The Rocket Science of Launching Stellar Disks

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The Rocket Science of Launching Stellar Disks
Stan Owocki UD Bartol Research Institute
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Disks in Space
Stan Owocki Bartol Research Institute University
of Delaware
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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

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Saturns rings
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Spiral Galaxies
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Disk in Center of Galaxy
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Beta Pictoris
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Lagoon Nebula
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Gaseous Pillars in M16
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Proto-stellar nebuale
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Protostellar Collapse
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Binary mass exchange
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Binary mass exchange
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Gravity
GMm F _____ r2
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Angular mometum
l m v r constant
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Centrifugal force
mv2 f ___ r
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Orbital motion
centrifugal force
f mv2/r 1 / r3
gravity
F GMm / r2
v2 GM/r
when Ff
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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
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The Rocket Science of Launching Stellar Disks
Stan Owocki UD Bartol Research Institute
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Spectral lines Doppler shift

• Atoms of a gas absorb emit light at discrete
  frequencies

• Motion of atoms shifts frequency by Doppler effect

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Be stars

• Hot, bright, rapidly rotating stars.
• Discovered by Father Secchi in 1868
• The e stands for emission lines in the stars
  spectrum

• 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.

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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??
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Key Puzzle Pieces

• Stellar Rotation
• Be stars are generally rapid rotators
• Vrot 200-400 km/s lt Vorbit 500 km/s

• Stellar Wind
• Driven by line-scattering of stars 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 lt l 1 - 4

• Here examine combination of these.

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Rotational Broadening of Photospheric Absorption
Lines
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Formation of a P-Cygni Line- Profile
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Wind Compressed Disk Model
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Hydrodynamical Simulations of Wind Compressed
Disks
Note Assumes purely radial driving of wind
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Inner Disk Infall

• WCD material lacks angular momentum for orbit
• Either Escapes in Wind or Falls Back onto star
• Limits disk density

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WCD Inhibition by Non-Radial Forces

• Oblateness implies polar tilt to radiative flux
• Poleward force reverses equatorward flow
• Inhibits WCD formation

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WCD Inhibition by Poleward Line-Force

• Stellar oblateness gt poleward tilt in radiative
  flux

• Net poleward line-force inhibits WCD

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Problems with WCD Model
N
r

• 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

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Launching into Earth Orbit

• Requires speed of 18,000 mi/h (5
  mi/s).
• Earths rotation is 1000 mi/h at
  equator.

• Launching eastward from equator requires only
  17,000 km/h.
• 1-(1- 1/18)2 2/18 gt 10 less Energy

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Launching into Be star orbit

• Requires speed of 500 km/sec.
• Be star rotation is often gt 250 km/sec
  at equator.
• Launching with rotation needs lt 250 km/sec
• Requires lt 1/4 the energy!
• Localized surface ejection self selects orbiting
  material.

DV250 km/sec
Vrot 250 km/sec
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SPH simulations - P. Kroll
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Line-Profile Variations from Non-Radial Pulsation
Line-Profile with
Wavelength (Vrot1)
NRP-distorted star (exaggerated)
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NRP Mode Beating
l4, m2
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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??

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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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First try Localized Equatorial Bright Spots
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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.
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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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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?

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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 m4 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???

DV250 km/s