The Upper Atmospheres of Extrasolar Gas Giants in 3D
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Transcript of The Upper Atmospheres of Extrasolar Gas Giants in 3D
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