Dark matter annihilation effects on the first stars.
The formation of the first generation of stars in the Universe is believed to be very different from all subsequent star formation episodes. The reduced cooling efficiency due to lack of metals at z> 20 together with the absence of magnetic fields and of relevant angular momentum effects, leads to the formation of a single massive star in the very centre of each halo (Ripamonti et al. 2002). In these conditions, the formation of a protostellar cloud at the centre of a halo could largely increase the central dark matter (DM) density through the adiabatic contraction mechanism (AC; Blumenthal et al. 1986 and Fig. 1, Iocco et al. 2008). This would boost the DM annihilation rate to a level that, eventually, the energy deposited in the surrounding gas would balance the one radiated away by H2 cooling (Spolyar, Freese and Gondolo, 2008). Apart from rough evaluations of the duration of the AC phase, that provide typical lifetimes ranging from hundred million years to a Hubble time (Freese et al. 2008, Natarajan et al. 2008), nothing is known on the evolution of such "dark" stars. Results: In this project we study the effects of weakly interacting massive DM particles (WIMPs) on the collapse and evolution of the first stars in the Universe (Iocco et al. 2008 a,b). Two main different processes may provide a significant energy source i) annihilation of DM particles that have adiabatically accreted during the collapse/contraction phase of the protostar and ii) annihilation of DM particles that are scattered/captured during the main phase of hydrogen burning. Both processes are followed in detail for a grid of metal-free stars forming in the center of a typical halo of 106 M⊙ at z=20.
The first process is important during the pre-main-sequence phase where DM particles of the parent halo are accreted in the protostellar interior by adiabatic contraction. Energy release from annihilation reactions cause a stalling phase which is transient, from a few 104 yr (M = 5 M⊙) to a few 103 yr (M = 600 M⊙), but much longer than the Kelvin-Helmholtz timescale (red line in Fig. 2) ii) The scattering/capture (SC) processes may or may not be important, depending on the assumed DM velocity dispersion, density and elastic scattering cross-section with baryons (blue line in Fig. 2). For our fiducial set of parameters (v=10km/s, ρ=1011 GeV cm-3 and σ0 =10-38 cm2) we find that the evolution of stars of mass M* < 40 M⊙ "freezes" on the HR diagram before reaching the zero-age main sequence (ZAMS). Stars with M* = 40 M⊙ ignite nuclear reactions; however, DM "burning" prolongs their lifetimes by a factor of 2 (5) for a 600 M⊙ (40 M⊙) star (Fig. 3). For ρ > 1012 GeV cm-3, and the same values of the other parameters, we find that all our models are entirely supported by DM annihilation and "freeze" on the HR diagram before igniting nuclear reactions.
Presently in our work we assume that stars of any mass between 3 and 600 M⊙ may form out of the initial cloud. With future studies we will address the following two particularly important points. A) The yet unexplored phases between virialization and the formation of the hydrostatic core, investigating whether the energy released by DM annihilation has an effect on the cooling of the gas, on the Jeans mass and on the fragmentation process (Ripamonti et al. 2008). B) The study of mass accretion onto the hydrostatic core during the AC phase in pre-main sequence stars, which will clarify the final mass range of primordial stars.
People:A. Bressan
Collaboration: A. Ferrara (Scuola Normale Superiore di Pisa), F. Iocco (CEA/Saclay), P. Marigo (UNIPD), E. Ripamonti (Universita' degli Studi dell'Insubria), R. Schneider (INAF- OAFI)
Publications: Iocco et al. (2008), MNRAS 390,1655; Iocco et al. (2008), IAUS 255,61; Ripamonti et al. (2008), PoS in press