosphereGenerating 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 atmosphereGenerating model atmosphereA 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 tableCalculate T and P as function of zModel convergedNew chemical abundances obtainedIteration until T, P, and chemical abundances

converged

1-D model technical detail1-D model technical detailmass 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 diffusionEddy diffusiondetermined from He distributiondensity-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

TimescalesTimescales

Radiative relaxation timescale of the atmosphere (cp/Teff

3)– 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 lossHydrodynamic loss

Escape parameter: esc (GMpm/r)/(kT)

Future ProspectFuture 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

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

Survey of extrasolar planetsSurvey of extrasolar planets

California & Carnegie Planet Search websitehttp://exoplanets.org/

Debra Fishcer 2003

Determination of planet’s orbital Determination of planet’s orbital and physical propertiesand physical properties

HD 209548HD 209548Mazeh et al. 2000

amplitude + period Msin i + Torbit

Charbonneau et al. 2000

duration + obscuration R + i

Atmospheric featuresAtmospheric featuresSodium lineSodium line

Na D lines detected, ~4 sigma detection (2.320.57)10-4

Charbonneau et al. 2002

Atmospheric featuresAtmospheric featuresAtomic hydrogenAtomic hydrogen

hydrogen in the atmosphere, – 15 4% detection

larger than Roche lobe (?), 3.6 RJ -> 10% maximum

Vidal-Madjar et al. 2003

planetover exaggerated

ResultsResultsH Productionhigh H/H2 ratio

H2O + h H + OHOH + H2 H2O + H

H2O Production

CO + h C + OO + H2 OH + HOH + H2 H2O + H

CO2 Production

OH + CO CO2 + H

CH4 Production

CO + h C + OC + H2 + M 3CH2 + M2 3CH2 C2H2 + 2HC2H2 + H + M C2H3 + MC2H3 + H2 C2H4 + HC2H4 + H + M C2H5 + MC2H5 + H 2CH3

CH3 + H + M CH4 + M

UV-flux limited

important for water-poor atmosphere

source of hydrocarbons

Barman et al. (2002) T-P profilesBarman et al. (2002) T-P profiles

Fortney et al. (2003) T-P profilesFortney et al. (2003) T-P profiles

this work

Barman et al. (2002) T-P profilesBarman et al. (2002) T-P profiles

Fortney et al. (2003) T-P profilesFortney et al. (2003) T-P profiles

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