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Formation of the First Stars Under Protostellar Feedback
Athena Stacy
First Stars IV 2012
Collaborators
• Volker Bromm (U.Texas)• Andreas Pawlik (U. Texas)• Thomas Greif (MPA)• Avi Loeb (Harvard/CfA)
Dark Ages
Open Questions
- What role did they play in reionization and metal enrichment?
- What feedback did they exert on later star formation? (Pop III to Pop II transition)
This depends on the Pop III IMF, SFR, and rotation rates…
- What were their typical masses?
- What was their typical multiplicity?
- When will a Pop III protostar’s accretion become shut off by feedback ?
or ???or
I. Pop III Star Formation Without Feedback
Stacy, Greif, & Bromm, 2010 MNRAS, 403, 45
???
Cosmological simulation:
- Gadget (SPH + N-body)
-initialized at z=100 according to CDM model
- followed formation of protostar (sink particle) and subsequent 5000 yr of accretion
- msph (gas) = 0.015 M - Mres ~ 1.5Nneighmsph ~ 1 M
= minimum allowed Jeans mass
Initial Collapse
IGM
minihalo sink
3-body reactions and H2
formation
(time)
Sink Particles
By using sink particles, we can continue following evolution of star-forming gas for thousands more years (~ 100 freefall times)!
• Msink = 1 M
• n = 1012 cm-3
• racc ~ 50 AU ~ 1015 cm• R ~ 1011 cm Density no longer evolved• Accrete gas particles
that fall within racc of
sink
50 AU
Pop III stars can form in multiples!
Multiple stars form within a disk that has grown to ~ 40 M (tacc = 5000 yrs)
5000 AUDensity[cm-3]
Binary and Multiple Formation
• Toomre Fragmentation criterion:• Q ~ 0.4 < 1
• tcool < trot and
(Gammie 2001, Kratter et al. 2010, 2011)• Multiple sinks form through disk fragmentation
Rapid Pop III Accretion Rates
Sink B:Msink ~ t0.25
dM/dt ~ t-0.75
Sink A:Msink ~ t0.5
dM/dt ~ t-0.5
B&L 2004
sink B
sink A
Mfinal > 100 M
II. Pop III Star Formation With Radiative Feedback
Stacy, Greif, & Bromm, 2012, MNRAS, 422, 290
Protostellar Feedback
• Repeat previous cosmological simulation, but with updated H2 cooling rates (Sobolev approximation)
• Model LW radiation and growth of surrounding HII region
• Also performed a comparison “no-feedback” simulation
• How will radiation alter the growth of the Pop III star?
-200 radial segments-105 angular sements-107 bins
The I-front Tracker
M* = Msink
R* = ?
The Protostellar Model
Hosokawa et al. 2010
Hosokawa et al. 2010
Adiabatic accretion
KH contraction
ZAMSAdiabatic expansion
KH contraction
ZAMS
The Protostellar Model
Adiabatic expansion
KH contraction
ZAMS
Slowcontractionmodel
Simulation model
M*=Msink
I-front breakout
Ionization rate
Mass infall rate
M* = 15 M
I-front Evolves in Morphology
1500 yr 2500 yr 4500 yr
Temperature Structure
2500 yr
3000 yr
With feedback
Sink potential well heating
I-front
Warm neutral bubble
Without feedback5000 yr
4000 yr
2500 yr
2500 yr
With Feedback
* = main sink
+ = secondary sink
Feedback Slows Disk Growth
No feedback
With feedback
Envelope = gas with n>108 cm-3
WithoutFeedback
Density,
x-y plane
Density,
x-z plane
Temperature,
x-y plane
Temperature,
x-z plane
Box = 10,000 AU
Disk disruptedby N-body dynamics
Reduced Accretion Rate
With feedback
Msink ~ t0.09
Without feedback
Msink ~ t0.13
2nd largest sink(with feedback)
Mfinal ~ 30 M
III. Pop III Formation Under Dark Matter Effects?
Stacy, Pawlik, Bromm, & Loeb 2012, MNRAS, 421, 894
WIMP annihilation important for Pop III stars?
(www.nasa.gov, Sky and Telescope, Gregg Dinderman)
Can it heat SF gas, or replace/supplement nuclear fusion?
Pop III stars form in regions of high DM density
May lead to extremely massive and luminous Pop III stars
(e.g., Freese et al. 2008, Spolyar et al. 2008, Ioccoet. al 2008, Natarajan et al. 2009)
-R* ~ 1 AU-Teff too low to ionize-Accretion unimpeded for long time
a.k.a. “dark stars”
DM heating and capture rates
Higher DM density greater effect on gas and stars
1. DM heating delayed protostellar contraction prolonged accretion (M* reaches 105 M?)
2. DM capture by MS star burn DM instead of hydrogen prolonged stellar lifetime (to z=0?)
But will this work in a Pop III MULTIPLE system?
New DM Initialization
1. Begin simulation immediately after the first sink has formed2. Align DM peak with main sink3. Continue simulation for 20,000 yrHow will these DM profiles evolve? Will density stay peaked?
Decline of DM Density on Stellar Disk Scales
10,000 yr
20,000 yr
10,000 yr
20,000 yr
Minimum DM density for DM capture to support a star
Sinks and DM density peak UNALIGNED
Rapid Decline of DM Effect on Gas and Stars
Blue – DM capture rate within sinks (-> DM density)Black – DM heating rate within sinks (-> DM density2)
Pop III Mass Growth Relatively Unaffacted
Sim A
Sim B
DM unrefined
Conclusions• Range of Pop III masses is likely very broad.
• Multiple mechanisms, particularly disk fragmentation, will contribute to formation of low mass stars.
• Fragmentation and broad mass range likely to describe Pop III stars even under radiative feedback! Possibly massive binaries
• Pop III stars can likely reach tens of solar masses, but hundreds of solar masses may be harder (see also Susa and Hosokawa’s talks)
Maybe explains why PISN signature has not been
observed (requires 140M < M* < 260M)
• Pop III multiplicity will strongly mitigate effects of DM annihilation - ‘dark stars’ unlikely (see also Smith’s poster and Iocco’s talk)
• Growing understanding of Pop III stars will ultimately increase physical realism of models of later star and galaxy formation
Questions?
THE END
Thank you!