Interacting Winds: Theory OverviewStan Owocki

Bartol Research InstituteUniversity of Delaware

with thanks for web slides from:D. Folini, K. Gayley, S. Lepine, M. MacLow, J. Pittard, I. Stevens, P. Tuthill, R. Walder

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OverviewHot-stars have massive, high-speed winds.

These interact: Internally

Large-scale, e.g. CIRs Small-scale, e.g., instability-generated turbulence

In high-mass binaries, e.g. WR-O With environs:

Previous epoch outflow, e.g. slow RSG wind ISM SNe

High-speed shocks, often unstable.

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HD64760 Monitored duringIUE “Mega” Campaign

Monitoring campaigns of P-Cygni lines formed in hot-star winds also often show modulation at periods comparable to the stellar rotation period.

These may stem from large-scale surface structure that induces spiral wind variation analogous to solar Corotating Interaction Regions.

Radiation hydrodynamicssimulation of CIRs in a hot-star wind

Rotational Modulation of Hot-Star Winds

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Hot-star winds intrinsically unstable at small-scales < Lsob vth/(dv/dr)

Growth rate g /vth v/L # e-folds v/vth

In 1D simulations, leads to formation of multiple shocks

In multi-D, expect supersonic “compressive turbulence”

Line-Driven Instability in Wind Acceleration Region

0.0 0.5 1.0

0

500

1000

1500

-15

-14

-13

-12

-11

-10

Height (R*

)

Velocity

Density

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WR Wind Blobs

Infer acceleration over extended scale: R* ~ 20-50 RO

grad ~ L*/4r2c

Requires radially increasing effective opacity ~ /m

Possible from desaturation of optically thick blobs

Yields ~ ~ r2 grad ~ constant!

Lepine & Moffat 1999

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Colliding Wind Binaries

Close binaries: X-ray attenuation Radiative forces

Inhibition Braking

Interface instabilities

Wide binaries: Cometary or Spiral

structure Radio Emission Dust formation

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Colliding Wind Momentum Balance

Wind-wind balance Wind-radiation balance

WR wind

O-star radiation

Symmetric or widely separated binaries

Asymmetric (e.g.WR+O) close binaries

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Sudden Radiative Braking Diagnostic potential for

line-driving opacity, e.g. in V444 Cyg

Scaling analyses suggests broad importance in close to moderately separated WR+O systems

Scaled Separation

Sca

led

Mom

entu

m R

atio

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Dust Spiral in WR 104

Tuthill et al. 1999

IR image from Keck

How does dust form?

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Wind-Blown Bubbles in ISM

Some key scalings:

WR wind bubble NGC 2359

dpc =V1000ø1000

_Møm ¥4º3Ω(Vøm)3

øm =100

s_M °6

V 31000n1

yrs

M =4º3Ωr3 º 0:1 MØ n1 r3pc

rpc =

√_M ° 6 ø5n1

! 1=3

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Formation of Prolate Nebulae

Frank et al. 1998:Prolate fast wind into spherical medium

-limit

Langer et al. 1999:Fast spherical wind into slow, dense equatorial flow

Gravity darkening

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Rayleigh-Taylor (heavy over light )

Vishniac & Thin-Shell (gas-ram) (ram-ram)

Shock Interface Instabilities Kelvin-Helmholtz (shear)

Cooling Overstabilty

g a

For summary, see J. Pittard Ph.D. thesis

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2D Planar Simulation of Interaction Layer

Walder & Folini

1998,1999

Density

Isothermal case:

Thin-shell instabilityRadiative cooling case:

Cooling overstability

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QuestionsInternal interactions

What induces large-scale DAC structure? NRP? B-fields?

What is lateral scale of instability structure?

What is origin of WR blobs? Instability? Pulsation?

What causes extended blob acceleration, >>1

Wind-wind collisions What reduces and softens X-ray emission?

Absorption? Conduction? Instability mixing? Braking?

Does Radiative Braking Occur? Even in clumped flows?

How does dust spiral form?

Wind-environs What determines nebula shape? e.g., in Car:

What causes the axisymmetry? Magnetic fields? Rotation? Radiation? -limit vs. gravity darkening

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Radiative Shocks Hot Gas Cools by

Line-Emission

In 1D ideally develops characteristic layers

µ ∂4

Ncool=7 £1017V

100km=scm2

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Reduction of X-ray emission

Instantaneous wind acceleration

3x1034 erg/s 8x1032 erg/s

Radiative wind acceleration

3D simulations of V444 Cygni (J. Pittard, Ph. Thesis, 1999):