Physics of Primordial Star Formation

Transcription

1 Physics of Primordial Star Formation Naoki Yoshida University of Tokyo SFDE17 Bootcamp, Quy Nhon, August 6th

2 So far, we learned 1. The standard cosmological model 2. Growth of density fluctuations 3. Assembly of dark matter halos

3 ELEMENTS NOW AND THEN Helium Hydrogen HUMAN BODY EARLY UNIVERSE

4 OBSERVATIONS OF HII REGIONS

5 HELIUM PRODUCTION simply read) why

6 HYDROGEN BURNING IN STARS The$net$result$of$hydrogen$burning$is$the$fusion$$ of$four$h$nuclei$into$one$ 4 He$nucleus.$ The$difference$in$binding$energy$is$ $MeV,$hence$~$7MeV$per$H.$ (Not$all$this$energy$is$emiEed$as$ photons$from$the$surface,$though.$ Think$about$ Where$does$the$rest$go$? )$

7 GALAXIES ARE NOT HELIUM FACTORIES One can calculate light-to-helium production ratio as follows: Let s take a galaxy with L ~ Lsun. Over a cosmological time of 10 Gyrs, it releases a total energy of ~ 7.5x10 73 ev. Its stellar mass would be ~ a few x10 11 Msun. Then it contains ~ 2 x hydrogen nuclei. This gives a mean energy output of 0.3 MeV per hydrogen nuclei. This is way too small compared with 7MeV per hydrogen burning. Galaxies (stars) are luminous, but not as much as it should, in order to produce helium to ~ 25% in mass. There must be another He factory.

8 BBN PREDICTION highly interesting

9

10 Contents 1. What Observations Tell Us 2. Formation of Primordial Gas Clouds 3. Thermal Evolution of a Primordial Gas 4. From Big Bang Ripples to a Protostar

11 STAR FORMATION AT HIGH-z WMAP

12 SURVIVING EARLY GENERATION STARS Almost no iron!! Number of stars Progress since last century Fe Fe Ca Iron abundance metallicity wrt sun There are stars in MW with very low metal content. Wavelength

13 Primordial Star Forma-on: Theory

14 Star forma-on in the early universe

15 Primordial gas chemistry e, H, H +, H -, H 2, H 2+, He, He +, He ++ D, D +, D -, HD, HD + Composition: 76% H, 24% He 10-5 D little Li Collisional ionization, recombination Formation of molecules (H2, HD, H3+, H2+, HD+, HeH) Photoionization, photo-dissociation Radiative cooling collisional excitation, collisional ionization, recombination, Bbremsstrahlung, compton cooling, CMB heating ~ reactions

16 Chemical reac-ons and rates Above is just a partial list.

17 PRIMORDIAL GAS CLOUD

18 Stars are formed We need a mechanism to cool a gas cloud. only How did all this happen in the early universe?

19 In the present-day universe

20 Primordial cooling func-on Galaxies H He+ H2

21 H2 Forma-on : Gas phase reac-ons Photo-a?achment H2 forma-on Slow reac-ons, relying on residual electrons as a catalyst. Become effec-ve at T > 1000 K (M ~ 10 5 M sun ) Molecular frac-on reaches ~ 0.001

22 Comological Recombination

23 H2 Forma-on : Gas phase reac-ons Photo-a?achment H2 forma-on Slow reac-ons, relying on residual electrons as a catalyst. Become effec-ve at T > 1000 K (M ~ 10 5 M sun ) Molecular frac-on reaches ~ 0.001

24 H2 Forma-on : Present-day Formation on dust grains H + H on dust H 2 + dust This reaction is much faster than the gas-phase reactions. Almost all the hydrogen atoms are converted to molecules at relatively low densities if there are a sufficient amount of dust grains.

25 H2 rota-onal transi-on A hydrogen atom hits a H 2 molecule in the ground-level (J=0). The H2 molecule got excited to J=2. After a while, it emits a photon spontaneously, to be back in the J=0.

26 H2 cooling rate A hydrogen atom hits a H 2 molecule in the ground-level (J=0). The H2 molecule got excited to J=2. After a while, it emits a photon spontaneously, to be back in the J=0.

27 H 2 cooling: A brief history

28 Primordial gas cloud Bromm, Coppi, Larson 2002 Physical properties of H2 molecules connected to thermal evolution. ΔE (J=2 0) ~ 512 K sets the minimum temperature. Non-LTE cooling to LTE cooling loitering.

29 Molecular frac-on What you get must be more than what you need what you need what you get = asymptotic fraction Temperature t_cool ~ t_hubble Tegmark et al. (1997) Left-over electroms H, H- Electron depletion + H, e-

30 Cosmological First Objects Matter distribution Web-like structure in the early universe. Halo mass ~ 1,000,000 Msun Gas clouds are ~ 1000 Msun. Strongly clustered. T age = 300 million years Computer simulation by Yoshida et al. (2003)

31 Result from a 3D cosmo. simula-on

32 THERMAL EVOLUTION OF A PRIMORDIAL GAS 10 4 T [K] adiabatic contraction H2 formation line cooling (non-lte) loitering (~LTE) number density 3-body reaction Heat release opaque to molecular line adiabatic, high-p Cooling collision rate from induced NLTE (~ density2) to LTE emission (~ density) dissociation opaque to continuum Jeans mass ~ T 1.5 /n 0.5 ~ 500 Msun

33 From a cloud to a protostar

34 0.3Mpc Self-gravitating cloud 5pc A new born proto-star with T * ~ 20,000K 0.01pc r ~ 10 Rsun! Fully-molecular core

35 THERMAL EVOLUTION OF A PRE-STELLAR GAS 10 4 T [K] The Physics adiabatic contraction H2 formation line cooling (non-lte) loitering (~LTE) 3-body reaction Heat release number density collision induced emission opaque to molecular line adiabatic, high-p dissociation opaque to continuum

36 A hydrogen molecule can Quantum energy levels Radiative cooling (gas temperature ) Chemical cooling and heating (T ) Interact with photons, cooling and heating

37 H2 formation by 3-body reactions At high densities, H2 molecules are formed by The reaction rate density3. The core becomes almost fully molecular Significant heat release (4.48eV per molecule)

38 Rot-vibra-onal transi-ons in LTE Par-cle number density ~ /cc, temperature ~1000K The level popula-on (energy distribu-on) is LTE for J = 0-20, v = 0-2 H 2 is a homo-nuclear molecule; no dipole transi-on quadropole transi-on (ΔJ = 2)

39 Radia-ve Transfer 1: molecular lines Example) H2 J=6 4 transi-on at T~1000 K For τ >0.1, the cloud core becomes optically thick to H2 lines and then line cooling is inefficient (τ 4,6 ~1 for Lcore~0.0001pc)

40 Net molecular cooling rate Λthick = Σ β escape n k,l A k,l hν optically thick / thin 3D calculation CIE cooling Omukai98 1D full RT log (n) NY, Omukai, Hernquist, Abel (2006)

41 isotropic Structure of a disk: with/without radiative transfer 3D effect important in post-collapse simulations cooling efficiency density line continuum

42 Collision Induced Emission hν continuum emission During collisions, a collision pair acts as a super-molecule, genera-ng an induced electric dipole.

43 Op-cally thin CIE cooling rate η (ν) = 2hν 3 c 2 σ n H2 exp(-hν/kt) H2-H2 (Borysow et al. 2001) H2-He (Jorgensen et al n ~

44 >10 16 Equilibrium chemistry: the Saha equa-ons A set of coupled equations including the energy equation are solved self-consistently: H-H+ H-H2

45 Cooling/hea-ng processes Chemical reaction

46 STATE OF THE ART COMPUTER SIMULATIONS Primordial Protostar A clump of dark matter Molecular gas cloud 1000 light-years 15 light-years Newborn proto-star Central core 0.1 astronomical unit 10 astronomical unit Yoshida, Omukai, Hernquist, 2008, Science