Two phases of WIMP dark matter annihilation in the First Stars Fabio Iocco INAF/Osservatorio Astrofisico di Arcetri TeVPA 2008, Beijing, september 25 th In collaboration with: A.Bressan, E. Ripamonti, R. Schneider, A. Ferrara, P. Marigo S.C. Yoon, S. Akiyama

The first stars (Population III) Form in halos of M h  10 6 Msun at z  20 (T vir < 10 4 K) First Stars  primordial chemical composition (BBN: no C,N,O -- A > 7) Weak cooling: H 2 vs CO  big masses No fragmentation, one star per halo Live fast, die young (30-300Msun go SNe) Hot: first engines for IGM Reionization (possibly) seed BH, correlated to quasars As of now, we have (very likely) not seen one yet Simulations! By courtesy of M. J. Turk

Gas (collisional) cooling and collapsing to the center “pulling in” DM (gravitationally) (modeled through adiabatic contraction) Spolyar, Freese & Gondolo 07 (PRL 100, 2008) Iocco + 08 (also Freese+ 08) Building the DM cusp

Powering the structure (with DM) Energy production DM profile critical! + Energy deposition Energy repartition for WIMP annihilation: 1/3 electrons 1/3 photons 1/3 neutrinos (lost) Absorption: Gas profile critical! Natarjan, Tan & O’ Shea 08

(only) First Stars can do it ! Spolyar, Freese & Gondolo 07 (PRL 100, 2008) At n gas ~10 12 #/cm 3 (and above) structure opaque to annihilation products

What happens, then? delicate equilibrium between accretion (or DM cusp build-up) and annihilation 100 Msun initial conditions: R=10 14 cm n gas =10 16 #/cm3

Evolving “Dark Stars” Adiabatic Contraction phase Acts at proto-stellar stages (or early pre-MS) at T  5000 K Adopted fiducial values: (m X =200GeV, shorter times)

Scattering and capture Halo WIMPs (originally outside the star) are captured by scattering off the gas of the star Captured WIMPs accumulate inside the star, thermalizing (needs some time) “sinking” to the center (most of annihilations is there)

Scattering: some more Capture rate depends on stellar characteristics And environemt (continuously capturing halo WIMPs)! Dark Matter annihilation inside first stars can overpower nuclear Needs continous replenishment!!!! Scott, Fairbairn & Edsjo 08 also excellent review DM in star can power only  10 yr!

So what, once they become “DM burner” (/Dark Stars)? They don’t “need” to burn nuclear Iocco + 08 See also: Salati & Silk 89 (low-mass-analitical) Fairbairn, Scott, Edsjo 08 (ZAMS low-mass) Taoso, Berone, Meynet & Esktrom 08 Yoon, Iocco & Akiyama 08 Scott, Fairbairn, Edsjo 08 Active in late stages of pre-MS on Hayashi track  KH <  DM

Prolonged lifetimes Taoso, Bertone, Meynet & Ekstrom 08 DM powered frozen stars are “frozen” as long as environmental DM stays supecritical frozen evolving  SD 0 = cm 2

Direct observation? (surviving the ages) (not actual size) Wechsler + 02 Bertone & Merritt 05 Halo merger DM cusp erosion (baryons + self-annihilation) z

Some more properties (useful for indirect detection) # ionizing photons! (tricky: increase or decrease depending on DM density) Chemistry (yeah: analize that!) Yoon, Iocco & Akiyama 08

Indirect observation? (surviving the models) Sources Structure Formation: Collapse of the cloud Metal Production H 2 dissociation UV Ionization of IGM Soft X-rays H 2 formation Cloud temperature IMF SFR Feedback! First studies on “Dark Stars” & Reionization; Schleicher, Benerjee & Klessen 08

Conclusions First phase of DM annihilation acts in proto-stellar phase, very likely transient. Yet to be explored in detail (depends mostly on ) Capture phase at ZAMS or slightly earlier, potentially everlasting (strongly depends on   SD -which is good-) Both phases are unlikely to be observed directly *but* studies on Reionization, 21cm, chemical evolution may help in the future…

It is a field in its infancy! Give us some more time, and stay tuned! Mauro Maiuri, born august 19 th 2008