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A Multiphase, Sticky Particle, Star Formation Recipe for Cosmology Craig Booth Tom Theuns & Takashi Okamoto
A Multiphase, Sticky Particle, Star Formation Recipe for
Cosmology
Craig BoothTom Theuns & Takashi Okamoto
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
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
Results From the One Zone Model
Results From the One Zone Model
Results From the One Zone Model
Results From the One Zone Model
Results From the One Zone Model
Results From the One Zone Model
Results From the One Zone Model
Good match with Milky Way properties
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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