Links Between Pulsations & Line-Driven Mass Loss in Massive Stars

Stan Owocki

Bartol Research Institute

University of Delaware

IAU Colloquium #185

Leuven, Belgium

July 26-31, 2001

Possible

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Outline• Key properties of line-driven mass loss

• Wind variability– Discrete Absorption Components– Periodic Absorption Modulations

• Candidates for Pulsation Wind Connection– BW Vul– EZ CMa = WR 6

• Possible role of pulsation in initiating mass loss– WR winds

– Be disks

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Optically Thick Line-Absorption in an Accelerating Stellar Wind

gthick~gthin

τ~

1ρ

dvdr τ≡κρ

vth

dv/dr

Lsob

For strong,

optically thick lines:

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< 1CAK ensemble ofthick & thin lines

CAK model of steady-state wind

inertia gravity CAK line-force

Solve for:

˙ M ≈Lc2

QΓ1−Γ⎛

⎝⎜

⎞

⎠⎟

1

−1Mass loss rate

˙ M v∞ ∝ L1

αWind-Momentum

Luminosity Law

≈0.6

v(r) ≈v∞(1−R∗/r)Velocity law

~vesc

Equation of motion: v ′ v ≈−GM(1−Γ)

r2 +Q Lr2

r2v ′ v ˙ M Q

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟ α

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Inward-propagating Abbott waves

±v ª ei(k r ° ! t)

°@v0i!±v =

@grad±v0

¥ U ik±v

w=k= ° U

U =@grad

@v0

ªgrad

v0 ªvv0

v0 ª v

ad@v@t

= gr

v

r

g~ v’

Abbott speed

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Line-Driven Instability

u=v/vth

for < Lsob:

g/g ~ u

Instability with growth rate

~ g/vth ~ v/Lsob ~100 v/R

=> e100 growth!

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Growth and phase reversal of periodic base perturbation

Radius (R*) Radius (R*)

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Time snapshot of wind instability simulation

0.0 0.5 1.0

0

500

1000

1500

-15

-14

-13

-12

-11

-10

Height (R*

)

Velocity

Density

CAK

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Wave-leakage into outflowing windCranmer 1996, PhD.

Pulsation-Induced Wind Variations

in BW Vul

Steve Cranmer, Harvard-Smithsonian

Stan Owocki, Bartol/U. of Del.

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BW Vul light curve

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Wind variations from base perturbations in density and brightness

log(Density)

Velocity

Radius

wind baseperturbed by

~ 50

radiative drivingmodulated by

brightness variations

Abbott-mode“kinks”

velocity “plateaus”

shockcompression

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Radial velocity

ModelObservations

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Observations vs. Model

C IV Model line

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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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Phase Bowing

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HD 64760 (B0Ia)

Si IV observationkinematic model with

m=l=4 NRP

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WR6 - Pulsational modelm=4 dynamical model NIV in WR 6

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The Puzzle of Be Disks

• And most Be stars are not in close binary systems.

• Be stars are too old to still have protostellar disk.

How do Be starsdo this??

• They thus lack outside mass source to fall into disk.

• So disk matter must be launched from star.

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Key Puzzle Pieces

• Stellar Wind– Driven by line-scattering of star’s radiation– Rotation can lead to Wind Compressed Disk (WCD) – But still lacks angular momentum for orbit

• Stellar Pulsation– Many Be stars show Non-Radial Pulsation (NRP) with m < l = 1 - 4

• Magnetic Spin-Up– Bdipole < 100 G => moment arm Ra < few R* ~ Rcorotation

• Here examine combination of #’s 1, 2, & 3

• Stellar Rotation– Be stars are generally rapid rotators

– Vrot ~ 200-400 km/s < Vorbit ~ 500 km/s

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Launching into Be star orbit• Requires speed of ~ 500 km/sec.

• Be star rotation is often > 250 km/sec at equator.

• Launching with rotation needs < 250 km/sec

• Requires < a quarter of the energy!

• Localized surface ejection self selects orbiting material.

Vrot = 250 km/sec

V=250 km/sec

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Line-Profile Variations from

Non-Radial Pulsation

Wavelength (Vrot=1)

Flux

Rotation

Rotation+

NRP

NRP-distorted star(exaggerated)

Line-Profile with:

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NRP Mode Beating

l=3, m=2

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l=2, m=1

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l=4, m=2

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NonRadial Radiative Driving

• Light has momentum.

• Pushes on gas that scatters it.

• Drives outflowing “stellar wind”.

• Pulsations distort surface and brightness.

• Could this drive local gas ejections into orbit??

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CIR from Symmetric Bright Spot on Rapidly Rotating Be Star

Vrot = 350 km/sVorbit= 500 km/s

Spot Brightness= 10Spot Size = 10 o

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Symmetric Spot with Stagnated Driving

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Symmetric Prominence/Filament

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Time Evolution of m=4 Prograde Spot Model

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Summary

• High-freq. p-modes feed line-driven instability

• g-modes can “leak” into wind

• Radial pulsational light curve modulates mass loss– distinctive Abbott kinks with velocity plateaus

• NRP light variations can lead to CIRs

• in Be stars NRP resonances might induce “orbital mass ejection”