13 March rd MAXI Symposium Be Be

Talk Outline 1.Introduction to Be stars 2.Decretion vs. accretion 3.Interactions in binaries 4.Concluding remarks 13 March rd MAXI Symposium

1.Introduction to Be stars 2.Decretion vs. accretion 3.Interactions in binaries 4.Concluding remarks 13 March rd MAXI Symposium

Be stars (Porter & Rivinius 2003; Martayan 2010) Definition 13 March rd MAXI Symposium (Martayan 2010) Non-supergiant B- type stars, which once has shown Balmer lines in emission (e is for emission)

Low-density, fast outflow emitting UV radiation Wind structure well explained by line- driven wind model (Castor+ 1975) Two-component circumstellar envelope Polar wind Equatorial disk High-density region with optical emission lines and IR excess 13 March rd MAXI Symposium

6 Be stars Intensity Wavelength Hydrogen spectrum Courtesy of Stan Owocki 13 March 20133rd MAXI Symposium

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 13 March rd MAXI Symposium

Population (Martayan 2010) MW SMC Be/B much higher in SMC than in MW Metalicity effect? 13 March rd MAXI Symposium

1.Introduction to Be stars 2.Decretion vs. accretion 3.Interactions in binaries 4.Concluding remarks 13 March rd MAXI Symposium

Disk formation by viscous diffusion (Lee, Saio, Osaki 1991) 13 March rd MAXI Symposium outward drift by viscosity mass ejection from star (Carciofi & Bjorkman 2006 for detailed disk structure) Viscous decretion disk model

11 Be stars Intensity Wavelength Hydrogen spectrum Courtesy of Stan Owocki 13 March 20133rd MAXI Symposium Viscous decretion disk Viscous decretion disk Keplerian Keplerian Equatorial disk Equatorial wind!

? Viscosity Mass injection: ON 13 March rd MAXI Symposium Mechanism unknown Origin unknown Decretion needs mass addition torque decretion accretion ang. mom. input Mass injection: OFF no ang. mom. input

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

1.Introduction to Be stars 2.Decretion vs. accretion 3.Interactions in binaries 1.Tidal truncation 2.Tidal/radiative precession and warping 3.Wind-wind collision 4.Concluding remarks 13 March rd MAXI Symposium

1.Introduction to Be stars 2.Decretion vs. accretion 3.Interactions in binaries 1.Tidal truncation 2.Tidal/radiative precession and warping 3.Wind-wind collision 4.Concluding remarks 13 March rd MAXI Symposium

The longer the orbital period, the larger the maximum disk size The longer the orbital period, the larger the maximum disk size Correlation between Max{EW(H )} and in Be/X-ray binaries (Reig+ 1997) Observational support for tidal truncation 13 March rd MAXI Symposium

Reig (2011) 13 March rd MAXI Symposium

(Zamanov+ 2001) Be disks denser in Be/X-ray binaries than in isolated Be stars (Zamanov+ 2001) Typical Be disks 13 March rd MAXI Symposium

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) (Artymowicz & Lubow 1994) Criterion of disk truncation: at a resonance radius Mechanism for tidal truncation 13 March rd MAXI Symposium

20 e = 0e = 0.34 e = 0.68 Tidal/resonant truncation Truncation radius decreases with increasing e (Be disk smaller than periastron distance) 13 March 20133rd MAXI Symposium

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 13 March rd MAXI Symposium

1.Introduction to Be stars 2.Decretion vs. accretion 3.Interactions in binaries 1.Tidal truncation 2.Tidal/radiative precession and warping 3.Wind-wind collision 4.Concluding remarks 13 March rd MAXI Symposium

Observational evidence(?) for disk precession and warping 13 March rd MAXI Symposium Spectacular profile changes in 4U (Negueruela ; Reig+ 2007)

Interpretation of spectacular profile changes Precessing warped disk (Negueruela+ 2001) radiation-driven warping? (Pringle 1996; Porter 1998) tidal warping of a misaligned disk? (Martin+ 2011) 13 March rd MAXI Symposium

5 types of torques on viscous disks Torques on Be disks in binaries torques tending to straighten the disk 13 March rd MAXI Symposium bending torque in retrograde direction, if disk is misaligned bending torque, if disk is optically thick. Direction depend on gradient of tilt angle internal (viscous) torque advective torque mass addition torque tidal torque radiative torque

Tidal precession/warping Tidal torques have alignment effect on a tilted disk toward the orbital plane. (Lubow & Ogilvie 2000; Martin+ 2011) : Disk aligns with orbital plane : Disk behaves like a rigid body and moves towards orb plane 13 March rd MAXI Symposium

Radiative precession/warping 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. (Pringle 1996; Wijers, Pringle 1999) 13 March rd MAXI Symposium

1.Introduction to Be stars 2.Decretion vs. accretion 3.Interactions in binaries 1.Tidal truncation 2.Tidal/radiative precession and warping 3.Wind-wind collision 4.Concluding remarks 13 March rd MAXI Symposium

TeV gamma-ray binary PSR B (Radio pulsar + O9.5e; Porb=3.4yr, e=0.87) 13 March rd MAXI Symposium (Credit: NASA)

Observations favor very dense Be disk 13 March rd MAXI Symposium Density on orbital plane Pulsar passes through Be disk PW truncates Be disk

Stellar evolution 13 March rd MAXI Symposium Disk physics interesting on its own: Mechanism leading to critical rotation High energy emission Decretion/accretion/truncation/warping/ precession, all affecting accretion onto NS/BH; Turbulence; Mass transfer Be-star binaries form largest subgroup of HMXBs and TeV binaries Concluding remarks: Why should we bother about Be stars?