Download Murray-Clay, R.

Photoevaporative Mass Loss
From Hot Jupiters
Ruth Murray-Clay
Eugene Chiang
UC Berkeley
~20% of the exoplanets
discovered to date are hot Jupiters
hot Jupiter
~0.05 AU
Star
Mercury
0.39 AU
Earth
1 AU
An example: HD 209458b,
a transiting hot Jupiter
Henry et al. 2000, Charbonneau et al. 2000
HST transit light curve
1.5% dip
Brown et al. 2001
Hydrogen absorption detected
around HD 209458b during transit
-100 km/s
100 km/s
Do Hot Jupiters lose a
significant fraction of
their mass?
Vidal-Madjar et al. 2003
0.05 AU is an extreme environment
• hot Jupiters probably formed further
out and migrated in
• once parked, they are bathed in UV
radiation
LUV ~ 10-6 Lbol
UV
Star
hot Jupiter
~0.05 AU
UV photons heat the atmosphere
by photoionization
Before:
After:
UV photon
e-
H
p+
collisions
distribute energy
from ejected
electron
The energy-limited maximum
mass-loss rate is large:
.
M ~ 5x1012 g/s
This would mean a Jupiter mass planet at 0.05 AU
evaporates completely in 5 Gyr
(Lammer et al. 2003; Baraffe et al. 2004, 2005;
Lecavelier des Etangs et al. 2004)
And observations show hot Jupiters are systematically less massive
than other exoplanets (Zucker & Mazeh 2002)
But Hubbard et al. (2007) are unable to reproduce the mass
distribution of hot Jupiters using mass-loss theories
Photoionization Heating Drives a
Hydrodynamic (Parker) Wind
wind
LOS
star
hot Jupiter
The Equations
Mass continuity:
Momentum:
Energy:
Ionization equilibrium:
Heating/Cooling Balance
Photoionization
Heating
UV photon
e-
H
Ly α Cooling
e-
Ly α photon
collisionally excited
line emission
collisions distribute
energy from
ejected electron
p+
PdV Work Cooling
wind
hot Jupiter
Ionization Balance
Photoionization
UV photon
e-
H
p+
Balanced by gas
advection not radiative
recombination
f+
f+
ionization fraction increases
outward
Boundary Conditions
density and temperature at depth
• Critical (sonic) point of
transsonic wind (2
conditions)
• ionization fraction at
depth
• self consistent optical
Burrows, Sudarsky, & Hubbard 2003
depth to ionization
hydrodynamic wind
exobase
mean free path to
Roche lobe radius
is 1
4.5 r0
Sonic point
2.9 r0
Twind ≈ 10,000 K
H, H+
Photoionization base (
τUV = 1)
1 bar surface of planet
1.1 r0
H2
r0 ~ 1010 cm
Teff ≈ 1300K
Hydrodynamic
Wind Model:
atomic and
ionized hydrogen
the energy-limited rate
Heating, Cooling, and Ionization Terms
From ray tracing through our model, we predict
that if observed at Lyα or Lyβ line center, the wind
will completely obscure the star during transit.
Ly α Line Center
-100 km/s
100 km/s
integrated ~10 km/s thermal broadening
centered on wind velocities of ~20 km/s
More absorbtion is observed blue-shifted
than red-shifted
Vidal-Madjar et al. 2003
Colliding Winds
Hydrodynamics of Colliding Stellar
Winds
Stevens, Blondin, & Pollock 1992
If red-shifted 100 km/s gas exists, we don’t
yet understand its origin
Vidal-Madjar et al. 2003
Mass Loss from Hot Jupiters
Conclusions
• Hot Jupiters lose mass in the form of hydrodynamic winds
driven by stellar UV heating.
• The mass loss rate for HD 209458b is < 5x1010 g/s. The planet
will lose ~1% of its mass over its lifetime.
• We predict that at line center, stellar Lyα is completely
obscured during transit.
• If observations of blue-shifted high-velocity gas are confirmed,
planetary wind/stellar wind interactions might provide a source.
• Currently observed red-shifted high-velocity gas is a mystery.