thick accretion diskastro.tsinghua.edu.cn/~xbai/teaching/studentseminar2017f/... · 2018. 2....
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THICK ACCRETION DISK
DOUWEI
2017/11/10GUIDEDBYBAIXUENING
CONTENT
• Basicmodel• Thickdisk• Properties• Discussion
PART I BASICMODELOFANACCRETIONDISK
WHAT IS ACCRETION DISK
•Central massive body• Protostars• Compact objects (white drawf, Neutron star and black holes)
• Surrounding material
HOW TO WORK?
ByNASA
SCALE (H/R)
Thindisk<<1Thickdisk~1
⇒ 𝜌 = 𝜌$ exp(−*+
,-+) and𝐻 = 01
2~ verticalscaleheight.
Ω5 =6789
�~Keplerianangularvelocity
𝐻𝑅 =
𝑐=𝛺𝑅~
𝑐=𝑣5~𝑣@A𝑣5
𝑐= isthevelocityofsound.
BASIC FUNCTIONS
• Continuity equation• Momentum equilibrium at radius• Angular momentum equilibrium
CONTINUITY EQUATION
• Massfluxconservation𝜕𝜌𝜕𝑡 + 𝛻 ⋅ 𝜌𝑣 = 0
CONT.
• Cylindricalcoordinate:𝜕𝛴𝜕𝑡 +
1𝑟𝜕𝜕𝑅 𝛴𝑣8𝑅 = 0
Where𝛴 = ∫ 𝜌𝑑𝑧 = 2𝜌𝐻 isdefinedassurfacedensity.
• Forastablesystem: OO@= O
OP= 0
• Forthesimplifiedequationisshownas:𝜕𝜕𝑅 𝜌𝑅𝐻𝑣 = 0
MOMENTUM EQUILIBRIUM AT R DIRECTION
• Cylindricalcoordinate• Thindisk:𝛺,𝑅 = 𝛺5,𝑅• Thickdisk:
𝑣d𝑣d𝑅 − 𝛺
,𝑅 = −𝛺5,𝑅 −1𝜌dd𝑅 𝜌𝑐R,
Accretion centrifugalforcegravitypressure• Advection-dominatedaccretiondisk(ADAF)
ANGULAR MOMENTUM EQUILIBRIUM
• Cylindricalwith𝜙 (AMbyviscosity)
Σ𝑣𝑅𝜕𝜕𝑅 (Ω𝑅
,) =𝜕𝜕𝑅 (Σ𝜈𝑅
𝑅𝜕Ω𝜕𝑅 𝑅)
Angularmomentumfluxchangedin2𝜋𝑅d𝑅 andviscosityWealsouseconstant𝛼 describingtheeffectofviscosity
𝜈 = 𝛼𝑐=𝐻 =𝛼𝑐=,
Ω5
PART II THICKDISK
ENERGY EQUATION
Σ𝑣𝑇d𝑠𝑑𝑅 = 𝑄f − 𝑄g
• Thindisk𝑄g = 4𝜋𝑅𝐹g
whenopticallythickandgeometrythinopticallythin?⇒ 𝜏 ≪ 1
ENERGY EQUATION
Opticallythin->• Free-freeemission(brakingradiation)• Emissivity~𝑁m,~𝜌,~𝛴,
Opticallythick->• Blackbodyradiation• Emissivity ~𝜎𝑇o~�̇�~𝛴
ENERGY EQUATION
Ifnocooling,justviscosity!
Σ𝑣𝑇d𝑠𝑑𝑅 = 𝑄f = 𝜈 𝑅
d𝛺d𝑅
,
Using𝑇d𝑠 = 𝐶|d𝑇 + 𝑝d𝑉3 + 3𝜖2 2𝜌𝐻𝑣
𝑑𝑐=,
𝑑𝑅 − 2𝑐=,𝐻𝑣𝑑𝜌𝑑𝑅 = 𝑄f
SOLVE : SELF-SIMILAR SOLUTION• O
O�𝜌𝑅𝐻𝑣 = 0
• 𝑣 ����− 𝛺,𝑅 = −𝛺5,𝑅 −
�����
𝜌𝑐R,
• Σ𝑣8𝑅OO8(𝛺,𝑅) = O
O8(Σ𝜈𝑅� O2
O8)
• 𝑇 �=��= 𝜈 𝑟 ��
�8
,
By wikiSolve the equations with varieties relative to 𝑅, we now have
𝜌 ∝ 𝑅g�,𝑣 ∝ 𝑅g
�,Ω ∝ 𝑅g
�,𝑐=, ∝ 𝑅g�
INTERESTING RESULTS
ThickDisk
• 𝑣 ≈ −���𝑣5
• Ω ≈ 0
• 01+
��+ ≈
,�
• �̇� ≈ −4𝜋𝑅𝐻𝑣𝜌 = 𝑐𝑜𝑛𝑠𝑡
ThinDisk
• 𝑣~𝛼 -�
,𝑣5
• Ω ≈ Ω5
• 01+
��+ =
-�
,≪ 1
INTERESTING RESULTS
Cooling does exist!• 𝑄f − 𝑄g = 𝑓𝑄f, 𝑓 = 0 representefficientcoolingand𝑓 = 1isnoradiativecooling.
• 𝐶| isrelativistic!Useafunctionof𝜖 = 0 representnorelativityand𝜖 = 0representrelativity.
• Using𝜖� = ��~ acriticalroleindeterminingthenatureofflow.
INTERESTING RESULTS
• 𝑣 ≈ − ���f,��
𝑣5
• Ω ≈ ,��
�f,��
�+ Ω5
• 𝑐=, ≈,��
�f,��𝑣5,
• -�~1 = 01
��
• Thindisk𝜖� → ∞• ADAF𝜖� < 1
PART III PROPERTIESOFTHICKDISK
BERNOULLI CONSTANT
• NormlizedBernoulliconstant(Energyconserved)
𝑏 =1𝑣5,
12 𝑣
, +12Ω
,𝑅, − Ω5,𝑅, +𝛾
𝛾 − 1 𝑐=, =
3𝜖 − 𝜖�
5 + 2𝜖′
𝑏 > 0 → gasreachinfinity(𝛺~𝑅g9+, 𝑣~𝑅g
�+)
𝑏 < 0 → cannotspontaneouslyescape
With𝑓 > 1/3 , 𝛾 < ��⇒ 𝑏 > 0!
• Winds?Jets?
• SeemoreinStone,Pringle(1999)
BRUNT -VÄISÄLÄ FREQUENCY
• 𝑁~Brunt- Väisälä frequency
𝜌$𝜕,𝑧�
𝜕𝑡, = −𝑔 𝜌 𝑧 − 𝜌 𝑧 + 𝑧� = −𝑔𝜕𝜌(𝑧)𝜕𝑧 𝑧′
⇒ 𝑧� = 𝑧$� 𝑒 g¥+� @ and𝑁 = − ¦�§
O�O*
�
O�O*> 0 →notstable!
INSTABILITY
• Dynamicalconvectiveinstability:
𝑁m��, = 𝑁, + 𝜅,
𝑁~Brunt- Väisälä frequency𝜅~ epicyclicfrequency
• Self-similar:𝑁m��, = �$��f©���g���(�f��)(�f,��)
⇒ 𝑓 > ,�+ ,
�𝜖
• Doublediffusiveinstability
PART IV DISCUSSION
�̇� > 𝑀ª̇ ⇒SLIM DISK
• 𝑃 = 𝑃¦¬= + 𝑃8¬�• 𝑇𝑑𝑠 = 𝑑𝐸�®=0¯=®@° − 𝑑𝐸8¬�®¬@®¯±
• Yuan(2003)
SUMMARY
• Non-standardmodel– ADAFisanopticallythingeometrythickmodelandcooledbyfree-freeemissionwhichisnotefficientcomparingtotheblackbodyradiation.
• Weuseaself-similarsolutionwhichisonlybasedontheradiustogetthefunctionofvelocity,density,angularvelocityandsoundspeed.
• Theremaybesomeinstabilityinthesimulationbuttheobservationareallstableuptonow.
THANKYOU!