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

Migration can acconunt for orbits of massive extrasolar planets – all within 5 AU – all within 5 AU Migration occurs by tidal interaction between planet and disk:  Type I: migration without gap opening – planet swallowed within 1 Myr. planet swallowed within 1 Myr.  Type II: migration after gap opening – planet locked to disk and migrates planet locked to disk and migrates at rate dictated by inner disk – again lost quickly 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.

Ionization: X-rays from star cosmic rays radioactive elements heating from central star 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)

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

Gap-opening masses of Planets Disk Radius [AU] Gap-opening mass [M J ] Jupiter Uranus or Neptune Earth

Dead Zones and Planet Migration (MPT 06) 1. eg. Type I migration (before gap- opening) → 10 M Earth (< M Uranus ) Dead Zone StarProtoplanet 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.

10 M E: Type I migration (No Gap-opening) Disk Radius [AU] 0 2×10 6 4×10 6 6×10 6 8× Time [years] (w/o Dead Zone)  = Disk Radius [AU] 0 2×10 6 4×10 6 6×10 6 8× Time [years] (w/ Dead Zone) Dead Zone  =10 -2

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:

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

 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

10 M E opens gap at 3.5 AU in dead zone Also: 1 M E opens gap near 0.1 AU

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 < Percent of planetary systems with planets migrating inside 5AU

Summary:  Earth mass planets, 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 < , can account for observed frequency of 5-20% of stellar systems with planets inside 5AU