Star Formation at
Very Low Metallicity
Anne-Katharina Jappsen
Collaborators
 Simon Glover, Heidelberg, Germany
 Ralf Klessen, Heidelberg, Germany
 Mordecai-Mark Mac Low, AMNH, New York
 Spyridon Kitsionas, AIP, Potsdam, Germany
The Initial Mass Function
From Pop III Stars to the IMF?
star formation in the early universe:
 30 Msun < M < 600 Msun (e.g. O’Shea & Norman 07)
 Z = 0 (Pop III) ➞ Z < 10-3 Zsun (Pop II.5)
 Mchar ~ 100 - 300 Msun
present-day star formation:
 0.01 Msun < M < 100 Msun
 Z > 10-5 Zsun , Z = Zsun
 Mchar ~ 0.2 Msun
Critical Metallicity
Bromm et al. 2001:
 SPH-simulations of
collapsing dark matter
mini-halos
 no H2 or other molecules
 no dust cooling
 only C and O atomic
cooling
10-4 Zsun < Zcr < 10-3 Zsun
Dependence on Metallicity
Omukai et al. 2005: one-zone model,
H2 , HD and other molecules, metal cooling, dust cooling
102 M
sun
1 Msun
10-2 Msun
t=1
Present-day star formation
Omukai et al. 2005: one-zone model,
H2 , HD and other molecules, metal cooling, dust cooling
t=1
Z=0
Dependence on Z at low r
Omukai et al. 2005: one-zone model,
H2 , HD and other molecules, metal cooling, dust cooling
t=1
Numerical Model
 Smoothed Particle Hydrodynamics
 Gadget-1 & Gadget-2 (Springel et al. 01,
Springel 05)
 Sink particles (Bate et al. 95)
 chemistry and cooling
 particle splitting (Kitsionas & Whitworth 02)
Chemical Model
Cooling and Heating
 gas-grain energy transfer
 photoelectric effect
 H collisional ionization
 H2 photodissociation
 H+ recombination
 UV pumping of H2
 H2 rovibrational lines
 H2 formation on dust grains
 H2 collisional dissociation
 Ly-alpha & Compton
cooling
 Fine-structure cooling from
C, O and Si
Dependence on Metallicity
at Low Density
 gas fully ionized
 initial temperature: 10000 K
 centrally condensed halo
 contained gas mass:
17% of DM Mass
 number of gas particles:
105 – 106
 resolution limit:
20 MSUN – 400 MSUN
Dependence on Metallicity
at Low Density
 halo size: 5 x 104 Msun – 107 Msun
 redshift: 15, 20, 25, 30
 metallicity: zero, 10-4 Zsun,
3
Zsun, 10-2 Zsun, 0.1 Zsun
 UV background: J21 = 0, 10-2,
10-1
 dust: yes or no
(Jappsen et al. 07)
10-
Dependence on Metallicity
at Low Density
Influence of Different Initial
Conditions
example I
centrally condensed halo
hot, ionized initial
conditions
example II
• NFW profile, rs = 29 pc
solid-body rotating top-hat
(cf. Bromm et al. 1999)
cold initial conditions
with dark matter
fluctuations
• T = 10000 K
• top-hat approximation
• z = 25
• T = 200 K
• MDM = 8 x 105 Msun
• MDM = 2 x 106 Msun
• Mres,
• Mres,
gas
= 1.5 Msun
gas
= 12 Msun
Example I
after 52 Myrs
CMB
Example II
 Rotating top-hat with dark
matter fluctuations and
cold gas initially:
gas fragments no matter
what metallicity, because
unstable disk builds up
(Jappsen et al. 09)
H2 is the dominant coolant!
“critical metallicity” only
represents point where
metal-line cooling
dominates molecular
cooling
Conclusions – so far
 H2 is the dominant and most effective coolant
 different initial conditions can help or hinder
fragmentation ⇒ we need more accurate
initial conditions from observations and
modeling of galaxy formation
 there is no “critical metallicity” for
fragmentation at densities below 105 cm-3
 Transition from Pop III to modern IMF maybe at
higher densities due to dust-induced
fragmentation:
Dependence on Z at high r
Omukai et al. 2005: one-zone model,
H2 , HD and other molecules, metal cooling, dust cooling
t=1
Dust-induced Fragmentation
 Clark et al. 2008 study dust-induced
fragmentation in 3D numerical simulations of star
formation in the early universe
 dense cluster of low-mass protostars builds up:
 mass spectrum peaks below 1 Msun
 cluster VERY dense (nstars = 2.5 x 109 pc-3)
 fragmentation at density ngas = 1012 - 1013 cm-3
Conclusions
 H2 is the dominant and most effective
coolant at n < 105 cm-3
 there is no “critical metallicity” for
fragmentation at densities below 105 cm-3
 different initial conditions can help or hinder
fragmentation ⇒ we need more accurate
initial conditions from observations and
modeling of galaxy formation
 Transition from Pop III to modern IMF maybe
at higher densities due to dust-induced
fragmentation at Z = 10-5 Zsun