Saving Planetary Systems: the Role of Dead Zones

1
Saving Planetary Systemsthe 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 (Gammie, 1998) - Magnetic turbulence
is inactive in poorly ionized regions of the
disk so the disks 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
Jupiter
Uranus or Neptune
Earth
6
1. eg. Type I migration (before gap-opening)
? 10 MEarth (lt 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
?10-2
?10-2
?10-5
8
If planet forms within the DZhalt 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
?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 lt 0.0001

Percent of planetary systems with planets
migrating inside 5AU
13
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 lt 0.0001, can account
  for observed frequency of 5-20 of stellar
  systems with planets inside 5AU