The Rocket Science of Launching Stellar Disks

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

2. Stan Owocki Bartol Research Institute University of Delaware Disks in Space

3. 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

4. Saturn’s rings

5. Spiral Galaxies

6. Disk in Center of Galaxy

7. Beta Pictoris

8. Gaseous Pillars in M16

9. Proto-stellar nebuale

10. Protostellar Collapse

11. Binary mass exchange

12. Binary mass exchange

13. Gravity GMm F = _____ r2

14. l = m v r ~ constant Angular mometum

15. Centrifugal force mv2 f = ___ r

16. Orbital motion centrifugal force f = mv2/r ~ 1 / r3 gravity F = GMm /r2 v2 = GM/r when F=f

17. 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

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

19. Spectral lines & Doppler shift • Atoms of a gas absorb & emit light at discrete frequencies • Motion of atoms shifts frequency by Doppler effect

20. Hb Ha Hydrogen spectrum Intensity lo Wavelength 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. • This is from Doppler shift of gas moving toward and away from the observer . • Indicates a disk of gas orbits the star.

21. So disk matter must be launched from star. 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??

22. 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.

23. Rotational Broadening of Photospheric Absorption Lines

24. Wind Compressed Disk Model

25. Hydrodynamical Simulations of Wind Compressed Disks Vrot (km/s) = 200 250 300 350 400 450 Note: Assumes purely radial driving of wind

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

27. Flux N Problems with WCD Model 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

28. Cannon atop high mountain DV ~ 18,000 mi/h Cannon at equator DV ~ 17,000 mi/h Launching into Earth Orbit • Requires speed of ~ 18,000 mi/h (5 mi/s). • 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

29. 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. DV=250 km/sec Vrot = 250 km/sec

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

31. NRP Mode Beating l=4, m=2

32. 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??

33. 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??

34. First try: Localized Equatorial Bright Spots

35. Symmetric Bright Spot on Rapidly Rotating Be Star Vrot = 350 km/s Vorbit= 500 km/s Spot Brightness= 10 Spot Size = 10 o

36. Assume localized distortion in surface height & brightness. If phase of brightness leads height, then can get “prograde flux”. Can this drive mass into orbit? RDOMERadiatively Driven Orbital Mass Ejection

37. Time Evolution of Single Prograde Spot

38. Prominence/Filament

39. Force Cutoff

40. Outward Viscous Diffusion of Ejected Gas

41. Time Evolution of m=4 Prograde Spot Model

42. Summary DV=250 km/s • 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???