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 MetallicityOmukai et al. 2005: one-zone model,H2 , HD and other molecules, metal cooling, dust cooling

t = 1

102 Msun

1 Msun

10-2 Msun

Present-day star formationOmukai et al. 2005: one-zone model,H2 , HD and other molecules, metal cooling, dust cooling

t = 1

Z=0

Dependence on Z at low rOmukai 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

H collisional ionization

H+ recombination

H2 rovibrational lines

H2 collisional dissociation

Ly-alpha & Compton cooling

Fine-structure cooling from C, O and Si

photoelectric effect

H2 photodissociation

UV pumping of H2

H2 formation on dust grains

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, 10-3 Zsun, 10-2 Zsun, 0.1 Zsun

UV background: J21 = 0, 10-2, 10-1

dust: yes or no

(Jappsen et al. 07)

Dependence on Metallicity at Low Density

Influence of Different Initial Conditions

example I

centrally condensed halo hot, ionized initial conditions

• NFW profile, rs = 29 pc

• T = 10000 K

• z = 25

• MDM = 8 x 105 Msun

• Mres, gas = 1.5 Msun

example II

solid-body rotating top-hat (cf. Bromm et al. 1999) cold initial conditions with dark matter fluctuations

• top-hat approximation

• T = 200 K

• MDM = 2 x 106 Msun

• Mres, gas = 12 Msun

Example I

CMB

after 52 Myrs

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 rOmukai 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