Outflows from YSOs and Angular Momentum Transfer

1
Outflows from YSOs and Angular Momentum Transfer

• National Astronomical Observatory (NAOJ)
• Kohji Tomisaka

2
Angular Momentum

• Fragmentation (Þ binary formation) is much
  affected by the amount of angular momentum in
  rotation supported disk bgtbcr.
• Angular Momentum Problem j ltlt j cl Specific
  angular momentum of a new-born star
• is much smaller than that of parent cloud

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Angular Momentum Transfer

• Magnetic Braking

• B-Fields do not play a role in angular momentum
  transfer in a contracting cloud?

4
Angular Momentum Redistribution in Dynamical
Collapse

• In outflows driven by magnetic fields
• The angular momentum is transferred effectively
  from the disk to the outflow.
• If 10 of inflowing mass is outflowed with
  having 99.9 of angular momentum, j would be
  reduced to 10-3 jcl.

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What we have done.

• Dynamical contraction of slowly rotating
  magnetized clouds is studied by ideal MHD
  numerical simulations with cylindrical symmetry.
• Evolution is as follows Run-away Collapse
  Increase in Central Density Formation of
  Adiabatic Core Accretion Phase

Shus Inside-out Solution Larson-Penston Solution
Outflow
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Numerical Method

• Ideal MHD Self-Gravity Cylindrical Symmetry
• Collapse nonhomologous
• Large Dynamic Range is attained by Nested Grid
  Method.
• Coarse Grids Global Structure
• Fine Grids Small-Scale Structure Near the Core

L0 L23
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Initial Condition

• Cylindrical Isothermal Clouds
• Magnetohydrostatic balance in r-direction
• uniform in z-direction
• B-Fields

• Slowly rotating ( rigid-body
  rotation)
• Added perturbation with l of the gravitationally
  most unstable mode lMGR.

lMGR
parameters
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Run-away Collapse Phase

• t0 0.6Myr
  1Myr

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Accretion Phase

• High-density gas becomes adiabatic.
• The central core becomes optically thick for
  thermal radiation from dusts.
• Critical density
• An adiabatic core is formed.
• To simulate, a double polytrope is applied
• isothermal
• adaiabatic

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Accretion Phase (II)

• Collapse time-scale in the adiabatic core becomes
  much longer than the infall time.
• Inflowing gas accretes on to the nearly static
  core, which grows to a star.
• Outflow emerges in this phase.

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Core Contracting Disk
B¹0, W0
Accretion Phase
Pseudo- Disk
Adiabatic (the first) Core
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A Ring Supported by Centrifugal Force
W¹0 , B0
Accretion Phase
r
r
W
W
Accretion Stage
Run-away Collapse Stage
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Accretion Phase
B¹0, W¹0
a1, W5
L10
300AU
Run-away Collapse Stage
t1000yr
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Why Does the Outflow Begin in the Accretion Stage?
B¹0, W¹0
Accretion Phase
Blandford Peyne 82
Mass Accretion Rate
Magneto-Centrifugal Wind
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Angular Momentum Distribution
Angular Momentum Problem
(1) Mass measured from the center
(2) Angular momentum in
(3) Specific Angular momentum distribution
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Specific Angular Momentum
Angular Momentum Problem
Initial
Core Formation
7000 yr after Core Formation
Mass
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Magnetic Torque, Angular Momentum Inflow/Outflow
Rate
Mass
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Ambipolar Diffusion?

• In weakly ionized plasma, neutral molecules have
  only indirect coupling with the B-fields through
  ionized ions.
• Neutral-ion collision time
• When , ambipolar diffusion is
  important.
• Assuming (on core
  formation), rotation period of centrifugal
  radius

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Molecular Outflow
Optical Jets
L1551 IRS5
Optical Jets
Edge of Hole made by Molecular Outflow
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Optical Jets
Jets and Outflows

• Flow velocity faster than molecular outflow.
• The width is much smaller.
• These indicate Optical jets are made and ejected
  from compact objects.
• The first outflow is ejected just outside the
  adiabatic (first) core.

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Temperature-Density Relation
Jets and Outflows

• Optical jets are formed just outside the second
  core?

Tohline 1982
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Jets and Outflows
L16
rc1019H2cm-3
2nd Runaway Collapse
Outflow
H2 Dissoc.
L8
rc1014.6H2cm-3
10R
X256
Jets
rc1021.3.H2cm-3
10AU
rs104H2cm-3
a1, w1/2
10R
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Summary

• In dynamically collapsing clouds, the outflow
  emerges just after the core formation (t1000yr).
• In the accretion phase, the centrifugal wind
  mechanism magnetic pressure force work
  efficiently.
• In t7000 yr ( ), the outflow
  reaches 2000 AU. Maximum speed reaches
  

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Summary(2)

• In the process, the angular momentum is
  transferred from the disk to the outflow and the
  outflow brings the excess j.
• This solves the angular momentum problem of
  new-born stars.
• The 2nd outflow outside the 2nd (atomic) core
  explains optical jets.

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Larson 1969, Penston 1969, Hunter
1977, Whitworth Summers 1985
Dynamical Collapse
Runaway Collapse
Accretion-associated Collapse
Shu 1977
Density increases infinitely
Inside-out Collapse
Hydrostatic Core
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Parameters

• Angular Rotation Speed
• Magnetic to thermal pressure ratio

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Nest (Self-Similar) Structure
Run-away Collapse Phase
Along z-axis
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Run-away Collapse

• Evolution characterized as self-similar

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Magnetocentrifugal Wind ModelBlandford Peyne
1982

• Consider a particle rotating with rotation speed
  w Kepler velocity and assume w is conserved
  moving along the B-fields.
• Along field lines with qlt60deg the particle is
  accelerated. For qgt60deg decelerated.

Effective potential for a particle rotating with
w.
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Momentum Flux (Observation)

• Low-Mass YSOs (Bontemps et al.1996)

Momentum

l
Luminosity
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Angular Momentum
Angular Momentum Problem
(1) Mass measured from the center
(2) Angular momentum in
(3) Specific Angular momentum distribution
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Effective Outflow Speed
a1
a0.1
W1
W5
W1
W5
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Outflow Driving Mechanism

• Rotating Disk Twisted Magnetic Fields
• Centrifugal Wind
• Pudritz Norman 1983
• Uchida Shibata 1985
• Shu et al.1994
• Ouyed Pudritz 1997
• Kudoh Shibata 1997
• Contraction vs Outflow?
• When outflow begins?
• Condition?

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Accretion/Outflow Rate

• Inflow Rate is Much Larger than Shus
  Rate (1977).
• LP Solution
• Outflow/Inflow Mass Ratio is Large 50 .
• Source Point of Outflow Moves Outward.

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Momentum Driving Rate

• Molecular Outflows (Class 01 Objects) show
  Momentum Outflow Rate (Bontemps et
  al.1996)

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Weak Magnetic Fields (a0.1,W5)
B¹0, W¹0
Accretion Phase
0 yr
2000 yr
4000 yr
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Effect of B-Field Strength
B¹0, W¹0
Accretion Phase

• In small a model, toroidal B-fields become
  dominant against the poloidal ones.
• Poloidal B-fields are winding.
• Small a and slow rotation lead less effective
  acceleration.

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Angular Momentum Problem
Angular Momentum Problem

• Typical specific angular momentum of T Tauri
  stars
• Angular momentum of typical molecular cores
• Centrifugal Radius

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Molecular Outflow
Saito, Kawabe, KitamuraSunada 1996
L1551 IRS5
Optical Jets