Transcript Links Between Pulsation and Line-Driven Mass Loss in Massive Stars

Possible 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
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
For strong,
optically thick lines:
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gthin 1 dv
gthick ~
~
  dr
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Lsob
v th
  
dv / dr
3
CAK model of steady-state wind
Equation of motion:
a
2


GM(1  ) Q L r vv
vv   
 2  Ý 
2
r
r  MQ 
inertia
gravity
0<a<1
CAK ensemble of
thick & thin lines
CAK line-force
Solve for:
Mass loss rate
L  Q 
Ý
M 2

c  1  
1
a
1
Wind-Momentum
Luminosity Law
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Velocity law
v(r)  v  (1  R / r)
~ vesc
1
Ýv  La
M

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a  0.6
4
Inward-propagating Abbott waves
@v
= gr ad
@t
° i! ±v =
±v ª ei (k r °
! t)
w=k = ° U
@gr ad 0
±v
0
@v
¥ U ik ±v
dg~ dv’
v
Abbott speed
@gr ad
U=
@v0
gr ad
vv 0
ª
ª
ª v
0
0
v
v
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r
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Line-Driven Instability
u=v/vth
for l < Lsob:
dg/g ~ du
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Instability with growth rate
W ~ g/vth ~ v/Lsob ~100 v/R
=> e100 growth!
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Growth and phase reversal of periodic base perturbation
Radius (R*)
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Radius (R*)
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Time snapshot of wind instability simulation
CAK
1500
-10
-11
1000
Velocity
-12
Density
-13
500
-14
0
-15
0.0
0.5
Height (R
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1.0
*
)
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Wave-leakage into outflowing wind
Cranmer 1996, PhD.
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Pulsation-Induced
Wind Variations
in BW Vul
Steve Cranmer, Harvard-Smithsonian
Stan Owocki, Bartol/U. of Del.
BW Vul light curve
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Wind variations from base perturbations in density and brightness
Abbott-mode“kinks”
Velocity
velocity “plateaus”
radiative driving
modulated by
brightness variations
log(Density)
shock
compression
Radius
wind base
perturbed by
d/ ~ 50
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Radial velocity
Observations
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Model
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Observations vs. Model
C IV
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Model line
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Rotational Modulation of Hot-Star Winds
HD64760 Monitored during
IUE “Mega” Campaign
Radiation hydrodynamics
simulation of CIRs in a hot-star wind
 These may stem from large-scale
surface structure that induces spiral
wind variation analogous to solar
Corotating Interaction Regions.
Monitoring campaigns of PCygni lines formed in hot-star
winds also often show modulation
at periods comparable to the stellar
rotation period.
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Phase Bowing
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HD 64760 (B0Ia)
kinematic model with
Si IV observation
m=l=4 NRP
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WR6 - Pulsational model
NIV in WR 6
m=4 dynamical model
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The Puzzle of Be Disks
• Be stars are too old to still
have protostellar disk.
• And most Be stars are not
in close binary systems.
• They thus lack outside
mass source to fall into
disk.
How do Be stars
do this??
• So disk matter must be
launched from star.
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Key Puzzle Pieces
• Stellar Rotation
– Be stars are generally rapid rotators
– Vrot ~ 200-400 km/s < Vorbit ~ 500 km/s
• 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
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Launching into Be star orbit
• Requires speed of
~ 500 km/sec.
DV=250 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!
Vrot = 250 km/sec
• Localized surface ejection
self selects orbiting material.
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Line-Profile Variations from
Non-Radial Pulsation
Line-Profile with:
Rotation
Flux
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Wavelength
(Vrot=1)
NRP-distorted star
(exaggerated)
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Rotation
+
NRP
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NRP Mode
Beating
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l=3, m=2
l=2, m=1
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/s
Vorbit= 500 km/s
Spot Brightness= 10
Spot 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”
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