Gas Dynamics in Protoplanetary Disks

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Hubble Fellow Symposium, STScI, 03/10/2014 Xuening Bai Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics Gas Dynamics in Protoplanetary Disks Collaborator: Jim Stone (Princeton)

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Hubble Fellow Symposium, STScI , 03/10/2014. Gas Dynamics in Protoplanetary Disks. Xuening Bai. Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics. Collaborator: Jim Stone (Princeton). Pathway to (giant) planets. Aerodynamic coupling. Gravitational coupling. - PowerPoint PPT Presentation

Transcript of Gas Dynamics in Protoplanetary Disks

Page 1: Gas Dynamics in Protoplanetary Disks

Hubble Fellow Symposium, STScI, 03/10/2014

Xuening Bai

Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics

Gas Dynamics in Protoplanetary Disks

Collaborator: Jim Stone (Princeton)

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Pathway to (giant) planets

Essentially all processes depend on the gas dynamics of protoplanetary disks.

μm cm km 103km 105km

Grain growth Planetesimal formation

Planetesimal growth to cores

growth/accretion to gas giants

Planet migration

Aerodynamic coupling Gravitational coupling

Most importantly, what are the structure and level of turbulence in PPDs?

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Observational facts

Typical 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)

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Goal: 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)

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What drives accretion?

Radial (viscous) transport by: Vertical transport by:

(Balbus & Hawley, 1991)

Magneto-rotational instability Magneto-centrifugal wind

(Blandord & Payne, 1982)

(turbulence generated by) (with large-scale external B-field)

angular momentum

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PPDs are extremely weakly ionized

cosmic raythermal ionization

Umibayashi & Nakano (1981)Igea & Glassgold (1999) Perez-Becker & Chiang (2011b)

far UV

stellar X-ray

(Bai, 2011a)

Ionization fraction rapidly decreases from surface to midplane.

Including small grains further reduce disk ionization.

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Non-ideal MHD effects in weakly ionized gas

DenseWeak B

SparseStrong B

Ohmic Hall Ambipolarinductive

Induction equation (no grain):

In the absence of magnetic field:

In the presence of magnetic field:

midplane region of the inner disk

inner disk surfaceand outer disk

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Dead zone: resistive quenching of the MRI

Active layer: resistivity negligible

Conventional picture of layered accretion

Armitage 2011, ARA&A

• Semi-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, 1996

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Athena MHD code (fully conservative)

Local shearing box simulations with orbital advection scheme (Gardiner & Stone, 2010)

More realistic simulations

x

y z

(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, 0.1μm grain abundance 10-4.

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Vast majority Poorly studied before

Zero net vertical magnetic flux

With net vertical magnetic flux

The importance of magnetic field geometry

βz0=Pgas,mid/Pmag,net

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Inner disk: simulations with Ohmic+AD+Hall

(Bai & Stone, 2013b, Bai 2013,2014)

By default, we consider βz0=105

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Ohmic resistivity ONLY Ohmic + ambipolar diffusion

azimuthal

radial

color: field strength

(Bai & Stone, 2013b)At 1 AU

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Ohmic + ambipolar diffusion

azimuthal

radial

color: velocity magnitude

Magnetocentrifugal outflow!

Wrong geometry?

(Bai & Stone, 2013b)

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Symmetry and strong current layer

Physical wind geometryUnphysical wind geometry

Br

Bz

Br

Bz

strong current layer

flipped horizontal

field

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Radial 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 radii

MRI sets in the (upper) far-UV ionization layer due to weak field

Wind is still the dominant mode to drive accretion.

wea

ker f

ield

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Adding the Hall effect (1AU)

BΩBΩ

(Bai, 2014, submitted)

BΩ>0BΩ<0

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Adding the Hall effect: range of stability

BΩBΩ

(Bai, 2014, submitted)

BΩ<0 BΩ>0

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Outer disk: simulations with Hall + AD

(Bai & Stone, 2014, in prep)

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Gas dynamics in the outer disk (15-60 AU)30 AU, weak vertical field β0=105

FUV layer (ideal MHD)

ambipolar diffusion

Hall

FUV layer (ideal MHD)

MRI turbulent,

disk outflow

MRI turbulent,

disk outflow

Aligned/anti-aligned field has stronger/weaker midplane magnetic activities compared with the Hall-free case.

BΩ>0

BΩ<0

No Hall

Disk outflow can also play a role, but its contribution is uncertain based on local simulations.

MRI in the FUV layer sufficient to drive rapid accretion.BrBϕ

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Gas dynamics in the outer disk (15-60 AU)

“dead zone”?

30 AU, weak vertical field β0=105

MRI turbulent,

disk outflow

MRI turbulent,

disk outflow

BΩ>0

BΩ<0

No Hall

Aligned field geometry has weakest midplane turbulence: suppressed by stronger magnetic field.

Anti-aligned field geometry has reduced midplane turbulence: MRI is suppressed in the midplane.

FUV layer (ideal MHD)

ambipolar diffusion

Hall

FUV layer (ideal MHD)

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Summary: a new paradigm

(Bai, 2013)

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Implications: planet formation & disk evolution

Grain growth and planetesimal formation

Planetesimal growth

Planet migration

Global disk evolution

Polarity dependent planet formation?

Inner disk is the favorable site for planetesimal formation.

Planetesimal growth does not suffer from turbulent excitation.

Gap opening is much easier, may slow down type-I migration.

Largely dictated by global magnetic flux distribution, heritage from star formation plus intrinsic magnetic flux transport within the disk.

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Conclusions and future work Non-ideal MHD effects play a crucial role in PPDs

MHD from midplane to disk surface dominated by Ohmic, Hall and AD

The inner PPD is purely laminar, launching an MCW. MRI suppressed by Ohmic and AD, external vertical field is essential. Hall effect modestly modifies the wind solution, depending on field polarity. Accretion proceeds through thin strong current layer.

The outer PPDs is likely to be turbulent with layered accretion. MRI is most active in the surface FUV layer, midplane is weakly turbulent.

Global simulations with resolved microphysics is essential: Issues with symmetry and strong current layer, kinematics of the wind Interplay between disk evolution and magnetic flux transport.