Photoevaporative Mass Loss From Hot Jupiters Ruth Murray-Clay Eugene Chiang UC Berkeley

~20% of the exoplanets discovered to date are hot Jupiters Star hot Jupiter ~0.05 AU Mercury 0.39 AU Earth 1 AU

An example: HD b, a transiting hot Jupiter HST transit light curve Brown et al Henry et al. 2000, Charbonneau et al % dip

Vidal-Madjar et al Hydrogen absorption detected around HD b during transit Do Hot Jupiters lose a significant fraction of their mass? -100 km/s100 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 L UV ~ L bol

e-e- p+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 ~ 5x10 12 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 starhot Jupiter LOS wind

The Equations Momentum: Mass continuity: Energy: Ionization equilibrium:

Heating/Cooling Balance e-e- p+p+ collisions distribute energy from ejected electron UV photon Photoionization Heating Ly α Cooling PdV Work Cooling hot Jupiter wind collisionally excited line emission e-e- Ly α photon H

Ionization Balance Photoionization Balanced by gas advection not radiative recombination ionization fraction increases outward f+f+ f+f+ e-e- p+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

T eff ≈ 1300K 1 bar surface of planet Photoionization base ( τ UV = 1) Sonic point Roche lobe radius exobase r 0 ~ cm 1.1 r r r 0 mean free path to ∞ is 1 T wind ≈ 10,000 K H2H2 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 Ly α or Ly β 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 More absorbtion is observed blue-shifted than red-shifted

Colliding Winds

Hydrodynamics of Colliding Stellar Winds Stevens, Blondin, & Pollock 1992

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

If the stellar wind is stronger, it may pressure-confine the planetary wind to a breeze.

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 b is < 5x10 10 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.