Planetesimal Formation in the Dead Zone of Protoplanetary Disks:

Hubble Fellow Symposium, STScI, 03/10/2014Xuening Bai

Institute for Theory and Computation, Harvard-Smithsonian Center for AstrophysicsGas Dynamics in Protoplanetary DisksCollaborator: Jim Stone (Princeton)1Pathway to (giant) planetsEssentially all processes depend on the gas dynamics of protoplanetary disks.

mcmkm103km105kmGrain growthPlanetesimal formationPlanetesimal growth to coresgrowth/accretion to gas giants

Planet migrationAerodynamic couplingGravitational couplingMost importantly, what are the structure and level of turbulence in PPDs?2Observational factsTypical mass: 10-3-10-1M.

Lifetime: 106-107 yr.

Typical accretion rate ~ 10-8 M yr -1.

Outflow is intimately connected to accretion:

Sicilia-Aguila et al. (2005)

3Goal:Understanding the gas dynamics in PPDs:

What is the radial and vertical structure of PPDs?Which regions of PPDs are turbulent / laminar?What drives accretion and outflow in PPDs?

The role of magnetic field:

Magneto-rotational instability (MRI)Magneto-centrifugal wind (MCW)(Balbus & Hawley 1991)(Blandford & Payne 1982, Pudritz & Norman 1983)4What drives accretion?Radial (viscous) transport by:Vertical transport by:(Balbus & Hawley, 1991)Magneto-rotational instabilityMagneto-centrifugal wind(Blandord & Payne, 1982)(turbulence generated by)(with large-scale external B-field)angular momentum5PPDs are extremely weakly ionizedcosmic raythermal ionizationUmibayashi & Nakano (1981)Igea & Glassgold (1999) Perez-Becker & Chiang (2011b)far UVstellar X-ray

(Bai, 2011a)Ionization fraction rapidly decreases from surface to midplane.Including small grains further reduce disk ionization.fiducial: CR: 10^(-17); X-ray: 10^(30), 5keV6Non-ideal MHD effects in weakly ionized gasDenseWeak BSparseStrong BOhmicHallAmbipolarinductive

Induction equation (no grain):

In the absence of magnetic field:

In the presence of magnetic field:

midplane region of the inner diskinner disk surfaceand outer disk7

Dead zone: resistive quenching of the MRIActive layer: resistivity negligibleConventional picture of layered accretionArmitage 2011, ARA&ASemi-analytical studies already indicated that MRI is insufficient to drive rapid accretion when including the effect of ambipolar diffusion (Bai & Stone, 2011, Bai, 2011a,b, Perez-Becker & Chiang, 2011a,b). Gammie, 19968Athena MHD code (fully conservative)Local shearing box simulations with orbital advection scheme (Gardiner & Stone, 2010)More realistic simulations

xyz(Stone et al., 2008)Magnetic diffusion coefficients obtained by interpolating a pre-computed lookup table based on equilibrium chemistry. (Bai & Goodman 2009, Bai 2011a,b) MMSN disk, CR, X-ray and FUV ionizations, 0.1m grain abundance 10-4.

9Vast majorityPoorly studied beforeZero net vertical magnetic fluxWith net vertical magnetic fluxThe importance of magnetic field geometryz0=Pgas,mid/Pmag,net10Inner disk: simulations with Ohmic+AD+Hall

(Bai & Stone, 2013b, Bai 2013,2014)By default, we consider z0=10511

Ohmic resistivity ONLYOhmic + ambipolar diffusionazimuthalradialcolor: field strength(Bai & Stone, 2013b)At 1 AU12

Ohmic + ambipolar diffusionazimuthalradialcolor: velocity magnitudeMagnetocentrifugal outflow!Wrong geometry?(Bai & Stone, 2013b)13Symmetry and strong current layerPhysical wind geometryUnphysical wind geometry

BrBBz

BrBBzstrong current layerflipped horizontal field14Radial dependence (Ohmic + ambipolar)

(Bai, 2013)Weak MRI turbulence sets in beyond ~5-10 AU.MRI sets in at midplane, where Ohmic-resistivity is no longer important at large radiiMRI sets in the (upper) far-UV ionization layer due to weak fieldWind is still the dominant mode to drive accretion.weaker field15Adding the Hall effect (1AU)BB(Bai, 2014, submitted)

B>0B0B0B