the instability of a self-gravitating gaseous disk
DESCRIPTION
The instability of a self-gravitating gaseous disk. by Chi Yuan Institute of Astronomy & Astrophysics Academia Sinica, Taipei 11-01-2006. Collaborators: David C.C. Yen, L.H. Lin, H.H. Wang, C.C. Yang. Two kinds of instability in astrophysical disks. - PowerPoint PPT PresentationTRANSCRIPT
The instability of aself-gravitating gaseous disk
by
Chi YuanInstitute of Astronomy & Astrophysics
Academia Sinica, Taipei11-01-2006
Collaborators: David C.C. Yen, L.H. Lin, H.H. Wang, C.C. Yang
Two kinds of instabilityin astrophysical disks
• Toomre’s instability: starburst rings, star formation
• Shear instability of Rayleigh’s type: associated with shocks, causing substructure, chaos and turbulence
Governing Equations
,0v
t
~,
,
,4
,Iv v v
2
02
bd0
d2
2
PaP
r
zG
at
The Antares Code
• Second-order 2-D Godunov codes
• Based on exact Riemann solver
• Coupled with the fft Poisson solver (non-periodic BC) for disk self-gravity
• True radiation (non-reflecting) boundary conditions.
• Cartesian and polar coordinates
• Isothermal and polytropic gas
• All codes parallelized
大火程序
Starburst Rings
NGC1512 NGC6782NGC4313
Circumnuclear Disk in ngc1068
Center of M51
NGC 1068
9.5 arcmin
• Type : Sb / AGN
• Distance : 14.4 Mpc
( H0 = 75 km s-1 Mpc-1 )
1 arcsec = 74 pc
Nearly Flat Rotation Curve
01.0,)( 2
1
0
r
rvv
Cases of Double Resonances: OLR-OILR
Toomre’s Instability
densitysurface
frequencyepicyclic
eeda:sound spG
aQ
:
:
1
Onset of Instability
G
aQ
Disk Instability
G
aQ
m
Sub-structure, feathers, chaos
Sub-structure of M81
M81
Shear Instability
Sub-structure, Patches, Chaos
Rayleigh’s Shear Instability: (vorticity in the axisymmetric flow flield has a local
extremum)
vorticity divided by surface density orSpecific vorticity has a local extremum
.0 ,0
problems D-2for
. ,
Fluids ibleIncompress Fluids leCompressib
Dt
D
Dt
D
uDt
Du
Dt
D
Specific Vorticity
STuhu
Dt
D
Dt
D
uuDt
D
zzz
pzsz
)2
1(
shockat created vorticity
0v)( y
u)( x
t
0,v)( y
u)( x
t
frame) (rotating .02
,0
problems D-2for
x ,
vorticityspecific
2
The Mechanism of Rayleigh’s Criterion for Shear Instability (C.C. Lin 1944)
Simple Explanation of Rayleigh’s shear instability
Specific Vorticity distribution
Vorticity Creation by Shock(Cocco, Truesdell, Lighthill, Hayes, Berndt, Kevlahan)
s
r
r
Dt
Du
Cs
C)(
1
)1(21
1
)1(21
1
22
sra
r
a Dt
Du
Cs
C)(
1
)1(21
1
)1(21
)1()(
2
2
2
1a
b
,ACCr
Creation of Vorticity along Shocks
Conclusions
• Disk instability is extremely important in astrophysics.• Toomre’s instability at work, giving rise starburst an
d star formation activities. It also explains the origin of the dense circumnuclear molecular disks observed in the center of many nearby galaxies.
• Shear instability creates substructures, patches and chaos in the middle of the galactic disk where shocks are present.
• We believe that the origin of the shear instability lies on the vorticity creation on shocks.
Evolution of Streamlines
Specific Vorticity at different resolution