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

Saturns rings

Spiral Galaxies

Disk in Center of Galaxy

Beta Pictoris

Lagoon Nebula

Gaseous Pillars in M16

Proto-stellar nebuale

Protostellar Collapse

Binary mass exchange

Gravity GMm F = _____ r 2

Angular mometum l = m v r ~ constant

Centrifugal force mv 2 f = ___ r

Orbital motion centrifugal force f = mv 2 /r ~ 1 / r 3 gravity F = GMm / r 2 v 2 = GM/r when 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 Owocki UD 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 stars spectrum Detailed spectra show emission intensity is split into peaks to blue and red of line-center. o Intensity Wavelength Indicates a disk of gas orbits the star. This is from Doppler shift of gas moving toward and away from the observer. Hydrogen spectrum H H

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 stars do 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 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 < l = Here examine combination of these. Stellar Rotation –Be stars are generally rapid rotators –V rot ~ km/s < V orbit ~ 500 km/s

Rotational Broadening of Photospheric Absorption Lines

Formation of a P-Cygni Line- Profile

Wind Compressed Disk Model

Vrot (km/s) = 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

WCD Inhibition by Non-Radial Forces Oblateness implies polar tilt to radiative flux Poleward force reverses equatorward flow Inhibits WCD formation

WCD Inhibition by Poleward Line-Force Net poleward line-force inhibits WCD Stellar oblateness => poleward tilt in radiative flux r Flux N

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 r N Flux

Launching into Earth Orbit Requires speed of ~ 18,000 mi/h (5 mi/s). Earths 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. V rot = 250 km/sec V=250 km/sec

SPH simulations - P. Kroll gas ejection at 100 km/s magnetic loop

Line-Profile Variations from Non-Radial Pulsation Wavelength (V rot =1) Flux Rotation + NRP NRP-distorted star (exaggerated) Line-Profile with:

l=4, m=2 NRP Mode Beating

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

HD64760 Monitored during IUE Mega Campaign Monitoring campaigns of P-Cygni lines formed in hot-star winds also often show modulation at periods comparable to the stellar rotation period. These may stem from large-scale surface structure that induces spiral wind variation analogous to solar Corotating Interaction Regions. Radiation hydrodynamics simulation of CIRs in a hot-star wind Rotational Modulation of Hot-Star Winds

Symmetric Bright Spot on Rapidly Rotating Be Star V rot = 350 km/s V orbit = 500 km/s Spot Brightness= 10 Spot Size = 10 o

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

Prominence/Filament

Force Cutoff

Outward Viscous Diffusion of Ejected Gas

Time Evolution of m=4 Prograde Spot Model

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