Transcript The Rocket Science of Launching Stellar Disks

The Rocket Science of
Launching Stellar Disks
Stan Owocki
UD Bartol Research Institute
Disks in Space
Stan Owocki
Bartol Research Institute
University of Delaware
Where do stars, planets, we,
come from??
• From collapse of interstellar gas clouds
• Gravity pulls together
• But clouds usually have small spin
• Amplified on collapse
• Leaves behind disk
• For proto-sun, this collapsed into planets, earth, us
Saturn’s rings
Spiral Galaxies
Disk in Center of Galaxy
Beta Pictoris
Gaseous Pillars in M16
Proto-stellar nebuale
Protostellar Collapse
Binary mass exchange
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Binary mass exchange
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Gravity
GMm
F = _____
r2
Angular mometum
l=mvr
~ constant
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Centrifugal force
2
mv
f = ___
r
Orbital motion
centrifugal force
f = mv2/r ~ 1 / r3
gravity
F = GMm / r2
when F=f
v2 = GM/r
Summary: Disks from Infalling Matter
• Star formation
– protostellar disk
– led to planets, Earth, us
• Binary stars
– overflow onto companion
– spirals down through disk
Key: Infalling matter must shed its angular momentum
The Rocket Science of
Launching Stellar Disks
Stan Owocki
UD Bartol Research Institute
Spectral lines & Doppler shift
• Atoms of a gas absorb
& emit light at discrete
frequencies
• Motion of atoms
shifts frequency by
Doppler effect
Be stars
• Hot, bright, & rapidly rotating stars.
• Discovered by Father Secchi in 1868
• The “e” stands for emission lines in the star’s spectrum
Hb
Ha
• Detailed spectra show
emission intensity is split
into peaks to blue and red
of line-center.
• This is from Doppler shift
of gas moving toward and
away from the observer .
• Indicates a disk of gas orbits the star.
Intensity
Hydrogen
spectrum
lo
Wavelength
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.
• So disk matter must be
launched from star.
How do Be stars
do this??
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
• Here examine combination of these.
Rotational Broadening of
Photospheric Absorption Lines
Wind Compressed Disk Model
Hydrodynamical Simulations of
Wind Compressed Disks
Vrot (km/s) =
200
250
300
350
400
450
Note: Assumes purely radial driving of wind
Inner Disk Infall
• WCD material lacks angular momentum for orbit
• Either Escapes in Wind or Falls Back onto star
• Limits disk density
Problems with WCD Model
• Inhibited by non-radial forces
• Lacks angular momentum for orbit
– inner disk infall
– outer disk outflow
• Thus, compared to observations:
– density too low
– azimuthal speed too low
– radial speed too high
• Need way to spin-up material into Orbit
N
r
Launching into Earth Orbit
• Requires speed of
~ 18,000 mi/h (5 mi/s).
Cannon atop
high mountain
DV ~ 18,000 mi/h
• Earth’s rotation is
~ 1000 mi/h at equator.
• Launching eastward from
equator requires only
~ 17,000 km/h.
• 1-(1- 1/18)2 ~ 2/18 =>
~10% less Energy
Cannon at
equator
DV ~ 17,000 mi/h
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 < 1/4 the energy!
Vrot = 250 km/sec
• Localized surface ejection
self selects orbiting material.
Line-Profile Variations from
Non-Radial Pulsation
Line-Profile with:
Rotation
Rotation
+
NRP
Flux
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Wavelength
(Vrot=1)
NRP-distorted star (exaggerated)
NRP Mode Beating
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l=4, m=2
Pulsation & Mass Ejection
• See occasional “outbursts” in circumstellar lines
• Tend to occur most when NRP modes overlap
• Implies NRPs trigger/induce mass ejections
• But pulsation speeds are only ~ 10 km/s.
• What drives material to ~ 250 km/s??
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??
First try: Localized Equatorial
Bright Spots
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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RDOME
Radiatively Driven Orbital Mass Ejection
• Assume localized distortion in
surface height & brightness.
• If phase of brightness leads height,
then can get “prograde flux”.
• Can this drive mass into orbit?
Time Evolution of Single Prograde Spot
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Prominence/Filament
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Force Cutoff
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Outward Viscous Diffusion of Ejected Gas
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Time Evolution of m=4 Prograde Spot Model
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Summary
• Disks often form from infall.
DV=250 km/s
• Be disks require high-speed surface launch.
• Like Earth satellites, get boost from rotation.
• Pulsation may trigger gas ejection.
• Driving to orbital speed by light,
perhaps from tilted bright spots???