The Rocket Science of Launching Stellar Disks Stan Owocki UD Bartol Research Institute.
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Transcript of The Rocket Science of Launching Stellar Disks Stan Owocki UD Bartol Research Institute.
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