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A Multiphase, Sticky Particle, Star Formation Recipe for Cosmology. Craig Booth Tom Theuns. Overview. Star Formation in Disk Galaxies & Properties of the ISM Simulating Star Formation & Feedback The Sticky Particle Model Results from a One Zone Simulation Summary. - PowerPoint PPT Presentation
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A Multiphase, Sticky Particle, Star Formation Recipe for
Cosmology
Craig BoothTom Theuns
Overview
1. Star Formation in Disk Galaxies & Properties of the ISM
2. Simulating Star Formation & Feedback
3. The Sticky Particle Model4. Results from a One Zone
Simulation5. Summary
Star Formation in Disk Galaxies
• Most stars form in Giant Molecular clouds
• Cloud dynamics are very complex and not very well understood
• The mechanism by which clouds form is unclear
Star Formation in Disk Galaxies
Blitz, 04
From the Astro-1 mission
• Schmidt (1959):
Kennicutt, 1998N~1.4
Star Formation in Disk Galaxies
Supernova Feedback
Model Overview
• The Physics we need to implement:– GMCs form in spiral arms
– Stars form from GMC collapse– Stellar winds destroy GMCs– Feedback drives superwinds &
regulates star formation
Simulating Star Formation
• Difficult problem for two reasons:– Scales of cosmological interest are
vastly different to those on which star formation takes place
– Simulation codes do not contain enough physics to accurately track star formation
• Take one of two approaches:1. Empirical rules2. Model the ISM statistically
Yepes et. al. 1997 Springel & Hernquist, 2003
2. Models of the ISM
• Need a simple ISM model:
Stars
HIMT~106
CNMT~100Kf~0.02
warmcloud coronaT~4000K
McKee & Ostriker, 1977
Three physical processes are important describing self-regulating star formation...
Models of the ISM
• Clouds form by the radiative cooling of the hot phase
Models of the ISM
• Clouds collapse into stars
Models of the ISM
• Stars go supernova and destroy clouds
Now treat each one in turn...
The Formation of Clouds
• Cooling Instability (Yepes et. al., 1997)
• if density > X and temperature allows for thermal instability then rather than cooling, hot gas is assumed to collapse into clouds
Sutherland & Dopita, 1993
The Multiphase Model
• Yepes et. al. formulated differential equations that describe:– the rate of formation of clouds– the rate of collapse of clouds to stars– the rate of supernova energy injection
cold
hot
The Multiphase Model
• Drawbacks:– coupling between
hot and cold gas– assumes pressure
equilibrium between hot & cold phases
– carries no information about the properties of the cold gas
Springel & Hernquist, 2003
The Sticky Particle Model
• Follow the same format with our model.
• Treat each process separately:– formation of clouds– coagulation of clouds into GMCs– collapse of GMCs– star formation
The Formation of Clouds
• In our simulations 'cloud particles' form as in Yepes et. al. 1997 (thermal instability)
• Store the mass function for every cloud. Evolve the 'clouds' and 'cloudlets' differently
• Unresolved clouds are called 'cloudlets'N
M
The Coagulation of Clouds
vm is a parameter in our simulations
• Clouds are treated as ballistic particles, following a couple of very simple rules upon collision:
– vapp < vm
– vapp> vm
Collision
Cooling
The Coagulation of Cloudlets
• We want the cloudlets to behave in exactly the same way as the clouds
• Integrate coagulation equation (and similar equations for energy evolution) to evolve system
Smoluchowski, 1916
GMC Collapse & Star Formation
• Giant Molecular Clouds are defined to be 106 solar masses
• When we form a GMC it lives for one dynamical time (~10Myr) then collapses.
• Some fraction of its mass becomes stars, the rest is fragmented into tiny clouds.
• This represents formation & coagulation of clouds and destruction of clouds by star formation
Results From the One Zone Model
• Set up 1kpc3 region• Density comparable to that in a MW
spiral arm• Evolve for 200Myr• Both as 'pure cloudlet' and hybrid sticky
particle/cloudlet
Results From the One Zone Model
pure cloudlet run
Observed cloud mass spectrum index -1.6 to -1.9
Results From the One Zone Model
Results From the One Zone Model
delay, SFR
Results From the One Zone Model
• SFR Shows little dependence on particle number• Cloudlets behave exactly like clouds• Higher resolution gives better spatial resolution
Schmidt Law
Summary
• Statistical star formation model• One zone simulation:
– reproduces cloud mass spectrum, velocity dispersion & SFR in Milky Way conditions
– Schmidt law as an output– Resolution independence
• Avoids some problems of the Multiphase model
• Provides a natural mechanism for delay
Thank you for listening!Thank you for listening!
Thank you for listening!
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