Showing papers by "Brian D. Fields published in 2008"

An update on the big bang nucleosynthesis prediction for 7Li: The problem worsens

Abstract: The lithium problem arises from the significant discrepancy between the primordial 7Li abundance as predicted by big bang nucleosynthesis (BBN) theory and the Wilkinson Microwave Anisotropy Probe (WMAP) baryon density, and the pre-Galactic lithium abundance inferred from observations of metal-poor (Population II) stars. This problem has loomed for the past decade, with a persistent discrepancy of a factor of 2–3 in 7Li/H. Recent developments have sharpened all aspects of the Li problem. Namely: (1) BBN theory predictions have sharpened due to new nuclear data; in particular, the uncertainty on the reaction rate for3He(α,γ)7Be has reduced to 7.4%, nearly a factor of 2 tighter than previous determinations. (2) The WMAP five-year data set now yields a cosmic baryon density with an uncertainty reduced to 2.7%. (3) Observations of metal-poor stars have tested for systematic effects. With these, we now find that the BBN+WMAP predicts7Li/H = (5.24−0.67+0.71) × 10−10. The central value represents an increase by 23%, most of which is due to the upward shift in the3He(α,γ)7Be rate. More significant is the reduction in the7Li/H uncertainty by almost a factor of 2, tracking the reduction in the3He(α,γ)7Be error bar. These changes exacerbate the Li problem; the discrepancy is now a factor 2.4 or 4.2σ (from globular cluster stars) to 4.3 or 5.3σ (from halo field stars). Possible resolutions to the lithium problem are briefly reviewed, and key experimental and astronomical measurements highlighted.

A Bitter Pill: The Primordial Lithium Problem Worsens

Abstract: The lithium problem arises from the significant discrepancy between the primordial 7 Li abun-dance as predicted by BBN theory and the WMAP baryon density, and the pre-Galactic lithiumabundanceinferred from observations of metal-poor (Population II) stars. Thisproblem has loomedfor the past decade, with a persistent discrepancy of a factor of 2−3 in 7 Li/H. Recent developmentshave sharpened all aspects of the Li problem. Namely: (1) BBN theory predictions have sharpeneddue to new nuclear data, particularly the uncertainty on 3 He(α,γ) 7 Be, has reduced to 7.4%, andwith a central value shift of ∼ +0.04 keV barn. (2) The WMAP 5-year data now yields a cosmicbaryon density with an uncertainty reduced to 2.7%. (3) Observations of metal-poor stars havetested for systematic effects, and have reaped new lithium isotopic data. With these, we now findthat the BBN+WMAP predicts 7 Li/H = (5.24 +0.71−0.67 )×10 −10 . The Li problem remains and indeedis exacerbated; the discrepancy is now a factor 2.4 – 4.3 or 4.2σ (from globular cluster stars) to5.3σ (from halo field stars). Possible resolutions to the lithium problem are briefly reviewed, andkey nuclear, particle, and astronomical measurements highlighted.

Supernova collisions with the heliosphere

Abstract: Nearby supernova explosions—within a few tens of pc of the solar system—have become a subject of intense scrutiny, due to the discovery of live undersea 60 Fe from an event 2.8 Myr ago. A key open question concerns the delivery of supernova ejecta to the Earth, in particular penetration of the heliosphere bythe supernova remnant (SNR). We present the first systematic numerical hydrodynamical study of the interaction between a supernova blast and the solar wind. Our simulations explore dynamic pressure regimes that are factors � 10 above those in other studies of the heliosphere under exotic conditions, for supernovae exploding at a range of distances through different interstellar environments, and interacting with solar winds of varying strengths. Our results are qualitatively consistent with the structure of the contemporary heliosphere modeled byprevious work,but compressedto within the inner solar system. We demonstrate that key characteristics of the resulting heliospheric structure follow simple scaling laws that can be understoodinterms of pressure-balancearguments,and which are inagreement withprevious work.Our modelsshow that a 10 pc supernova event, incident on a solar-wind outflow with the mean observed properties, compresses the heliopause to just beyond 1 AU. We also demonstrate scenarios where the supernova remnant compresses the heliopause to within 1 AU, in which cases supernova material will be directly deposited on Earth. Since 8 pc marks the nominal ‘‘kill radius’’ for severe biosphere damage, any extinction-level events should have left terrestrial deposits of supernova debris. We conclude with a brief discussion of the effect of our approximations and the impact of additional physics. Subject headingg astrobiology — interplanetary medium — solar wind — supernovae: general Online material: color figures

Superluminous Supernovae: No Threat from η Carinae

Abstract: Recently, Supernova 2006gy was noted as the most luminous ever recorded, with a total radiated energy of ∼1044 Joules. It was proposed that the progenitor may have been a massive evolved s...

FUSE deuterium observations: a strong case for galactic infall

Abstract: Measurements of deuterium in the local interstellar medium have revealed large variations in D/H along different lines of sight. Moreover, recent far-ultraviolet spectroscopic explorer (FUSE) measurements find D/H to be anticorrelated with several indicators of dust formation and survival, suggesting that interstellar deuterium suffers significant depletion onto dust grains. This in turn implies that the total deuterium abundance in the local Galactic disc could be as high as ~84% of the primordial D abundance. Because deuterium is destroyed in stars, its abundance in the interstellar medium (ISM) also tells us about the fraction of material which has never been processed through stellar environments. Therefore, the new report of such high ISM deuterium abundance implies that most present-day interstellar baryons are unprocessed. It was proposed that the infall/accretion of pristine gas is needed to explain such a high deuterium abundance. However, we point out that the infall needed to maintain a high present-day D/H is, within the preferred models, in tension with observations that gas represents only some ~20% of Galactic baryons, with the balance in stars. This small gas fraction implies that, integrated over Galactic history, most baryons have been sequestered into stars and stellar remnants. We study this tension in the context of a wide class of Galactic evolution models for baryonic processing through stars, which show that deuterium destruction is strongly and cleanly correlated with the drop in the gas fraction. We find that FUSE deuterium observations and Galactic gas fraction estimates can be reconciled in some models; these demand a significant infall rate of pristine material that almost completely balances the rate of star formation. These successful models also require that the average fraction of gas that is returned by dying stars is less than 40% of the initial stellar mass. Cosmological implications of dust depletion of D in high-redshift systems are discussed.

The Heliosphere in Time

Abstract: Because of the dynamic nature of the interstellar medium, the Sun should have encountered a variety of different interstellar environments in its lifetime. As the solar wind interacts with the surrounding interstellar medium to form a heliosphere, different heliosphere shapes, sizes, and particle contents result from the different environments. Some of the large possible interstellar parameter space (density, velocity, temperature) is explored here with the help of global heliosphere models, and the features in the resulting heliospheres are compared and discussed. The heliospheric size, expressed as distance of the nose of the heliopause to the Sun, is set by the solar wind - interstellar pressure balance, even for extreme cases. Other heliospheric boundary locations and neutral particle results correlate with the interstellar parameters as well. If the H0 clouds identified in the Millennium Arecibo survey are typical of clouds encountered by the Sun, then the Sun spends ~99.4% of the time in warm low density ISM, where the typical upwind heliosphere radii are up to two orders of magnitude larger than at present.

Unresolved Unidentified Source Contribution to the Gamma-Ray Background

Abstract: The large majority of EGRET point sources remain without an identified low-energy counterpart, and a large fraction of these sources are most likely extragalactic. Whatever the nature of the extragalactic EGRET unidentified sources, faint unresolved objects of the same class must have a contribution to the diffuse extragalactic gamma-ray background (EGRB). Understanding this component of the EGRB, along with other guaranteed contributions from known sources, is essential if we are to use this emission to constrain exotic high-energy physics. Here, we follow an empirical approach to estimate whether a potential contribution of unidentified sources to the EGRB is likely to be important, and we find that it is. In addition, we show how upcoming GLAST observations of EGRET unidentified sources, as well as of their fainter counterparts, can be combined with GLAST observations of the Galactic and extragalactic diffuse backgrounds to shed light on the nature of the EGRET unidentified sources even without any positional association of such sources with low-energy counterparts.

FUSE Deuterium Observations: A Strong Case For Galactic Infall

Abstract: Measurements of deuterium in the local interstellar medium have revealed large variations in D/H along different lines of sight. Moreover, recent {\it Far Ultraviolet Spectroscopic Explorer} (FUSE) measurements find D/H to be anticorrelated with several indicators of dust formation and survival, suggesting that interstellar deuterium suffers significant depletion onto dust grains. This in turn implies that the total deuterium abundance in the local Galactic disk could be as high as $\sim 84 %$ of the primordial D abundance. It was proposed that the infall/accretion of pristine gas is needed to explain such a high deuterium abundance. However, we point out that the infall needed to maintain a high present-day D/H is, within the preferred models, in tension with observations that gas represents only some $\sim 20%$ of Galactic baryons, with the balance in stars. We study this tension in the context of a wide class of Galactic evolution models for baryonic processing through stars, which show that deuterium destruction is strongly and cleanly correlated with the drop in the gas fraction. We find that FUSE deuterium observations and Galactic gas fraction estimates can be reconciled in some models; these demand a significant infall rate of pristine material that almost completely balances the rate of star formation. These successful models also require that the average fraction of gas that is returned by dying stars is less than 40% of the initial stellar mass. Cosmological implications of dust depletion of D in high-redshift systems are discussed.

Penetration of supernova ejecta in the solar system

Abstract: In this paper, we investigate the mechanism by which supernova ejecta can penetrate the solar system, and in particular, directly deposit live radioactivities on Earth. A purely hydrodynamic interaction between a supernova blast and the solar wind yields a limit of 10 pc as the maximum supernova explosion distance in order for supernova plasma to penetrate within 1AU. However, there exists evidence that the vast majority of heavy elements in a supernova remnant may be depleted onto grains, hence they can be considered as charged particles which do not participate in the plasma dynamics of the interaction of the supernova plasma and the solar wind. We examine the motion of these charged particles as they decouple from the supernova plasma and are influenced by the solar magnetic, radiation and gravitational field. We find that given the large incoming velocities of the charged grains, they suffer little or no deflection within the solar system. Consequently, the dust penetration to 1 AU has essentially 100% transmission probability, and the dust capture onto the earth should have a geometric cross section.

New metallicity independent constraints of galactic infall from deuterium observations

Abstract: Local deuterium observations have shown large variations over different lines of sight. However, it has been recently proposed that such variations can be explained by strong depletion of deuterium onto dust grains. Consequently, recent Far Ultraviolet Spectroscopic Explorer (FUSE) deuterium observations represent only a lower bound on the true local deuterium abundance which has thus been estimated to be as high as ∼ 85% of the primordial D abundance, as opposed to previous estimates of ∼ 55%. Such high local deuterium abundance could be explained with Galactic infall. Within our analytical model we demonstrate that such high local D abundance in fact requires a significant infall. Our constraint comes from the FUSE deuterium observations AND Galactic gas fraction estimates, which, when used in concert, demand infall rate comparable to the star-formation rate. Moreover, our analysis also constrains the fraction of stellar mass that is returned to the ISM to a range 0.1 < R <∼ 0.4 which is just marginally consistent with modern initial mass functions. Finally, the requirement of infall is broadly consistent with hierarchical structure formation. Thus, our results offer new qualitative and quantitative ways of placing Galactic evolution in the larger cosmological context.