1. FORMATION OF THE FIRST STARS IN THE UNIVERSE
Origin of Matter and Evolution of Galaxies, Sapporo, December 4, 2007
FORMATION OF THE FIRST STARS IN THE UNIVERSE
NAOKI YOSHIDA
DEPARTMENT OF PHYSICS NAGOYA UNIVERSITY

2. CONTENTS Cosmological Background Thermal Evolution of a Primordial Gas
- Dark matter, dark energy, and light elements
Thermal Evolution of a Primordial Gas
- Physics at high densities (chemistry, radiative transfer)
- Chemo-thermal instability and fragmentation
- Accretion physics and the mass of the first stars
Formation of Primordial Stars in a Reionized Gas
Msun primordial stars and hypernovae/GRB
Primordial Stars and Dark Matter

3. PRIMORDIAL STAR FORMATION A ’SIMPLE’ PROBLEM
The Initial Condition:
CDM model, Gaussian random density field
dark matter + hydrogen-helium gas + CMB　　　　
The Physics:
gravity, hydro.,
atomic/molecular processes
14 species, equilibrium, non-eq.
　e, H, H+, H-, H2, H2+,
　He, He+, He++, D, D+, D-, HD, HD+
50 reactions
+ many radiative processes
Density evolution to ~1021 cm-3
Initial density field

4. CONCORDANCE +COLD DARK MATTER MODEL
Gaussian random field (inflation)
=> We need only a power spectrum
Energy content
Mpc Gpc
Length scale

5. BRIEF HISTORY OF THE UNIVERSE
１３.７ Gyr
FIRST STARS
★★★
１ Gyr
Galaxies
Big Bang
Dark Age
Recombination,
CMB last scattered
Cosmic
Reionization

6. STAR-FORMATION IN THE EARLY UNIVERSE
Cosmological recombination at z = (380Kyrs)
　　　　　　　　　↓
Non-linear structure formation and dark halo assembly
Virialization and H2 formation
(gas-phase reaction by left-over e)
Molecular cloud formation at the center of DM halos
Runaway collapse when a cloud gets large enough
(=What cosmology people call “star-formation”)

7. COSMOLOGICAL MOLECULAR GAS CLOUDS
Yellow spots at
the intersections
of filaments
“1 cloud per halo”
Host dark halos:
M ~ 106 Msun
Tvir ~ 1000 K
Strongly clustered,
large bias
Gas
NY, Abel, Hernquist, Sugiyama (2003, ApJ)

8. THERMAL EVOLUTION OF A COLLAPSING PRIMORDIAL GAS
hydrostatic core
real gas effect
104
collision
induced
emission
H2 formation
line cooling
　(NLTE)
Effective Equation of State
from a first-principle calculation
T [K]
3-body
reaction
Heat
release
opaque to
continuum
and
dissociation
103
loitering
(~LTE)
opaque to
molecular
line
adiabatic
contraction
102
number density

9. COSMOLOGICAL SIMULATIONS
Standard CDM model
Multi-level zoom-in technique
final mass resolution 100Mpluto
final spatial resolution ~Rsun
Hydro, eq/neq-chemistry ,
radiative processes, etc. etc.
NY, Omukai, Hernquist, Abel (2006, ApJ)
Gao et al. (2007, MNRAS)
NY, Omukai, Hernquist (2007, ApJL)

10. 5pc Self-gravitating cloud 0.3Mpc 0.01pc A new born proto-star
with T* ~ 20,000K
0.01pc
r ~ 10 Rsun!
Fully-molecular core

11. First stars likely very massive
Abel+02, Bromm+02, Omukai & Palla03, Yoshida+06
1 Large Jeans-mass at the onset of collapse
(a reservoir of ~ 1000 Msun gas,
but overall very low star-formation efficiency)
2 No vigorous fragmentation during
the final collapse
3 Large accretion rate (high Tenvelope)
dM/dt ~ cs3/G > Msun/yr

12. CHEMO-THERMAL INSTABILITY: NUMERICAL RESULTS2
1.5 1
0.5
growth parameter
tff/tg becomes larger than 1,
but always below 2.
Perturbation growth and
gravitational collpase occur
on a similar time scale.
The cloud does not
fragment to multiple objects
Collapse is just
accelerated.
tfree fall / tgrowth
log (n)
NY+06,07 ApJ

13. PRIMORDIAL PROTO-STAR - A TINY SEED IN A LARGE CLOUD
Gas accretion on to
A hydrostatic core
(a proto star)
There stands a strong
accretion shock
原始星の半径(accretion shock)
と質量の進化を計算
accretion shock
hydrostatic
core
outer envelope

14. ACCRETION RATE AND PROTOSTAR EVOLUTION
dM/dt = Msun/yr MZAMS = Msun
NY, Omukai, Hernquist, Abel (2006)
We used the obtained accretion rate as an input
to the proto-stellar evolution calculation

15. PRIMORDIAL STAR FORMATION IN A CDM UNIVERSE
No fragmentation is observed during the prestellar collapse. The parent cloud is a single ~300 Msun
cloud at the center of a cosmological mini-halo.
The collapsing cloud is stable against gravitational deformation, too.
A tiny proto-stellar seed with mass ~ 0.01 Msun
is formed first.
Proto-stellar evolution calculations give
MZAMS ~ 100 Msun

16. PRIMORDIAL BINARIES: ROTATION-INDUCED BREAK UP
Simulations by Machida+ 07
Binaries and multiples are
common products in the
final collapse stage
Tabulated EoS,
highly symmetic set-up
Cosmological set-up
1AU

17. RADIATION STOPS ACCRETION
Semi-analytic model calculation
by McKee & Tan
Msun
Mstar

18. First stars massive or very massive ?
Recent theoretical works and simulations mostly
suggest the first generations stars are rather massive
(> 100Msun)
In the elemental abundance patterns of galactic
very metal-poor stars, there is no indication for
pair-instability supernova
(with progenitor mass Msun).
Why ?

19. The first light and 2nd generation stars
Radiation-hydrodynamic simulation
of early cosmic reionization
Effect of HD cooling
in the ‘late’ collapsing object
T [K]
TCMB at z=16
NY, Oh, Kitayama, Hernquist (2007, ApJ)

20. 2nd generation primordial star
NY, Omukai, Hernquist (2007, ApJL, 667, 117)
Primordial stars in a
reionized gas
are not very massive
1st star
Hydrogen-burning
starts at M~30Msun
MZAMS ~ 40Msun
Mcloud ~ 40Msun
2nd. gen. star
with HD cooling

21. Reionization and 2nd gen. stars
1 Parent gas cloud mass, accretion rate both
substantially smaller than “ordinary” PopIII cases
=> M < 40 Msun
2 The progenitor mass of the supernova that
triggered the formation of the OBSERVED HMP
stars is suggested to be ~ Msun
(Iwamoto et al. 2005)
3 Also in a hypernova-GRB progenitor mass range:
Massive primordial stars formed during/after reionization
might trigger high-z GRBs.

22. SF IN A LOW-METALLICITY GAS
Omukai et al. 2005
Dust thermal emission is likely to be a key
mechanism for low-mass star formation

23. First stars and dark matter
Warm Dark Matter simulation
Gao & Theuns (2007, Science)
Neutralinos as dark matter
Spolyer et al. (2007, PRD)

24. Primordial star formation Things to explore further:
Theory: accretion process,
feedback from proto-star
(early formation stage done.)
Observation:
HMP search and hunting for evidence
of (non-)existence of very massive
stars and pair-instability SN.