Be 星ガス円盤の物理

1. Be星ガス円盤の物理 岡崎敦男 （北海学園大学工学部） 3rd MAXI Symposium

2. Talk Outline • Introduction to Be stars • Decretion vs. accretion • Interactions in binaries • Concluding remarks 3rd MAXI Symposium

3. Introduction to Be stars • Decretion vs. accretion • Interactions in binaries • Concluding remarks 3rd MAXI Symposium

4. Be stars (Porter & Rivinius2003; Martayan 2010) • Definition Non-supergiant B-type stars, which once has shown Balmer lines in emission (“e” is for emission) (Martayan 2010) 3rd MAXI Symposium

5. Two-component circumstellar envelope Polar wind • Low-density, fast • outflow emitting UV radiation • Wind structure well explained by line-driven wind model (Castor+ 1975) Equatorial disk • High-density region with optical emission lines and IR excess 3rd MAXI Symposium

6. Bestars Hydrogen spectrum Intensity Wavelength Courtesy of Stan Owocki 3rd MAXI Symposium

7. Central star • Rapid rotation Conventional interpretation: ~70-80% of critical rotation Current idea: almost critical rotation (~95% or more) increasing stellar rotation Courtesy of Stan Owocki 3rd MAXI Symposium

8. Population • Be/B much higher in SMC than in MW SMC Metalicity effect? MW (Martayan 2010) 3rd MAXI Symposium

9. Introduction to Be stars • Decretion vs. accretion • Interactions in binaries • Concluding remarks 3rd MAXI Symposium

10. Disk formation by viscous diffusion(Lee, Saio, Osaki 1991) Viscous decretion disk model outward drift by viscosity mass ejection from star (Carciofi& Bjorkman 2006 for detailed disk structure) 3rd MAXI Symposium

11. Equatorial disk Equatorial wind! Bestars Hydrogen spectrum Viscous decretion disk Intensity • Keplerian Wavelength Courtesy of Stan Owocki 3rd MAXI Symposium

12. Decretion needs mass addition torque Mass injection: ON ang. mom. input decretion Mass injection: OFF accretion no ang. mom. input ? Viscosity Mechanismunknown Origin unknown 3rd MAXI Symposium

13. Disk formation and dissipation (Haubois+ 2012) Formation Dissipation 3rd MAXI Symposium

14. Introduction to Be stars • Decretion vs. accretion • Interactions in binaries • Tidal truncation • Tidal/radiative precession and warping • Wind-wind collision • Concluding remarks 3rd MAXI Symposium

15. Introduction to Be stars • Decretion vs. accretion • Interactions in binaries • Tidal truncation • Tidal/radiative precession and warping • Wind-wind collision • Concluding remarks 3rd MAXI Symposium

16. Observational support for tidal truncation • Correlation between Max{EW(H )} and • in Be/X-ray binaries (Reig+ 1997) The longer the orbital period, the larger the maximum disk size 3rd MAXI Symposium

17. Be disks denser in Be/X-ray binaries than in isolated Be stars (Zamanov+ 2001) Typical Be disks (Zamanov+ 2001) 3rd MAXI Symposium

18. Mechanism for tidal truncation • Viscous torques provide ang. mom. to disk (Lin & Papaloizou 1986) • Tidal (e=0)/resonant (e>0) torques remove ang. mom. from disk (Goldreich & Tremaine 1979, 1980) Criterion of disk truncation: at a resonance radius (Artymowicz & Lubow 1994) 3rd MAXI Symposium

19. Tidal/resonant truncation e = 0 e = 0.34 e = 0.68 Truncation radius decreases with increasing e (Be disk smaller than periastrondistance) 3rd MAXI Symposium

20. Evolution of truncated Be disks • Initial disk growth similar to single Be stars • Density breaks at a resonant radius • Mass transfer rate increases as disk density increases. • Disk reaches a quasi-equilibrium state Surface density evolution 3rd MAXI Symposium

21. Introduction to Be stars • Decretion vs. accretion • Interactions in binaries • Tidal truncation • Tidal/radiative precession and warping • Wind-wind collision • Concluding remarks 3rd MAXI Symposium

22. Observational evidence(?) for disk precession and warping Spectacular profile changes in 4U 0115+63 (Negueruela+ 2011; Reig+ 2007) 3rd MAXI Symposium

23. Interpretation of spectacular profile changes Precessing warped disk • radiation-driven warping?(Pringle 1996; Porter 1998) • tidal warping of a misaligned disk?(Martin+ 2011) (Negueruela+ 2001) 3rd MAXI Symposium

24. Torques on Be disks in binaries bending torque in retrograde direction, if disk is misaligned torques tending to straighten the disk 5 types of torques on viscous disks • internal (viscous) torque • advective torque • mass addition torque • tidal torque • radiative torque bending torque, if disk is optically thick. Direction depend on gradient of tilt angle 3rd MAXI Symposium

25. Tidal precession/warping (Lubow & Ogilvie 2000; Martin+ 2011) Tidal torques have alignment effect on a tilted disk toward the orbital plane. : Disk aligns with orbital plane : Disk behaves like a rigid body and moves towards orb plane 3rd MAXI Symposium

26. Radiative precession/warping (Pringle 1996; Wijers, Pringle 1999) • Stellar photons with relatively long wavelengths, which in Be stars is a small fraction of total photons, exert torques on optically thick part of disk. • Even so, radiative torques can be comparable with or stronger than tidal torques in outer disk regions. 3rd MAXI Symposium

27. Introduction to Be stars • Decretion vs. accretion • Interactions in binaries • Tidal truncation • Tidal/radiative precession and warping • Wind-wind collision • Concluding remarks 3rd MAXI Symposium

28. TeV gamma-ray binary PSR B1259-63(Radio pulsar + O9.5e; Porb=3.4yr, e=0.87) 3rd MAXI Symposium (Credit: NASA)

29. Observations favor very dense Be disk PW truncates Be disk Pulsar passes through Be disk Density on orbital plane 3rd MAXI Symposium

30. Concluding remarks: Why should we bother about Be stars? • Stellar evolution Mechanism leading to critical rotation • Disk physics interesting on its own: Decretion/accretion/truncation/warping/precession, all affecting accretion onto NS/BH; Turbulence; Mass transfer • High energy emission Be-star binaries form largest subgroup of HMXBs and TeV binaries 3rd MAXI Symposium