Showing papers by "Brian D. Fields published in 2005"

Deep-Ocean Crusts as Telescopes: Using Live Radioisotopes to Probe Supernova Nucleosynthesis

Abstract: Live 60Fe has recently been detected in a deep-ocean ferromanganese crust, isolated in layers dating from about 3 Myr ago. Since 60Fe has a mean life of 2.2 Myr, a near-Earth supernova is the only likely source for such a signal, and we explore here the consequences of a supernova origin. We combine the 60Fe data with several supernova nucleosynthesis models to calculate the supernova distance as a function of progenitor mass, finding an allowed range of 15-120 pc. We also predict the signals expected for several other radioisotopes, which are independent of the supernova distance. Species likely to be present near or above background levels are 10Be, 26Al, 53Mn, 182Hf, and 244Pu. Of these, 182Hf and 244Pu are nearly background-free, presenting the best opportunities to provide strong confirmation of the supernova origin of the 60Fe signal and to demonstrate that at least some supernovae are sources for the r-process. The accuracies of our predictions are hampered by large uncertainties in the predicted 60Fe yields for supernovae of different masses, so the new crust data motivate a redoubled theoretical attack on this problem.

Implications of a new temperature scale for halo dwarfs on LiBeB and chemical evolution

Abstract: Big bang nucleosynthesis (BBN) and the cosmic baryon density from cosmic microwave background anisotropies together predict a primordial 7Li abundance a factor of 2-3 higher than that observed in galactic halo dwarf stars. A recent analysis of 7Li observations in halo stars, using significantly higher surface temperature for these stars, found a higher Li plateau abundance. These results go a long way toward resolving the discrepancy with BBN. Here we examine the implications of the higher surface temperatures on the abundances of Be and B that are thought to have been produced in galactic cosmic-ray nucleosynthesis by spallation of CNO together with Li (produced in α + α collisions). While the Be abundance is not overly sensitive to the surface temperature, the derived B abundances and more importantly the derived oxygen abundances are very temperature-dependent. If the new temperature scale is correct, the implied increased abundances of these elements pose a serious challenge to models of galactic cosmic-ray nucleosynthesis and galactic chemical evolution.

6Li and Gamma Rays: Complementary Constraints on Cosmic-Ray History

Abstract: The rare isotope 6Li is made only by cosmic rays, predominantly in αα → 6Li fusion reactions with interstellar medium (ISM) helium. Consequently, this nuclide provides a unique diagnostic of the history of cosmic rays in our Galaxy. The same hadronic cosmic-ray interactions also produce high-energy γ-rays (mostly via pp → π0 → γγ). Thus, hadronic γ-rays and 6Li are intimately linked. Specifically, 6Li directly encodes the local cosmic-ray fluence over cosmic time, while extragalactic hadronic γ-rays encode an average cosmic-ray fluence over lines of sight out to the horizon. We examine this link and show how 6Li and γ-rays can be used together to place important model-independent limits on the cosmic-ray history of our Galaxy and the universe. We first constrain γ-ray production from ordinary Galactic cosmic rays, using the local 6Li abundance. We find that the solar 6Li abundance demands an accompanying extragalactic pionic γ-ray intensity that exceeds that of the entire observed extragalactic γ-ray background (EGRB) by a factor of 2-6. Possible explanations for this discrepancy are discussed. We then constrain Li production using recent determinations of the EGRB. We note that cosmic rays created during cosmic structure formation would lead to pre-Galactic Li production, which would act as a "contaminant" to the primordial 7Li content of metal-poor halo stars; the EGRB can place an upper limit on this contamination if we attribute the entire EGRB pionic contribution to structure-forming cosmic rays. Unfortunately, the uncertainties in the determination of the EGRB are so large that the present γ-ray data cannot guarantee that the pre-Galactic Li contribution is small compared to primordial 7Li; thus, an improved determination of the EGRB will shed important new light on this issue. Our limits and their more model-dependent extensions will improve significantly with additional observations of 6Li in halo stars and with improved measurements of the EGRB spectrum by GLAST.

Double distribution of dark matter halos with respect to mass and local overdensity

Abstract: We present a double distribution function of dark matter halos, with respect to both object mass and local over- (or under-) density. This analytical tool provides a statistical treatment of the properties of matter surrounding collapsed objects, and can be used to study environmental effects on hierarchical structure formation. The size of the ``local environment'' of a collapsed object is defined to depend on the mass of the object. The Press-Schechter mass function is recovered by integration of our double distribution over the density contrast. We also present a detailed treatment of the evolution of overdensities and underdensities in ${\ensuremath{\Omega}}_{\mathrm{m}}+{\ensuremath{\Omega}}_{\ensuremath{\Lambda}}=1$ and ${\ensuremath{\Omega}}_{\mathrm{m}}=1$ universes according to the spherical evolution model. We explicitly distinguish between true and linearly extrapolated overdensities and provide conversion relations between the two quantities.

Structure Formation Cosmic Rays: Identifying Observational Constraints

Abstract: Shocks that arise from baryonic in-fall and merger events during the structure formation are believed to be a source of cosmic rays. These "structure formation cosmic rays" (SFCRs) would essentially be primordial in composition, namely, mostly made of protons and alpha particles. However, very little is known about this population of cosmic rays. One way to test the level of its presence is to look at the products of hadronic reactions between SFCRs and the ISM. A perfect probe of these reactions would be Li. The rare isotope Li is produced only by cosmic rays, dominantly in αα → 6Li fusion reactions with the ISM helium. Consequently, this nuclide provides a unique diagnostic of the history of cosmic rays. Exactly because of this unique property is Li affected most by the presence of an additional cosmic ray population. In turn, this could have profound consequences for the Big-Bang nucleosynthesis: cosmic rays created during cosmic structure formation would lead to pre-Galactic Li production, which would act as a "contaminant" to the primordial 7Li content of metalpoor halo stars. Given the already existing problem of establishing the concordance between Li observed in halo stars and primordial 7Li as predicted by the WMAP, it is crucial to set limits to the level of this "contamination". However, the history of SFCRs is not very well known. Thus we propose a few model-independent ways of testing the SFCR species and their history, as well as the existing lithium problem: 1) we establish the connection between gamma-ray and Li production, which enables us to place constraints on the SFCR-made lithium by using the observed Extragalactic Gamma-Ray Background (EGRB); 2) we propose a new site for testing the primordial and SFCR-made lithium, namely, low-metalicity High-Velocity Clouds (HVCs), which retain the pre-Galactic composition without any significant depletion. Although using one method alone may not give us strong constraints, using them in concert will shed a new light on the SFCR population and possibly give some answers about the pressing lithium problem. .

Analytical Models for the Energetics of Cosmic Accretion Shocks, their Cosmological Evolution, and the Effect of Environment

Abstract: We present an analytical description of the energetics of the population of cosmic accretion shocks, for a concordance cosmology (Ωm + ΩΛ = 1). We calculate how the shock-processed accretion power and mass current are distributed among different shock Mach numbers and how they evolve with cosmic time. We calculate the cumulative energy input of cosmic accretion shocks of any Mach number to the intergalactic medium as a function of redshift and compare it with the energy output of supernova explosions, as well as with the energy input required to reionize the universe. In addition, we investigate and quantify the effect of environmental factors, such as local clustering properties and filament preheating, on the statistical properties of these shocks. We find that the energy processed by accretion shocks is higher than the supernova energy output for all z < 3 and that it becomes more than an order of magnitude higher in the local universe. The energy processed by accretion shocks alone becomes comparable to the energy required to reionize the universe by z ~ 3.5. Finally, we establish both qualitatively and quantitatively that local clustering, as well as filament compression and preheating, are important factors in determining the statistical properties of the cosmic accretion shock population.