The chemical signature of surviving Population III stars in the Milky Way

Cosmological simulations of Population (Pop) III star formation suggest that the primordial initial mass function may have extended to sub-solar masses. If Pop III stars with masses < 0.8 M_Sun did form, then they should still be present in the Galaxy today as either main sequence or red giant stars. Despite broad searches, however, no primordial stars have yet been identified. It has long been recognized that the initial metal-free nature of primordial stars could be masked due to accretion of metal-enriched material from the interstellar medium (ISM). Here we point out that while gas accretion from the ISM may readily occur, the accretion of dust from the ISM can be prevented due to the pressure of the radiation emitted from low-mass stars. This implies a possible unique chemical signature for stars polluted only via accretion, namely an enhancement in gas phase elements relative to those in the dust phase. Using Pop III stellar models, we outline the conditions in which this signature could be exhibited, and we derive the expected signature for the case of accretion from the local ISM. Intriguingly, due to the large fraction of iron depleted into dust relative to that of carbon and other elements, this signature is similar to that observed in many of the so-called carbon-enhanced metal-poor (CEMP) stars. We therefore suggest that some fraction of the observed CEMP stars may, in fact, be accretion-polluted Pop III stars.Comment: 9 pages, 9 figures, 1 table; accepted for publication in MNRA

The Early Growth of the First Black Holes

With detections of quasars powered by increasingly massive black holes (BHs) at increasingly early times in cosmic history over the past decade, there has been correspondingly rapid progress made on the theory of early BH formation and growth. Here we review the emerging picture of how the first massive BHs formed from the primordial gas and then grew to supermassive scales. We discuss the initial conditions for the formation of the progenitors of these seed BHs, the factors dictating the initial masses with which they form, and their initial stages of growth via accretion, which may occur at super-Eddington rates. Finally, we briefly discuss how these results connect to large-scale simulations of the growth of supermassive BHs over the course of the first billion years following the Big Bang.Comment: 13 pages, 9 figures, invited review accepted for publication in PAS

Suppression of accretion onto low-mass Population III stars

Motivated by recent theoretical work suggesting that a substantial fraction of Population (Pop) III stars may have had masses low enough for them to survive to the present day, we consider the role that the accretion of metal-enriched gas may have had in altering their surface composition, thereby disguising them as Pop II stars. We demonstrate that if weak, Solar-like winds are launched from low-mass Pop III stars formed in the progenitors of the dark matter halo of the Galaxy, then such stars are likely to avoid significant enrichment via accretion of material from the interstellar medium. We find that at early times accretion is easily prevented if the stars are ejected from the central regions of the haloes in which they form, either by dynamical interactions with more massive Pop III stars, or by violent relaxation during halo mergers. While accretion may still take place during passage through sufficiently dense molecular clouds at later times, we find that the probability of such a passage is generally low (< 0.1), assuming that stars have velocities of order the maximum circular velocity of their host haloes and accounting for the orbital decay of merging haloes. In turn, due to the higher gas density required for accretion onto stars with higher velocities, we find an even lower probability of accretion (~ 0.01) for the subset of Pop III stars formed at z > 10, which are more quickly incorporated into massive haloes than stars formed at lower redshift. While there is no a priori reason to assume that low-mass Pop III stars do not have Solar-like winds, without them surface enrichment via accretion is likely to be inevitable. We briefly discuss the implications that our results hold for stellar archaeology.Comment: 9 pages; 2 figures; MNRAS accepte

The cooling of shock-compressed primordial gas

We find that at redshifts z > 10, HD line cooling allows strongly-shocked primordial gas to cool to the temperature of the cosmic microwave background (CMB). This temperature is the minimum value attainable via radiative cooling. Provided that the abundance of HD, normalized to the total number density, exceeds a critical level of ~ 10^{-8}, the CMB temperature floor is reached in a time which is short compared to the Hubble time. We estimate the characteristic masses of stars formed out of shocked primordial gas in the wake of the first supernovae, and resulting from the mergers of dark matter haloes during hierarchical structure formation to be ~ 10 M_{solar}. In addition, we show that cooling by HD enables the primordial gas in relic H II regions to cool to temperatures considerably lower than those reached via H_2 cooling alone. We confirm that HD cooling is unimportant in cases where the primordial gas does not go through an ionized phase, as in the formation process of the very first stars in z ~ 20 minihaloes of mass ~ 10^{6} M_{solar}.Comment: 10 pages, 10 figures, accepted for publication in MNRAS with minor revisions, new table adde

Uncovering The Chemical Signature Of The First Stars In The Universe

The chemical abundance patterns observed in metal-poor Galactic halo stars contain the signature of the first supernovae, and thus allow us to probe the first stars that formed in the universe. We construct a theoretical model for the early chemical enrichment history of the Milky Way, aiming in particular at the contribution from pair-instability supernovae (PISNe). These are a natural consequence of current theoretical models for primordial star formation at the highest masses. However, no metal-poor star displaying the distinct PISN signature has yet been observed. We here argue that this apparent absence of any PISN signature is due to an observational selection effect. Whereas most surveys traditionally focus on the most metal-poor stars, we predict that early PISN enrichment tends to "overshoot,'' reaching enrichment levels of [Ca/H] similar or equal to -2.5 that would be missed by current searches. We utilize existing observational data to place constraints on the primordial initial mass function (IMF). The number fraction of PISNe in the primordial stellar population is estimated to be 90%) contribution from PISNe is merely similar to 10(-4) to 5 x 10(-4). The corresponding fraction of stars formed from gas exclusively enriched by PISNe is a factor of similar to 4 smaller. With the advent of next-generation telescopes and new, deeper surveys, we should be able to test these predictions.NSF AST 07-08795Astronom

Properties of Gamma-Ray Burst Progenitor Stars

We determine some basic properties of stars that produce spectacular gamma-ray bursts at the end of their life. We assume that accretion of the outer portion of the stellar core by a central black hole fuels the prompt emission, and that fall-back and accretion of the stellar envelope later produces the plateau in the X-ray light curve seen in some bursts. Using X-ray data for three bursts we estimate the radius of the stellar core to be ~ 1-3 x 10^10 cm, and that of the stellar envelope to be ~ 1-2 x 10^11 cm. The density profile in the envelope is fairly shallow, with \rho ~ r^-2. The rotation speeds of the core and envelope are ~ 0.05 and ~ 0.2 of the local Keplerian speed, respectively.Comment: Science in pres

Towards the First Galaxies

The formation of the first galaxies at redshifts z~10-15 signaled the transition from the simple initial state of the universe to one of ever increasing complexity. We here review recent progress in understanding their assembly process with numerical simulations, starting with cosmological initial conditions and modelling the detailed physics of star formation. In particular, we study the role of HD cooling in ionized primordial gas, the impact of UV radiation produced by the first stars, and the propagation of the supernova blast waves triggered at the end of their brief lives. We conclude by discussing promising observational diagnostics that will allow us to probe the properties of the first galaxies, such as their contribution to reionization and the chemical abundance pattern observed in extremely low-metallicity stars.Comment: 12 pages, 14 figures, appeared in "First Stars III", eds. B. O'Shea, A. Heger and T. Abel, a high resolution version (highly recommended) can be found at http://www.ita.uni-heidelberg.de/~tgreif/files/gjb07.pd

The First Supernova Explosions: Energetics, Feedback, And Chemical Enrichment

We perform three-dimensional smoothed particle hydrodynamics simulations in a realistic cosmological setting to investigate the expansion, feedback, and chemical enrichment properties of a 200M(circle dot) pair-instability supernova in the high-redshift universe. We find that the SN remnant propagates for a Hubble time at z similar or equal to 20 to a final mass-weighted mean shock radius of 2.5 kpc (proper), roughly half the size of the H II region, and in this process sweeps up a total gas mass of 2: 5; 10(5) M-circle dot. The morphology of the shock becomes highly anisotropic once it leaves the host halo and encounters filaments and neighboring minihalos, while the bulk of the shock propagates into the voids of the intergalactic medium. The SN entirely disrupts the host halo and terminates further star formation for at least 200 Myr, while in our specific case it exerts positive mechanical feedback on neighboring minihalos by shock-compressing their cores. In contrast, we do not observe secondary star formation in the dense shell via gravitational fragmentation, due to the previous photoheating by the progenitor star. We find that cooling by metal lines is unimportant for the entire evolution of the SN remnant, while the metal-enriched, interior bubble expands adiabatically into the cavities created by the shock, and ultimately into the voids with a maximum extent similar to the final mass-weighted mean shock radius. Finally, we conclude that dark matter halos of at least M-vir greater than or similar to 10(8) M-circle dot must be assembled to recollect all components of the swept-up gas.Astronom