Properties of the Structures formed by Parker - Jeans Instability
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Transcript of Properties of the Structures formed by Parker - Jeans Instability
Properties of the Structuresformed by
Parker-Jeans Instability
Y.M. Seo1, S.S. Hong1, S.M. Lee2 and J. Kim3
1 ASTRONOMY, SEOUL NATIONAL UNIVERSITY2 SUPERCOMPUTING CENTER , KiSTI
3 KOREA ASTRONOMY & SPACE SCIENCE INSTITUTE
Previous Works of Parker Instability
Results Summary
Under Uniform External Gravity
→ convective motion everywhere in the disk.
Under Non-uniformExternal Gravity[ Kim & Hong 1998; Kim, et al. 2004 ]
Under Self-gravity [ Lee & Hong 2006, accepted ]
→ ISM turned into thin sheets due to interchange mode
→ compatible with HI super-clouds, but not with GMCs.
This workIsothermal, magnetized, and self gravitating disk under influence of external gravity
1. Giant Molecular Cloud [Blitz 1993, PP III]
2x105 ~1x106 MSUN ; ~50 H2 cm-3 ; separation 0.4~0.6 Kpc
Star forming rate → Gas consumption rate
→ Need about 33 Myrs [Larson 1994]
2. HI Super-cloud [Elmegreen & Elmegreen in 1981]
1x106 ~4x107MSUN ; ~10 H cm-3 ; separation 1~4 Kpc
mean separation of 106Msun clouds 1.2Kpc
[Alfaro, Cabrear-Cano, Delgado 1992]
Arm Crossing time → about 120 Myrs
- All HI super-clouds have GMCs inside ; not all GMCs are
located inside HI super-clouds.
Observations
Dispersion Relation : Undular Mode
ΩJEANS < ΩPARKER
ΩJEANS > ΩPARKER
ΩJEANS ≈ ΩPARKER
Self Gravity + External Gravity
Solar Neighborhood
Code: Isothermal MHD TVD
MHD + Poisson
(Nx, Ny, Nz) = (256, 512, 256)
→ (1Kpc, 2Kpc, 1Kpc) , with 4pc pixel resolution
o = 2mHcm-3 , = 0.3 , cs = 5.0 km/s
H = 156pc , h = 0.94Kpc , b = 20, 15, 10
Time is in units of [H/cs] , which is 28.3 Myr.
Cs is observed velocity dispersion of cloudlets.
Non Linear Simulations
Non Linear Simulations• ΩPARKER > ΩJEANS
78 Myrs after transient phase 210 Myrs after transient phase
AzimuthMagnetic Field
Radial
Cylinder-like structure perpendicular to B
(Parker Cylinder)
Non-linear Simulations• ΩJEANS > ΩPARKER
130 Myrs after transient phase170 Myrs after transient phase
AzimuthMagnetic Field
Radial
Parker cylinders form first. Parker cylinders merge witheach other.
Fourier Analysis
ΩJEANS > ΩPARKERΩPARKER > ΩJEANS
• ΩPARKER > ΩJEANS : Several peaks
• λy ≈ 2 kpc → HI super-cloud scale structure
λy ≈ 705 pc, λy ≈ 445 pc → GMCs scale structure• ΩPARKER < ΩJEANS : A broad peak
Properties of Clumps
Clump Identification code [Jonathan P. Williams, Eugene J. De Geus, & Leo Blitz]
AzimuthalMagnetic Field
Radial
93 Myrs after transient phase
Mass Distribution
Mean SeparationΩParker > Ω Jean
Projected distance of the peaks
→ 500pc & 1140pc
Averaged distances between all of each clumpsin three dimension
dl 3/2
Mean density → a little lower than GMCs
Radius → a little larger than GMCs
The clumps are precursorsof GMCs
Properties of Clumps
Parker instability dominates in the early stage of clump formation.
Principle Axis of Clumps
Total 22 Clumps at t = 9.8
Longest axis
Shortest Axis
Radialdirection
15 1
Azimuthaldirection
7 0
Verticaldirection
0 21
Clumps are made by the Parker instability
Energies of Clumps
dVw
dVcs 2
2
3
dVuT 2
Clumps are in the virial equilibrium.
Surface energy ≈ Internal energy
Cs is NOT thermal sound speed
[unit in erg]
dVB
M8
2
dSxPE surfp
,
dSxB
E surfm
8
2
,
ID W Π T M Ep,surf Em,surf
1 1.02E+50 2.23E+50 3.70E+48 3.15E+49 3.33E+50 5.33E+49
11 2.85E+47 6.87E+48 2.58E+47 6.91E+47 1.29E+49 1.37E+48
22 1.92E+43 2.69E+46 5.21E+44 1.96E+45 5.37E+46 4.13E+45
Fourier Analysis
Slope: -5
Kolmogorov’s slope : -5/3
Slope : -3
Discussion & Summary
2. Parker - Jeans instability tends to steepen the power spectrum.
1. Structures & Formation Time Scales ΩPARKER > ΩJEANS
→Formation of HI superclouds scale structures and GMCs scale structures within 130Myrs
Parker cylinders and GMCs form first.This Work
HI super-clouds
Fragmentation
GMCs & HI super-clouds
Parker cylinder & GMCs
Collect Parker cylinder and GMCs
GMCs in HI-superclouds
Initial Equilibrium Configuration• (1 + ) cs
2 ism(z) = - ism(z) tot
► 2 ism = 4G ism(z), ext = tot-ism
ism(z) = osech2 (z/ H) sech2b (z/ h)
► o= 2mHcm-3, = 0.3, cs = 5.0 km/s H = 156pc, b = 20, h = 0.94Kpc
• In Simulations
► b = 20, 15, 10 The other parameters are fixed.
tot [Bienayme, Robin, & Creze 1987, A.Ap., 180, 94.]
Observational Facts
ρ [Boulares, A. & Cox, D.P. 1990, ApJ, 365, 544.]
B = 4 ±1 μG (local regular field) [Rainer Beck, 2001, Sp Rev. 99: 243-260]
Synthesized HI Profiles
• < σV(x,y,z) >x,y ≈ 5.0 km/s BUT FWHM ≈ 2.5 km/s < Cs
→ Clump velocity is much too small compared to observed cloud velocity.
Time vs
< Vx(x,y,z) >xy /Cs
Cs = 5.0 km/s < Vx(z) > ≈ 0.0 km/s < σVx(z) > ≈ 5.0 km/s
Time vs
< σVx (x,y,z) >xy /Cs
Slop :0.75
Magnetic to Gas Pressure
P
B
8
2
2scP ,
Velocity dispersion of cloudlets