Interacting Winds: Theory Overview

1. Interacting Winds: Theory Overview with thanks for web slides from: D. Folini, K. Gayley, S. Lepine, M. MacLow, J. Pittard, I. Stevens, P. Tuthill, R. Walder Stan Owocki Bartol Research Institute University of Delaware

2. Overview Hot-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.

3. Rotational Modulation of Hot-Star Winds Radiation hydrodynamics simulation of CIRs in a hot-star wind • 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. HD64760 Monitored during IUE “Mega” Campaign

4. Line-Driven Instability in Wind Acceleration Region Velocity Density • Hot-star winds intrinsically unstable at small-scales l < Lsobºvth/(dv/dr) • Growth rate » g /vth »v/L # e-folds »v/vth »100 • In 1D simulations, leads to formation of multiple shocks • In multi-D, expect supersonic “compressive turbulence” t=430 ksec

5. WR Wind Blobs Lepine & Moffat 1999 • Infer acceleration over extended scale: bR* ~ 20-50 RO • grad ~ k L*/4p r2c • Requires radially increasing effective opacity k ~ s/m • Possible from desaturation of optically thick blobs • Yieldsk ~ s ~ r2 grad ~ constant!

6. Colliding Wind Binaries • Close binaries: • X-ray attenuation • Radiative forces • Inhibition • Braking • Interface instabilities • Wide binaries: • Cometary or Spiral structure • Radio Emission • Dust formation

7. Colliding Wind Momentum Balance Wind-radiation balance Wind-wind balance O-star radiation WR wind Symmetric or widely separated binaries Asymmetric (e.g.WR+O) close binaries

8. Sudden Radiative Braking • Scaling analyses suggests broad importance in close to moderately separated WR+O systems • Diagnostic potential for line-driving opacity, e.g. in V444 Cyg Scaled Momentum Ratio Scaled Separation

9. Dust Spiral in WR 104 Tuthill et al. 1999 IR image from Keck How does dust form?

10. Wind-Blown Bubbles in ISM d = V ø pc 1000 1000 4 º 4 º _ 3 3 3 M = Ωr º 0 : 1 M n r M ø ¥ Ω ( V ø ) Ø 1 m m pc 3 3 WR wind bubble NGC 2359 √ ! 1 = 3 _ M ø ° 6 5 r = pc n 1 Some key scalings: s _ M ° 6 ø = 100 yrs m 3 V n 1 1000

11. Formation of Prolate Nebulae W-limit Gravity darkening Frank et al. 1998: Prolate fast wind into spherical medium Langer et al. 1999: Fast spherical wind into slow, dense equatorial flow

12. Shock Interface Instabilities g a • Kelvin-Helmholtz (shear) • Cooling Overstabilty • Rayleigh-Taylor (heavy over light ) • Vishniac & Thin-Shell (gas-ram) (ram-ram) For summary, see J. Pittard Ph.D. thesis

13. 2D Planar Simulation of Interaction Layer Walder & Folini 1998,1999 Isothermal case: Thin-shell instability Radiative cooling case: Cooling overstability Density

14. Questions Internal 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, b>>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 h Car: • What causes the axisymmetry? Magnetic fields? Rotation? Radiation? • W-limit vs. gravity darkening

15. Radiative Shocks µ ∂ 4 V 17 2 N = 7 £ 10 cm cool 100 km = s • Hot Gas Cools by Line-Emission • In 1D ideally develops characteristic layers

16. Reduction of X-ray emission 3D simulations of V444 Cygni (J. Pittard, Ph. Thesis, 1999): Instantaneous wind acceleration Radiative wind acceleration 3x1034 erg/s 8x1032 erg/s