Download Spiral shock triggering of star formation

Spiral Triggering of Star Formation
Ian Bonnell, Clare Dobbs Tom Robitaille,
University of St Andrews
Jim Pringle
IoA, Cambridge
Dynamical Models of Star Formation
• Local regions of GMCs
• Models for the origin of
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Stellar clusters
Massive stars
Brown dwarfs
Initial Mass Function
– But not the initial conditions
for star formation
Giant Molecular Clouds
• Stars form in molecular clouds
• Molecular cloud properties
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Mass: 1000’s to >105 Msun
Sizes: ~ 10 pc
Densities: 10-19 to 10-22 g cm-3
Cold: T ~ 10 K
– Located in spiral arms
– Lots of structure
– Supersonic ‘turbulence’
» Larson relation:
v  R
0 .5
Spiral Shocks and Star Formation
• Do spiral shocks
control star formation?
» Roberts 1971
• Gas dynamics in
2 (4) armed spiral potential
» External potential
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SPH simulations (4 x 105 to 4 x 106 particles)
Isothermal (100 K)
Clumpy : average 10-3 Msun /pc3 ; max 10-1 Msun /pc3
Self gravity
Star formation modeled with sink-particles
Initial Conditions
• Test particle simulation in
spiral potential
– Inside co-rotation
• Region of over-density of
100 pc chosen
• Proto-GMC traced
backwards
• Replace by self-gravitating
SPH particles
• Surface density 0.1 to 1 Msun
pc-2
Spiral Triggering of star formation
• Follow gas flow through spiral arm
• Shocks leaving pot. minimum
• Form dense clouds
– GMCs
• Onset of gravitational
collapse and SF
• Forms stellar clusters
– At r > 103 Msun pc-3
• Masses 102 to 104 Msun
Low surface
density simulation
S = 0.1 Msun pc-2
(105 Msun)
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Low surface
density simulation
S = 0.1 Msun pc-2
(105 Msun)
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High surface
density simulation
S = 1.0 Msun pc-2
(106 Msun)
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Size ~ 500 pc
Formation of Giant Molecular Clouds
• Convergent gas streams
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Due to spiral potential
Clumpy shock forms substructure (GMCs?)
Dissipate kinetic energy in shock
Forms bound substructure
Star Formation
– Structures due to instabilities
» Self-gravity ? Probably not
» Kelvin-Helmholtz ?
Size ~ 50 pc
– Edges sharper on
upwind side
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GMC Kinematics
• Convergent gas streams
– Clumpy gas
– Broadens shock
• Post-shock velocity depends on
– Density of incoming clump
– Mass loading in shock
– generates velocity dispersion
Velocity dispersion in plane of galaxy
v  R
0 .5
Star Formation and Efficiencies
• Star formation requires:
– Orbit crowding
– shock
– Enough gas mass
• GMC lifetimes ~ 107 years (few dynamical times)
• Star Formation Efficiencies Low
– 5 to 30 % of gas mass formed into stars
» Without any feedback
• Why?
– Clouds globally unbound
– Majority of mass escapes
– Clouds disperse leaving spiral arms
Unbound Clouds and SF Efficiency
Clark et al 2004
•Globally unbound GMCs
•Local dissipation of turbulence
•Star formation
• SF involves ~10% of mass
Global disk simulations
• Clare Dobbs poster (no. 18)
• Goal: explore gas dynamics through multiple spiral
arm passages
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Non self-gravitating
4 armed spiral
Gas ring: 5 to 10 kpc (co-rotation 10 kpc)
Mass: 5 x 108 Msun
Isothermal (100 to 104 K)
Distribution: globally uniform, locally clumpy
– Post-processed H2 formation
• Bergin et al (2004)
T=100 K
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Location of
H2 gas
Size scale:
22kpc, 11kpc,
6kpc, 3kpc
Formation of Molecular Clouds
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Size ~ 4 kpc
Formation of H2
• Molecular gas formed in
spiral arms
– Higher density
– Higher extinction
• Giant Molecular Clouds:
– Almost completely in spiral
arms
• Mass components:
• 10 % over full disk
• 30-50 % in spiral arms
Azimuthal distribution of gas and H2
Spiral shocks and structure
generation
• Molecular cloud spacing~ 500 pc
– Not due to self-gravity
• Simulation produces spurs and
feathering
– Due to clumps in arms
– Sheared in the inter-arm region
– Disappears at higher gas temperatures
Velocity dispersion
• Velocity dispersion driven
by spiral shocks
• Due to clumpy shocks
• Velocity dispersion
increases in each spiral arm
passage
• Lower in interarm regions
Azimuthal distribution of velocity dispersion
A local viewpoint of spiral shocks
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Spot: motion of one gas particle
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1kpc region centred on gas particle
Conclusions
• Spiral shocks can trigger star formation
• Produce realistic GMCs
– Structures
– Kinematics (not turbulence)
• Low star formation efficiencies (clouds unbound)
• Global disk simulations
– Generates Spurs and feathering (when cold)
– Produce GMCs in spiral arms 10% of gas in H2
– Observable signatures
» as gas passes through shocks
Modelling Spiral Galaxies
Pass gas through Galactic potential, consisting of 3
components:
Disc: Logarithmic potential (Binney & Tremaine)
- Flat rotation curve, v0=220km/s
Spiral: Cox & Gomez (2002) (sum of 3 perturbations)
- Milky Way parameters with 4 arms
- Pattern speed of 210 -8 rad/yr -1
Halo: Caldwell & Ostriker (1981)
No self-gravity/ magnetic fields
r
rc
1  r rc 
2
Velocity dispersion in clumpy shocks
- Gas through 1D sinusoidal potential.
- Velocity dispersion flat and subsonic for uniform shock (-)
- Velocity size-scale relation  (v)  r 0.5 for clumpy shock (-)