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The Rocket Science of Launching Stellar Disks
Stan OwockiUD Bartol Research Institute
Disks in SpaceStan Owocki
Bartol Research InstituteUniversity 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
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Binary mass exchange
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Binary mass exchange
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Gravity
GMmF = _____ r2
Angular mometum
l = m v r~ constantQuickTime™ and a
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Centrifugal force
mv2f = ___ r
Orbital motion
centrifugal force f = mv2/r ~ 1 / r3
gravity F = GMm / r2
v2 = GM/rwhen F=f
Key: Infalling matter must shed its angular momentum
Summary: Disks from Infalling Matter
• Star formation– protostellar disk
– led to planets, Earth, us
• Binary stars– overflow onto companion
– spirals down through disk
The Rocket Science of Launching Stellar Disks
Stan OwockiUD Bartol Research Institute
Spectral lines & Doppler shift
• Motion of atoms shifts frequency by Doppler effect
• Atoms of a gas absorb & emit light at discrete frequencies
Be stars• Hot, bright, & rapidly rotating stars.• Discovered by Father Secchi in 1868• The “e” stands for emission lines in the star’s spectrum
• Detailed spectra show emission intensity is split into peaks to blue and red of line-center.
o
Inte
nsi
ty
Wavelength
• Indicates a disk of gas orbits the star.
• This is from Doppler shift of gas moving toward and away from the observer .
Hydrogenspectrum
HH
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.
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
• Here examine combination of these.
• Stellar Rotation– Be stars are generally rapid rotators
– Vrot ~ 200-400 km/s < Vorbit ~ 500 km/s
Rotational Broadening of Photospheric Absorption Lines
Wind Compressed Disk Model
Vrot (km/s) = 200 250 300 350 400 450
Hydrodynamical Simulations of Wind Compressed Disks
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
rN
Flux
Launching into Earth Orbit
• Requires speed of ~ 18,000 mi/h (5 mi/s).
• Earth’s rotation is ~ 1000 mi/h at equator.
Cannon atop high mountain
V ~ 18,000 mi/h
V ~ 17,000 mi/h
Cannon at equator• Launching eastward from
equator requires only ~ 17,000 km/h.
• 1-(1- 1/18)2 ~ 2/18 => ~10% less Energy
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 < 1/4 the energy!
• Localized surface ejection self selects orbiting material.
Vrot = 250 km/sec
V=250 km/sec
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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l=4, m=2
NRP Mode Beating
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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/sVorbit= 500 km/s
Spot Brightness= 10Spot Size = 10 o
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RDOMERadiatively 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.
• 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???
V=250 km/s