Photoevaporative Mass Loss From Hot Jupiters

Ruth Murray-ClayEugene Chiang

UC Berkeley

~20% of the exoplanets discovered to date are hot Jupiters

Star

hot Jupiter~0.05 AU

Mercury0.39 AU

Earth1 AU

An example: HD 209458b, a transiting hot Jupiter

HST transit light curve

Brown et al. 2001

Henry et al. 2000, Charbonneau et al. 2000

1.5% dip

Vidal-Madjar et al. 2003

Hydrogen absorption detected around HD 209458b during

transit

Do Hot Jupiters lose a significant fraction of

their mass?

-100 km/s 100 km/s

0.05 AU is an extreme environment

Star

hot Jupiter~0.05 AU

• hot Jupiters probably formed further out and migrated in• once parked, they are bathed in UV radiation

UV

LUV ~ 10-6 Lbol

e-

p+

UV photons heat the atmosphere by photoionization

collisions distribute

energy from ejected

electron

UV photon

Before: After:

H

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

star hot Jupiter

LOS

wind

The Equations

Momentum:

Mass continuity:

Energy:

Ionization equilibrium:

Heating/Cooling Balance

e-

p+

collisions distribute energy

from ejected electron

UV photon

Photoionization Heating

Ly α CoolingPdV Work Cooling

hot Jupiter

wind

collisionally excited line

emission

e-

Ly α photon

H

Ionization BalancePhotoionization

Balanced by gas advection not

radiative recombination

ionization fraction increases outward

f+ f+

e-

p+

UV photon

H

Boundary Conditions

Burrows, Sudarsky, & Hubbard 2003

density and temperature at depth• Critical (sonic) point of

transsonic wind (2 conditions)

• ionization fraction at depth

• self consistent optical depth to ionization

Teff ≈ 1300K 1 bar surface of planet

Photoionization base (τUV = 1)

Sonic point

Roche lobe radius

exobase

r0 ~ 1010 cm

1.1 r0

2.9 r0

4.5 r0

mean free path to ∞ is 1

Twind ≈ 10,000 K

H2

H, H+

hydrodynamic wind

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

integrated ~10 km/s thermal broadening

centered on wind velocities of ~20 km/s

-100 km/s 100 km/s

Vidal-Madjar et al. 2003

More absorbtion is observed blue-shifted

than red-shifted

Colliding Winds

Hydrodynamics of Colliding Stellar Winds

Stevens, Blondin, & Pollock 1992

Vidal-Madjar et al. 2003

If red-shifted 100 km/s gas exists, we don’t yet understand its origin

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.