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3rd MAXI Symposium 113 March 2013
岡崎敦男(北海学園大学工学部)
Be 星ガス円盤の物理
3rd MAXI Symposium 2
Talk Outline
1. Introduction to Be stars
2. Decretion vs. accretion
3. Interactions in binaries
4. Concluding remarks
13 March 2013
3rd MAXI Symposium 3
1. Introduction to Be stars
2. Decretion vs. accretion
3. Interactions in binaries
4. Concluding remarks
13 March 2013
3rd MAXI Symposium 4
Be stars
(Porter & Rivinius 2003; Martayan 2010)
Definition
13 March 2013
(Martayan 2010)
Non-supergiant B-type stars, which once has shown Balmer lines in emission (“e” is for emission)
3rd MAXI Symposium 5
Low-density, fastoutflow emitting UV radiation
(Vr ≈103 kms-1 )
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 excess13 March 2013
3rd MAXI Symposium 6
Be stars
Inte
nsity
Wavelength
Hydrogenspectrum
Courtesy of Stan Owocki
13 March 2013
λ0
3rd MAXI Symposium 7
Central star
Rapid rotationConventional interpretation: ~70-80%
of critical rotationCurrent idea: almost critical rotation
(~95% or more)increasing stellar rotation
Courtesy of Stan Owocki13 March 2013
3rd MAXI Symposium 8
Population
(Martayan 2010)M
WS
MC
Be/B much higher in SMC than in MW
Metalicity effect?
13 March 2013
3rd MAXI Symposium 9
1. Introduction to Be stars
2. Decretion vs. accretion
3. Interactions in binaries
4. Concluding remarks
13 March 2013
3rd MAXI Symposium 10
Disk formation by viscous diffusion(Lee, Saio, Osaki 1991)
13 March 2013
outward drift by viscosity
mass ejection from star
(Carciofi & Bjorkman 2006 for detailed disk structure)
Viscous decretion disk model
3rd MAXI Symposium 11
Be stars
Inte
nsity
Wavelength
Hydrogenspectrum
Courtesy of Stan Owocki
13 March 2013
λ0
Viscous decretion disk Keplerian
Equatorial disk Equatorial wind!≠
3rd MAXI Symposium 12
? Viscosity
Mass injection: ON
13 March 2013
Mechanismunknown
Origin unknown
Decretion needs mass addition torque
decretion
accretion
ang. mom. inputMass injection: OFF
no ang. mom. input
3rd MAXI Symposium 1313 March 2013
Disk formation and dissipation(Haubois+ 2012)
Formation Dissipation
α =0.1
α =1
3rd MAXI Symposium 14
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 2013
3rd MAXI Symposium 15
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 2013
3rd MAXI Symposium 16
The longer the orbital period, the larger the maximum disk size
Correlation between Max{EW(H )} and in Be/X-ray binaries (Reig+ 1997)Porb
α
Observational support for tidal truncation
13 March 2013
3rd MAXI Symposium 18
(Zamanov+ 2001)
Be disks denser in Be/X-ray binaries than in isolated Be stars (Zamanov+ 2001)
Typical Be disks
13 March 2013
3rd MAXI Symposium 19
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:Tvis +Tres < 0 at a resonance radius
Mechanism for tidal truncation
13 March 2013
3rd MAXI Symposium 20
e = 0 e = 0.34 e = 0.68
Tidal/resonant truncation
Truncation radius decreases with increasing e (Be disk smaller than periastron distance)
13 March 2013
3rd MAXI Symposium 21
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 2013
3rd MAXI Symposium 22
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 2013
3rd MAXI Symposium 23
Observational evidence(?) for disk precession and warping
13 March 2013
Spectacular profile changes in 4U 0115+63
(Negueruela+ 2011; Reig+ 2007)
3rd MAXI Symposium 24
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 2013
3rd MAXI Symposium 25
5 types of torques on viscous disks
Torques on Be disks in binariestorques tending to straighten the disk
13 March 2013
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
3rd MAXI Symposium 26
Tidal precession/warping
Tidal torques have alignment effect on a tilted disk toward the orbital plane.
(Lubow & Ogilvie 2000; Martin+ 2011)
talign = tvis : Disk aligns with orbital plane
talign ? tvis
talign =
Σ(GM*r)1/2
Ttid
: tprec
: Disk behaves like a rigid body and moves towards orb plane
13 March 2013
3rd MAXI Symposium 27
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 2013
3rd MAXI Symposium 28
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 2013
29
TeV gamma-ray binary PSR B1259-63(Radio pulsar + O9.5e; Porb=3.4yr, e=0.87)
13 March 2013 3rd MAXI Symposium (Credit: NASA)
3rd MAXI Symposium 30
Observations favor very dense Be disk
ρ0 = 10−10 g cm−3
13 March 2013
ρ0 = 10−9 g cm−3
Density on orbital plane
Pulsar passes through Be disk
PW truncates Be disk
3rd MAXI Symposium 31
Stellar evolution
13 March 2013
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 transferBe-star binaries form largest subgroup of HMXBs and TeV binaries
Concluding remarks: Why should we bother about Be stars?