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Disks of Be Stars & Their Pulsations &

Disks of Be Stars & Their Pulsations &. Qingkang Li Department of Astronomy Beijing Normal University The Third Workshop of SONG, April, 2010. Collaborators: J. C. Brown, Dept. of Physics and Astronomy, University of Glasgow, UK

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Disks of Be Stars & Their Pulsations &

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  1. Disks of Be Stars & Their Pulsations & Qingkang Li Department of Astronomy Beijing Normal University The Third Workshop of SONG, April, 2010

  2. Collaborators: • J. C. Brown, Dept. of Physics and Astronomy, University of Glasgow, UK • J. P. Cassinelli, Dept of Astronomy, University of Wisconsin-Madison, USA • R. Ignace, Dept. of Physics and Astronomy, University of East Tennessee, USA • And many others.

  3. Contents • 1. Introduction • 2. The mystery of the Be stars • 3. Be star disk models • 4. Discussion

  4. 1. Introduction • Over the course of their lifetimes, hot, luminous, massive (OB-type) stars lose large amount of mass in nearly continuous outflow called stellar winds. • These winds are driven by scattering of the star’s continuum radiation in a large ensemble of spectral lines (Castor, Abbott & Klein 1975). • There is extensive evidence for variability and structure on both small and large scales.

  5. SOHO Extreme ultraviolet (171 Angstrom) Solar Activities: Coronal Expansion

  6. WR wind bubble NGC 2359 Superbubble in the Large Magellanic Cloud Wind-Blown Bubbles in ISM

  7. Eta Carinae

  8. OB stars in the HR diagram • O, B spectral type stars • Teff > 10,000 K • L* > 100 Lsun • M* > 3 Msun

  9. Hot massive pulsators

  10. Pulsating starsin the HR diagram

  11. βCep variables: pulsation periods P~2-10 hrs, low-order p and g modes, B0-B2 Slowly pulsating B (SPB) stars: P∽10-50 hrs, high-order g modes, B3-B9 Pulsating Be stars: P~similar to SPB up to 100-200 hrs

  12. 2. The Mystery of the Be Stars Be stars are non-supergiant B-type stars whose spectra have, or had at some time, one or more Balmer lines in emission. The mystery of the "Be phenomenon" is that the emission, which is well understood to originate from a flattened circumstellar disk (e.g. Struve, 1931), can come and go episodically on time scales of days to decades.

  13. Hb Ha Hydrogen spectrum Intensity lo Wavelength Be stars • Hot, bright, & rapidly rotating stars. • 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.

  14. Cartoon of a Be star and its emission lines From http://www.astrosurf.org/buil/us/bestar.html

  15. Variability of line profile From http://www.astrosurf.org/buil/us/bestar.html

  16. Key Properties • Strong, radiatively driven stellar winds • ---Mdot ~ 10-10 to 10-6MO/yr; v¥ > 1000 km/s • Driven by line-scattering of the star’s radiation

  17. Magnetic Activity • Some have observed dipole field ~103-104 G • stable; from convective dynamo or fossil?

  18. Stellar Rotation • Be stars are generally rapid rotators • Vrot ~ 200-400 km/s ~ Vcrit/2 • Prot ~ few days • oblateness and gravity darkening

  19. Stellar Pulsation • Many Be stars show Non-Radial Pulsations (NRP) with m < l = 1 – 4 • Give rise to the line variability.

  20. So disk matter must be launched from the star. The Key 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?

  21. The key theoretical concerns on the Be star disks are as follows: 1. What source of energy elevates matter to orbits well above the stellar surface? 2. How does the matter obtain the angular momentum and orbit around the star with Keplerian speed with no observed outflow? 3. How are the very high observed disk density attained?

  22. 3. Be star disk models (1) Wind compressed disk model (Bjorkman, Cassinelli, 1993, ApJ, 409, 429)

  23. (2) Episodic mass ejection model (Ando, 1986, A&A, 163, 97; Owocki & Cranmer, 2002, ASP 259, 512, 951; Cranmer, 2009, ApJ, 701, 396) Observations confirm that Be star line-profile variability is due to the non-radial pulsations (NRP). Some mixed modes of NRP (retrograde) are associated with the transport of angular momentum up to the surface and beyond, saying, pulsational energy may leak out of a hot star into circumstellar medium. • low-frequency evanescent pulsations; • high-frequency resonant wave excitation at the acoustic cutoff; • shock steepening and dissipation; • wave pressure & angular momentum transfer.

  24. (3) Magnetically torqued disk (MTD) model (Cassinelli, et al., 2002, ApJ, 578, 951)

  25. MTD+GD (Brown, et al., 2004, MNRAS, 352, 1061)

  26. X-ray emission from MTD+GD Li, et al., 2008, ApJ, 672,1174

  27. X-ray emission from MTD+GD Li, et al., 2008, ApJ, 672,1174

  28. (4) Combination of all stuff Wind + B-field + Rotation + Pulsation ?

  29. Painted by John C. Brown

  30. 4. Discussion • The disk formation mechanism of Be stars is one of the key issues to hot star research both observational and theoretical. • We need new observational facilities, such as SONG. • We require the development of simulations. • We require the advancement of model atmospheres and radiative transfer techniques • We need correct stellar parameters (mass loss rate, rotation, magnetic fields, and pulsations).

  31. Thank you very much for your time!

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