Saving Planetary Systems: the Role of Dead Zones

1. Saving Planetary Systems:the Role of Dead Zones Ralph Pudritz, Soko Matsumura (McMaster University), & Ed Thommes (CITA) AAS 208, Calgary

2. Migration can acconunt for orbits of massive extrasolar planets – all within 5 AU Migration occurs by tidal interaction between planet and disk: • Type I: migration without gap opening – planet swallowed within 1 Myr. • Type II: migration after gap opening – planet locked to disk and migrates at rate dictated by inner disk – again lost quickly Why do planetary systems survive it? • Absence of disk turbulence in “dead zone” in central disk significantly slows planetary migration (Matsumura, Pudritz, & Thommes 2006: MPT06). Can even reverse it.

3. Dead Zone (low viscosity region in a disk) Dead Zone (Gammie, 1998): - Magnetic turbulence is inactive in poorly ionized regions of the disk: so the disk’s viscosity is very low there. - The DZ stretches out to about 13 Astronomical Units (1AU = Earth-Sun difference). Eg. Matsumura & Pudritz 2006 (MNRAS) Ionization: X-rays from star cosmic rays radioactive elements heating from central star

4. Gap opens in a disk when Tidal Torque ~ Viscous Torque Level of magnetic turbulence responsible for the “viscosity” of the gas Protoplanet Tidal Torque Disk Viscous Torque Disk

5. Gap-opening masses of Planets 100 10 1 Gap-opening mass [MJ] 0.1 0.01 0.001 0.0001 0.01 0.1 1 10 100 Disk Radius [AU] Jupiter Uranus or Neptune Earth

6. Dead Zones and Planet Migration (MPT 06) 1. eg. Type I migration (before gap-opening) → 10 MEarth (< MUranus) Dead Zone Star Protoplanet Numerical Technique: We use a hybrid numerical code combining N-body symplectic integrator SYMBA (Duncan et al 1998) with evolution equation for gas (Thommes 2005) - Allows us to follow evolution of planet and disk for disk lifetime: 3 – 10 Million years.

7. 10 ME: Type I migration (No Gap-opening) 30 30 20 20 Dead Zone Disk Radius [AU] Disk Radius [AU] 10 10 0 0 0 2×106 4×106 6×106 8×106 107 0 2×106 4×106 6×106 8×106 107 Time [years] Time [years] (w/o Dead Zone) (w/ Dead Zone) =10-2 =10-2 =10-5

8. If planet forms within the DZ:halt migration of terrestrial planets by opening a gap in the DZ 10 M_E planet started in dead zone; Left 2 million yrs Viscosity:

9. Type II migration of Jupiter mass planet 30 30 20 20 Dead Zone Disk Radius [AU] Disk Radius [AU] 10 10 0 0 0 2×106 4×106 6×106 8×106 107 0 2×106 4×106 6×106 8×106 107 Time [years] Time [years] (w/o Dead Zone) (w/ Dead Zone) =10-3 =10-3 =10-5

10. Migration of a Jovian planet over 10 Myr. • Note extent of gap opened by planet once inside dead zone. • Planet started at 20 AU settles into orbit at 4AU after 10 Myr

11. 10 ME opens gap at 3.5 AU in dead zone Also: 1 ME opens gap near 0.1 AU

12. Percentage of planets that migrate and stop within 5 AU • Assume uniform distribution of disks with temperatures (1AU) between 150 and 450 K; and lifetimes between 1 – 10 Million yrs • Observe 5-20% of stars with planets in this regime: - arises if disk viscosity < 0.0001 Percent of planetary systems with planets migrating inside 5AU

13. Summary: • Earth massplanets, that start migration outside of DZ, are reflected to larger radii • Earth mass planets that are formed inside DZ halt migration because they can open a gap in the disk (eg. Earth mass at around 0.1 AU). • Massive planets open gaps, but their Type II migration very slow in low viscosity DZ • If viscosity parameter is < 0.0001, can account for observed frequency of 5-20% of stellar systems with planets inside 5AU