Feedback Effects of the First Stars

on Nearby Halos

Kyungjin AhnThe University of Texas at Austin

The End of the Dark Ages

STSCI

March 13, 2006

Outline

Introduction

Code Description

Initial Setup

Result

Conclusion

Dark Ages and Reionization End of dark ages – reionization – is only observed indirectly

WMAP 1st year result : Need for high-redshift reionization sources Gunn-Peterson Effect Ly Forest Temperature

First Stars Prime candidate for early reionization sources Forms by H2 cooling Feedback effects may be self-regulating (e.g. Haiman, Abel, Rees 2000)

Feedback effects of the First Stars

Feedback Effects (positive vs. negative for further star formation) Negative – star formation quenched

H2 is fragile: dissociation by Lyman-Werner band photons (Haiman, Abel, Rees 2000; Machacek, Bryan, Abel 2001)

Positive – star formation promoted Hard photons partially ionize IGM to create H2

(Haiman, Rees, Loeb 1996; Ricotti, Gnedin, Shull 2002)

Feedback effects of the First Stars Feedback Effects of the First Stars onto Nearby

Collapsed Objects (study by 3-D simulations) O'shea et al. (2005)

Assume full ionization of nearby halos of M~5*105 Msolar

Quick formation of H2 after source dies Inner core collapses; Outer region evaporates

Alvarez, Bromm, Shapiro (2005) Track I-front propagation through nearby halos of

M~5*105 Msolar

I-front slows down and being trapped. I-front fails to reach the center: Center remains neutral Neutral center: no further formation of H2 after source

dies at the center Negative feedback then??

Feedback effects of the First Stars Alvarez, Bromm, Shapiro (2005)

Fig. 8.— Volume visualization at z = 20 of neutral density field (blue – low density, red – high density) and I-front (translucent white surface). Top row panels show a cubic volume ∼ 13.6 kpc (proper) across, middle row ∼ 6.8 kpc, and bottom row ∼ 3.4 kpc. Left column is at the initial time, middle column shows simulation at t∗ = 3 Myr for the run with stellar mass M∗ = 80M⊙, and the right column shows simulation at t∗ = 2.2 Myr for the run with stellar mass M∗ = 200M⊙. The empty black region in the lower panels of middle and right columns indicates fully ionized gas around the source, and is fully revealed as the volume visualized shrinks to exclude the I-front that obscures this region in the larger volumes above.

Feedback effects of the First Stars(In collaboration with Paul Shapiro)

Feedback Effects of the First Stars onto Nearby Collapsed Objects (study by 1-D simulation) Use 1-D radiation, hydrodynamics code

Full treatment of primordial chemistry, radiative transfer, cooling/heating, hydrodynamics

1-D spherical geometry Ultra high resolution possible Analysis relatively easier than 3-D

Follow I-front propagation of the radiation from outer source in detail Is I-front trapped? What happens to the center? Any H2 formation/dissociation interesting? Is it positive or negative feedback effect?

1-D Spherical, Radiation-Hydro Code

Gravity Dark matter: use fluid approximation. Better than radial

shells. Ocasionally frozen gravity is not a bad approximation.

Baryon: Gravity involved hydrodynamics. Chemistry

Solve primordial chemistry, neglecting HD and HLi. H, H-, H+, H2, He, He+, He++, e

ionization, dissociation, recombination, radiative transfer Cooling/Heating

excitation, recombination, free-free, H2

photoheating adiabatic compression/rarefaction

Initial Setup Experiment 1 (Artificial)

Test O'shea et al. result Fully ionize the target halo without disturbing the

structure Let it evolve without source

Experiment 2 (Realistic) Start out with a halo profile (TIS profile) Abundance of electron and H2 molecule with equilibrium

value of a given halo: Departs from primodial values xe=10-4, xH2=2x10-6

Send plane-parallel, black-body radiation from outside

120Msolar, 105K, 106.24Lsolar, tstar=2.5 Myr Place different-mass target halos at 360pc.

Rule of Thumbs H atomic cooling : down to ~10000K collisional ionization : at > ~10000K photo-ionization front : thickness ~ mean free path of

ionizing photons R-type ionization front: I-front moves supersonically into

neutral region; gas doesn’t respond dynamically D-type ionization front: I-front moves subsonically into

neutral region; gas responds, shock-front develops Primary H2 formation mechanism

H + e H- + H- + H H2 + e H2 cooling : down to ~100K at H2/H >~ 10-4

Low temperature, T <~ 1000K, required to have H2/H >~ 10-

4 to be safe from collisional dissociation H2 self-shielding effective at N(H2)>~1014 cm-2 in static.

Experiment 1 (O’shea et al. type) No radiation

(after star died) Fully-ionized gas

quickly forms a lot of H2

Core region quickly cools to ~100K

Outer region evaporates

Again, can this full ionization happen in the first place?

Initial Setup Experiment 2 (Realistic)

Result: Experiment 2 (collapse fails)

105 Msolar target halo

Movie during the lifetime of star (2.5 Myr)

Enet=kinetic energy + thermal energy + potential energy

Enet<0 collapse

Enet>0 exodus

105 Msolar target halo

Movie after the lifetime of star

Result: Experiment 2 (collapse fails)

Result: Experiment 2 (collapse successful)

4x105 Msolar target halo

Movie during the lifetime of star (2.5 Myr)

4x105 Msolar target halo

Movie after the lifetime of star

Cooling at the center & in H2 precursor shell leads to negative net energy collapse of neutral region.

Result: Experiment 2 (collapse successful)

Result: Features to note

I-front slows down, finally gets trapped. Transition to D-type front Precursor H2 shell formation

Long mean-free-path of ionizing photons Partial ionization -> H- formation -> H2 formation Shielding + H2 molecule cooling

Shock-front is driven, with T~1000 - 10000 K Heating!! Shock-front accelerates in constant-density core Accelerates up to >~ 10000K heated enough to lead to

collisional ionization electron, H2 formation: Basically identical to Shapiro & Kang 1987, with vs~15 km s-1

H atomic cooling + H2 molecule cooling

H2 Shell Forms through electrons in the partially

ionized region Gains substantial column density, of

order ~1016-1018 cm-2. Self-shields against dissociating photons. (Not completely, though, because of peculiar motion of H2 precursor shell)

Neutral region sees weakned dissociating photons.

Helps cool the gas against shock-heating fragmentation?

H2 Shell Structure

Ricotti, Gnedin, Shull 2001 Into static IGM

Our result Into minihalos

Conclusion Minihalos (target) nearby the first Stars (source) I-front trapped; Ionized gas evaporates H2 formation in evaporating gas doesn’t help, just

evaporates H2 shell forms ahead of I-front: shielding

dissociation + cooling Shock is driven to the neutral region: active heating

collisional ionization. (universal?) Competition between H2 cooling & shock-heating

determines the fate of neutral region. Higher the mass, more efficient the cooling -> critical minihalo mass for hosting 2nd generation stars.

In preparation Wider parameter search Jeans mass? IMF? Sequential star formation? Photon budget?

See Brian O’shea’s talk too (Another feedback).