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

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 - 10-40Msun primordial stars and hypernovae/GRB Primordial Stars and Dark Matter

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

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

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

STAR-FORMATION IN THE EARLY UNIVERSE Cosmological recombination at z = 1089 (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”)

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 distribution@z=17 NY, Abel, Hernquist, Sugiyama (2003, ApJ)

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

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)

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

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 > 0.01 - 0.001 Msun/yr

CHEMO-THERMAL INSTABILITY: NUMERICAL RESULTS 2 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 9 10 11 12 log (n) NY+06,07 ApJ

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

ACCRETION RATE AND PROTOSTAR EVOLUTION dM/dt = 0.1-0.001 Msun/yr MZAMS = 60-100 Msun NY, Omukai, Hernquist, Abel (2006) We used the obtained accretion rate as an input to the proto-stellar evolution calculation

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

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

RADIATION STOPS ACCRETION Semi-analytic model calculation by McKee & Tan 60-300 Msun 10 100 1000 Mstar

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 140-260 Msun). Why ?

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)

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

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 ~ 20-40 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.

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

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

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.