13 March 201313rd MAXI Symposium Be Be. Talk Outline 1.Introduction to Be stars 2.Decretion vs....

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

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Be stars

(Porter & Rivinius 2003; Martayan 2010)

Definition

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(Martayan 2010)

Non-supergiant B-type stars, which once has shown Balmer lines in emission (“e” is for emission)


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


Be stars

Inte

nsity

Wavelength

Hydrogenspectrum

Courtesy of Stan Owocki

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λ0


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


Population

(Martayan 2010)M

WS

MC

Be/B much higher in SMC than in MW

Metalicity effect?

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Disk formation by viscous diffusion(Lee, Saio, Osaki 1991)

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outward drift by viscosity

mass ejection from star

(Carciofi & Bjorkman 2006 for detailed disk structure)

Viscous decretion disk model


Be stars

Inte

nsity

Wavelength

Hydrogenspectrum

Courtesy of Stan Owocki

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λ0

Viscous decretion disk Keplerian

Equatorial disk Equatorial wind!≠


? Viscosity

Mass injection: ON

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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





1. Tidal truncation

2. Tidal/radiative precession and warping

3. Wind-wind collision


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1. Tidal truncation




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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

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(Zamanov+ 2001)

Be disks denser in Be/X-ray binaries than in isolated Be stars (Zamanov+ 2001)

Typical Be disks

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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

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e = 0 e = 0.34 e = 0.68

Tidal/resonant truncation

Truncation radius decreases with increasing e (Be disk smaller than periastron distance)

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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

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1. Tidal truncation




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Observational evidence(?) for disk precession and warping

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Spectacular profile changes in 4U 0115+63

(Negueruela+ 2011; 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)

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5 types of torques on viscous disks

Torques on Be disks in binariestorques tending to straighten the disk

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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)

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

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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)

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1. Tidal truncation




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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)


Observations favor very dense Be disk

ρ0 = 10−10 g cm−3

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ρ0 = 10−9 g cm−3

Density on orbital plane

Pulsar passes through Be disk

PW truncates Be disk


Stellar evolution

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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?

13 March 201313rd MAXI Symposium Be Be. Talk Outline 1.Introduction to Be stars 2.Decretion vs....

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