The Upper Atmospheres of Extrasolar Gas Giants in 3D

Transit of HD17156b (Antonio Cagnoli)

T.T.Koskinen, A.D.Aylward, S.MillerCentre for Planetary Sciences

Department of Physics and Astronomy, University College LondonMolecules 2008, Paris, France

EXOTIM output

EXOTIM3D equations of motion in spherical pressure coordinates (Eulerian co-rotating frame)

Continuity

Momentum Viscosity (due to molecular diffusion)

Energy equation

Equations return u and T at every point in the 3D grid.

Hydrostatic equilibrium

Pressure range: 2 bar - 3.7 pbar

Reaction rates

Evaporation

Exobase: ncHc ≈ 1

Escape velocity

Jupiter: vesc 60 kms-1

Thermal escape parameter

Jupiter c ~ 480

Jeans escape flux

Limiting flux

Orbital Distance and Temperature

Exobase and effective temperatures vs. orbital distance for a Jupiter-like EGP orbiting a Sun-like star

H3+ cooling efficiency and low-

pressure (0.008 nbar) mixing ratio of H2 averaged over the dayside

Koskinen et al., ApJ, 661, 515-526 (2007)

At 0.2 AU

Temperatures and circulation at the upper boundary of the model at 0.2 AU. The maximum winds reach ~2 km s-1.

Mixing ratio of atomic hydrogen near the upper boundary of the model as a function of longitude along the equator

Energy equation terms at the substellar point of the model at 0.2 AU

Substellar electron densities

Hydrostatic ‘Stability’ Limit

Fig. 2, Koskinen et al., Nature, 450, 845-848 (2007)

Mass Loss

With parameters appropriate for HD209458b (onset of hydrodynamic escape cooling efficiency ~60 %)

0.045 AU: dM/dt ~ 1.7 x 1010 gs-1

Koskinen, T.T, et al., Nature, 450, 845-848 (2007)

Escape flux (sr-1s-1):

(Watson, 1981)

HD17156b

The Planet1,2

Mp ≈ 3.1 MJ , Rp ≈ 1.0 - 1.23 RJ

p ≈ 1.66 - 3.8 J

a ≈ 0.16 AU , P ≈ 21.2 days

e ≈ 0.67 , i ≈ 85o - 86.5o

1Gillon, M. et al., A&A, 485, 871-875 (2008)

2Irwin, J. et al., ApJ, 681, 636-643 (2008)

The Star

Fischer, D.A. et al., ApJ, 669, 1336-1344 (2007):

G0

Dist ≈ 78.24 pc , MV ≈ 3.7

Age ≈ 5.7 Gyr , Teff ≈ 6079 K

M* ≈ 1.2 MSun , R* ≈ 1.47 RSun

Fe/H ≈ 0.24 , Prot ≈ 12.8 days

Discovered by the N2K consortium (Fischer et al. 2007), transit first detected by amateur astronomers (Barbieri, M. et al., A&A, 476, L13-L16, 2007)

The Orbit

Earth

HD17156b: ≈ 121o

Kepler’s equation for the mean anomaly

E = Eccentric anomaly

True anomaly

Pseudo-synchronous spin

sp ≈ 5.6 <>orbit

The orbital distance varies between 0.052 and 0.27 AU. One quarter orbit is reached at -153 degrees.

Average Temperatures

EXOTIM globally averaged temperature at p = 0.04 nbar vs. orbital true anomaly

72-91 % cooling function (Exo-1)

0.01-0.24 % cooling function (Exo-2)

Mixing ratio of atomic hydrogen determines the cooling efficiency. The plot shows apastron mixing ratios for two different lower boundary values, 2 x 10-4 (solid line) and 0.01 (dotted line).

vesc ~ 108 km s-1

Koskinen et al., ApJ, accepted

Exo-1

Substellar electron density profiles at apastron (solid line) and at periastron (dotted line)

Substellar density profiles of the dominant neutral species

Substellar ion density profiles: H+ (dotted), H3

+ (solid), H2+ (dashed), He+

(dash-dotted)

Above: Substellar P-T profiles at apastron (solid), 1/4 orbit (dotted), periastron (dashed), and 3/4 orbit (dash-dotted)

Exo-2

Substellar electron density profiles at apastron (solid line) and at periastron (dotted line)

Substellar density profiles of the dominant neutral species

Substellar ion densitiesAbove: Substellar P-T profiles at apastron (solid), 1/4 orbit (dotted), periastron (dashed), and 3/4 orbit (dash-dotted)

Insert 3D sphere plot

Evaporation

Exo-1:

zc 1.03 Rp

c (H) 200

Hc 180 km

Exo-2:

zc 1.55 Rp

c (H) 15

Hc 5000 km

Exobase characteristics (periastron)

Evaporation of H at periastron (Exo-2)

wJ 0.45 cm s-1, dMJ 104 gs-1

[wL 1.9 cm s-1] Mass loss from the Exo-2 simulation based on thermal Jeans escape

Hydrogen Cloud

Altitude of the 0.04 nbar pressure level in Exo-1 (solid) and Exo-2 (dotted)

H3+ emissions

Total H3+ infrared emissions from

HD17156b vs. orbital true anomaly

Line cm-1) Power

(x 1013 W)

Flux at Earth

(x 10-25 Wm-2)

Q (1,0) 2529.73 2.34 3.19

Q (3,0) 2509.08 3.85 5.26

R (3,0) 2930.17 4.09 5.59

R (4,4) 2894.5 2.68 3.65

Line fluxes at periastron

Total emitted power: 8.6 x 1015 W [~1.17 x 10-22 Wm-2]

Conclusions

• We have developed a TGCM for extrasolar giant planets, including an ionosphere in photochemical equilibrium

• The nature of the upper atmosphere depends on the composition and the details of the photochemistry, especially on the mixing ratio of H2

• We have applied the model to HD17156b, and find that the atmosphere of this planet is not likely to undergo such fast hydrodynamic escape as has been postulated for close-in giants like HD209458b

• Observations of the upper atmosphere can constrain the properties of the lower atmosphere, stellar XUV activity and stellar wind conditions in the vicinity of planets