arXiv subjects

Brian D. Fields

Publications and source records attributed to Brian D. Fields.

At least 109 records · Page 6Linked to original sources

arXiv2000

Standard Cosmic Ray Energetics and Light Element Production

The recent observations of Be and B in metal poor stars has led to a reassessment of the origin of the light elements in the early Galaxy. At low it is metallicity ([O/H] < -1.75), it is necessary to introduce a production mechanism which is independent of the interstellar metallicity (primary). At higher metallicities, existing data might indicate that secondary production is dominant. In this paper, we focus on the secondary process, related to the standard Galactic cosmic rays, and we examine the cosmic ray energy requirements for both present and past epochs. We find the power input to maintain the present-day Galactic cosmic ray flux is about 1.5e41 erg/s = 5e50 erg/century. This implies that, if supernovae are the sites of cosmic ray acceleration, the fraction of explosion energy going to accelerated particles is about 30%, a value which we obtain consistently both from considering the present cosmic ray flux and confinement and from the present 9Be and 6Li abundances. Using the abundances of 9Be (and 6Li) in metal-poor halo stars, we extend the analysis to show the effect of the interstellar gas mass on the standard galactic cosmic ray energetic constraints on models of Li, Be, and B evolution. The efficiency of the beryllium production per erg may be enhanced in the past by a factor of about 10; thus the energetic requirement by itself cannot be used to rule out a secondary origin of light elements. Only a clear and undisputable observational determination of the O-Fe relation in the halo will discriminate between the two processes. (abridged)

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arXiv1999

Testing Spallation Processes With Beryllium and Boron

The nucleosynthesis of Be and B by spallation processes provides unique insight into the origin of cosmic rays. Namely, different spallation schemes predict sharply different trends for the growth of LiBeB abundances with respect to oxygen. ``Primary'' mechanisms predict BeB $\propto$ O, and are well motivated by the data if O/Fe is constant at low metallicity. In contrast, ``secondary'' mechanisms predict BeB $\propto$ O$^2$ and are consistent with the data if O/Fe increases towards low metallicity as some recent data suggest. Clearly, any primary mechanism, if operative, will dominate early in the history of the Galaxy. In this paper, we fit the BeB data to a two-component scheme which includes both primary and secondary trends. In this way, the data can be used to probe the period in which primary mechanisms are effective. We analyze the data using consistent stellar atmospheric parameters based on Balmer line data and the continuum infrared flux. Results depend sensitively on Pop II O abundances and, unfortunately, on the choice of stellar parameters. When using recent results which show O/Fe increasing toward lower metallicity, a two-component Be-O fits indicates that primary and secondary components contribute equally at [O/H]$_{eq}$ = -1.8 for Balmer line data; and [O/H]$_{eq}$ = -1.4 to -1.8 for IRFM. We apply these constraints to recent models for LiBeB origin. The Balmer line data does not show any evidence for primary production. On the other hand, the IRFM data does indicate a preference for a two-component model, such as a combination of standard GCR and metal-enriched particles accelerated in superbubbles. These conclusions rely on a detailed understanding of the abundance data including systematic effects which may alter the derived O-Fe and BeB-Fe relations.

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arXiv1999

Chemical Abundance Constraints on White Dwarfs as Halo Dark Matter

We examine the chemical abundance constraints on a population of white dwarfs in the Halo of our Galaxy. We are motivated by microlensing evidence for massive compact halo objects (Machos) in the Galactic Halo, but our work constrains white dwarfs in the Halo regardless of what the Machos are. We focus on the composition of the material that would be ejected as the white dwarfs are formed; abundance patterns in the ejecta strongly constrain white dwarf production scenarios. Using both analytical and numerical chemical evolution models, we confirm that very strong constraints come from Galactic Pop II and extragalactic carbon abundances. We also point out that depending on the stellar model, significant nitrogen is produced rather than carbon. The combined constraints from C and N give $Ω_{WD} h < 2 \times 10^{-4}$ from comparison with the low C and N abundances in the Ly$α$ forest. We note, however, that these results are subject to uncertainties regarding the nucleosynthesis of low-metallicity stars. We thus investigate additional constraints from D and $^4$He, finding that these light elements can be kept within observational limits only for $Ω_{WD} \la 0.003$ and for a white dwarf progenitor initial mass function sharply peaked at low mass (2$M_\odot$). Finally, we consider a Galactic wind, which is required to remove the ejecta accompanying white dwarf production from the galaxy. We show that such a wind can be driven by Type Ia supernovae arising from the white dwarfs themselves, but these supernovae also lead to unacceptably large abundances of iron. We conclude that abundance constraints exclude white dwarfs as Machos. (abridged)

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arXiv1999

The Galactic Evolution of Beryllium and Boron

The galactic chemical evolution of Be and B provides unique information about the origin and history of cosmic rays. The available Pop II data demonstrate that Be and B have a Galactic source, probably in one or more kinds of spallation processes. However, the data are not unequivocal about the nature of Be and B origin, as encoded in the primary or secondary (linear or quadratic) scaling with metallicity. We summarize a careful analysis of the trends among Be, B, Fe, and O observations. We show that if O/Fe is constant, some other cosmic ray origin or component is needed. On the other hand, if O/Fe is not constant, as recent data suggest, then the data could indicate a standard cosmic ray origin, wherein the abundances of cosmic rays scale with those of the ISM. We suggest future observational tests which will distinguish several proposed scenarios of LiBeB and cosmic ray origin.

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arXiv1999

LiBeB and Big Bang Nucleosynthesis

The dual origin of population II Li7, in both big bang nucleosynthesis and galactic cosmic-ray nucleosynthesis is discussed. It is argued that with additional Li6 data, stringent limits on the degree of Li7 depletion can be obtained. Li7 depletion is also constrained by the concordance of big bang predictions with observational determinations of light element abundances. Stringent limits can also be obtained for a fixed primordial D/H abundance.

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arXiv1998

On Deep-Ocean Fe-60 as a Fossil of a Near-Earth Supernova

Live $^{60}$Fe has recently been reported in a deep-ocean ferromanganese crust. Analysis of the isotopic ratios in the sample suggests that the measured $^{60}$Fe abundance exceeds the levels generated by terrestrial and cosmogenic sources, and it has been proposed that the excess of $^{60}$Fe is a signature of a supernova that exploded near the earth several Myr ago. In this paper, we consider the possible background sources, and confirm that the measured $^{60}$Fe is significantly higher than all known backgrounds, in contrast with the reported abundance of live $^{53}$Mn. We discuss scenarios in which the data are consistent with a supernova event at a distance $D \sim 30$ pc and an epoch $t_{\rm SN} \sim 5$ Myr ago. We propose tests that could confirm or refute the interpretation of the $^{60}$Fe discovery, including searches for $^{10}$Be, $^{129}$I and $^{146}$Sm. Such a nearby supernova event might have had some impact on the earth's biosphere, principally by enhancing the cosmic-ray flux. This might have damaged the earth's ozone layer, enhancing the penetration of solar ultraviolet radiation. In this connection, we comment on the Middle Miocene and Pliocene mini-extinction events. We also speculate on the possibility of a supernova-induced "cosmic-ray winter," if cosmic rays play a significant role in seeding cloud formation.

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arXiv1998

The Evolution of Li6 in Standard Cosmic-Ray Nucleosynthesis

We review the Galactic chemical evolution of Li6 and compare these results with recent observational determinations of the lithium isotopic ratio. In particular, we concentrate on so-called standard Galactic cosmic-ray nucleosynthesis in which Li, Be, and B are produced (predominantly) by the inelastic scattering of accelerated protons and α's off of CNO nuclei in the ambient interstellar medium. If O/Fe is constant at low metallicities, then the Li6 vs Fe/H evolution-as well as Be and B vs Fe/H-has difficulty in matching the observations. However, recent determinations of Population II oxygen abundances, as measured via OH lines, indicate that O/Fe increases at lower metallicity; if this trend is confirmed, then the Li6 evolution in a standard model of cosmic-ray nucleosynthesis is consistent with the data. We also show that another key indicator of Li6BeB origin is the Li6/Be ratio which also fits the available data if O/Fe is not constant at low metallicity. Finally we note that Li6 evolution in this scenario can strongly constrain the degree to which Li6 and Li7 are depleted in halo stars.

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arXiv1998

The Revival of Galactic Cosmic Ray Nucleosynthesis?

Because of the roughly linear correlation between Be/H and Fe/H in low metallicity halo stars, it has been argued that a ``primary'' component in the nucleosynthesis of Be must be present in addition to the ``secondary'' component from standard Galactic cosmic ray nucleosynthesis. In this paper we critically re-evaluate the evidence for the primary versus secondary character of Li, Be, and B evolution, analyzing both in the observations and in Galactic chemical evolution models. While it appears that [Be/H] versus [Fe/H] has a logarithmic slope near 1, it is rather the Be-O trend that directly arises from the physics of spallation production. Using new abundances for oxygen in halo stars based on UV OH lines, we find that the Be-O slope has a large uncertainty due to systematic effects, rendering it difficult to distinguish from the data between the secondary slope of 2 and the primary slope of 1. The possible difference between the Be-Fe and Be-O slopes is a consequence of the variation in O/Fe versus Fe: recent data suggests a negative slope rather than zero (i.e., Fe $\propto$ O) as is often assumed. In addition to a phenomenological analysis of Be and B evolution, we have also examined the predicted LiBeB, O, and Fe trends in Galactic chemical evolution models which include outflow. Based on our results, it is possible that a good fit to the LiBeB evolution requires only traditional the Galactic cosmic ray spallation, and the (primary) neutrino-process contribution to B11. We thus suggest that these two processes might be sufficient to explain Li6, Be, and B evolution in the Galaxy, without the need for an additional primary source of Be and B.

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arXiv1998

Testing the Relation Between the Local and Cosmic Star Formation Histories

Recently, there has been great progress toward observationally determining the mean star formation history of the universe. When accurately known, the cosmic star formation rate could provide much information about Galactic evolution, if the Milky Way's star formation rate is representative of the average cosmic star formation history. A simple hypothesis is that our local star formation rate is proportional to the cosmic mean. In addition, to specify a star formation history, one must also adopt an initial mass function (IMF); typically it is assumed that the IMF is a smooth function which is constant in time. We show how to test directly the compatibility of all these assumptions, by making use of the local (solar neighborhood) star formation record encoded in the present-day stellar mass function. Present data suggests that at least one of the following is false: (1) the local IMF is constant in time; (2) the local IMF is a smooth (unimodal) function; and/or (3) star formation in the Galactic disk was representative of the cosmic mean. We briefly discuss how to determine which of these assumptions fail, and improvements in observations which will sharpen this test.

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arXiv1998

Massive Compact Halo Objects Viewed from a Cosmological Perspective: Contribution to the Baryonic Mass Density of the Universe

[Abridged] We estimate the contribution of Massive Compact Halo Objects (Machos) and their stellar progenitors to the mass density of the Universe. If the Machos that have been detected reside in the Halo of our Galaxy, then a simple extrapolation of the Galactic population (out to 50 kpc) of Machos to cosmic scales gives a cosmic density $ρ_{Macho} = (1-5) \times 10^9 h \msun \Mpc^{-3}$, which in terms of the critical density corresponds to $Ω_{Macho}=(0.0036-0.017) h^{-1}$. Such a mass density is comparable to the baryon density implied by Big Bang Nucleosynthesis. If we take the central values of the estimates, then Machos dominate the baryonic content of the Universe today, with $Ω_{Macho}/Ω_{Baryon} \sim 0.7 h$. However, the cumulative uncertainties in the density determinations only require that $Ω_{Macho}/Ω_{Baryon} \geq 1/6 h f_{gal}$, where the fraction of galaxies that contain Machos $f_{gal} > 0.17$, and $h$ is the Hubble constant in units of 100 km s$^{-1}$ Mpc$^{-1}$. Our best estimate for $Ω_{Macho}$ is hard to reconcile with the current best estimates of the baryonic content of the intergalactic medium indicated by measurements of the Lyman-$α$ forest. We explore the addition constraints that arise if the Machos are white dwarfs as suggested by the present microlensing data. We discuss the challenges this scenario presents at both the local and cosmic scales, emphasizing in particular the constraints on the required mass budget and nucleosynthesis products (particularly carbon).

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arXiv1998

On the Evolution of Helium in Blue Compact Galaxies

We discuss the chemical evolution of dwarf irregular and blue compact galaxies in light of recent data, new stellar yields and chemical evolution models. We examine the abundance data for evidence of HII region self-enrichment effects, which would lead to correlations in the scatter of helium, nitrogen, and oxygen abundances around their mean trends. The observed helium abundance trends show no such correlations, though the nitrogen--oxygen trend does show strong evidence for real scatter beyond observational error. We construct simple models for the chemical evolution of these galaxies, using the most recent yields of \he4, C, N and O in intermediate- and high-mass stars. The effects of galactic outflows, which can arise both from bulk heating and evaporation of the ISM, and from the partial escape of enriched supernova ejecta are included. In agreement with other studies, we find that supernova-enriched outflows can roughly reproduce the observed He, C, N, and O trends; however, in models that fit N versus O, the slopes $ΔY/Δ$O and $ΔY/Δ$N consistently fall more than $2σ$ below the fit to observations. We discuss the role of the models and their uncertainties in the extrapolation of primordial helium from the data. We also explore the model dependence arising nucleosynthesis uncertainties associated with nitrogen yields in intermediate mass stars, the fate of $8-11 \msol$ stars, and massive star winds.

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arXiv1996

Halo White Dwarfs and the Hot Intergalactic Medium

We present a schematic model for the formation of baryonic galactic halos and hot gas in the Local Group and the intergalactic medium. We follow the dynamics, chemical evolution, heat flow and gas flows of a hierarchy of scales, including: protogalactic clouds, galactic halos, and the Local Group itself. Within this hierarchy, the Galaxy is built via mergers of protogalactic fragments. We find that early bursts of star formation lead to a large population of remnants (mostly white dwarfs), which would reside presently in the halo and contribute to the dark component observed in the microlensing experiments. The hot, metal-rich gas from early starbursts and merging evaporates from the clouds and is eventually incorporated into the intergalactic medium. The model thus suggests that most microlensing objects could be white dwarfs ($m \sim 0.5 \msol$), which comprise a significant fraction of the halo mass. Furthermore, the Local Group could have a component of metal-rich hot gas similar to, although less than, that observed in larger clusters. We discuss the known constraints on such a scenario and show that all local observations can be satisfied with present data in this model. The best-fit model has a halo that is 40% baryonic, with an upper limit of 77%.

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arXiv1996

Geological Isotope Anomalies as Signatures of Nearby Supernovae

Nearby supernova explosions may cause geological isotope anomalies via the direct deposition of debris or by cosmic-ray spallation in the earth's atmosphere. We estimate the mass of material deposited terrestrially by these two mechanisms, showing the dependence on the supernova distance. A number of radioactive isotopes are identified as possible diagnostic tools, such as Be-10, Al-26, Cl-36, Mn-53, Fe-60, and Ni-59, as well as the longer-lived I-129, Sm-146, and Pu-244. We discuss whether the 35 and 60 kyr-old Be-10 anomalies observed in the Vostok antarctic ice cores could be due to supernova explosions. Combining our estimates for matter deposition with results of recent nucleosynthesis yields, we calculate the expected signal from nearby supernovae using ice cores back to $\sim 300$ kyr ago, and we discuss using deep ocean sediments back to several hundred Myr. In particular, we examine the prospects for identifying isotope anomalies due to the Geminga supernova explosion, and signatures of the possibility that supernovae might have caused one or more biological mass extinctions.

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arXiv1996

Model Independent Predictions of Big Bang Nucleosynthesis from \he4 and \li7: Consistency and Implications

We examine in detail how BBN theory is constrained, and what predictions it can make, when using only the most model-independent observational constraints. We avoid the uncertainties and model-dependencies that necessarily arise when solar neighborhood D and \he3 abundances are used to infer primordial D and \he3 via chemical and stellar evolution models. Instead, we use \he4 and \li7, thoroughly examining the effects of possible systematic errors in each. Via a likelihood analysis, we find near perfect agreement between BBN theory and the most model-independent data. Given this agreement, we then {\it assume} the correctness of BBN to set limits on the single parameter of standard BBN, the baryon-to-photon ratio, and to predict the primordial D and \he3 abundances. We also repeat our analysis including recent measurements of D/H from quasar absorption systems and find that the near perfect agreement between theory and observation of the three isotopes, D, \he4 and \li7 is maintained. These results have strong implications for the chemical and stellar evolution of the light elements, in particular for \he3. In addition, our results (especially if the D/H measurements are confirmed) have implications for the stellar depletion of \li7. Finally, we set limits on the number \nnu\ of neutrino flavors, using an analysis which carefully and systematically includes all available experimental constraints. The value \nnu = 3.0 fits best with BBN and a 95\% CL upper limit of \nnu $\la 4$ is established.

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arXiv1996

Nuclear and Gamma-ray Production by Supernova Ejecta

The fastest ejecta of supernova explosions propagate as a precursor to the main supernova shock wave, and can be quite energetic. The spectrum of such fast ejecta is estimated based on recent analytic and numerical supernova models, and found to be a power law having a cutoff at an energy of order 10 MeV/nucl, the precise value of which depends on the supernova mass and energetics. With cutoffs in this range there can be significant flux with energies above the thresholds for $γ$-ray and Li, Be, and B production. These nuclear interactions should inevitably accompany the passage of prompt supernova ejecta through the surrounding medium. The fast particle composition is that of the outermost layers of the progenitor; if the progenitor experienced significant loss of its envelope to a companion or by mass loss, then the composition is nonsolar, and in particular, metal-rich. Such a composition and spectrum of fast particles is required to explain the recent COMPTEL observations of $γ$-ray emission in Orion. Supernova ejecta from one progenitor having lost a large amount of mass are shown to quantitatively account for the Orion $γ$ rays, and to imply the presence of other, weaker lines and of a supernova explosion in Orion in the last $\la 10^4$ years. Implications for this mechanism as a mode of nucleosynthesis, and as a potential supernova diagnostic, are discussed. Model dependences and uncertainties are noted, and the need is shown for accurate measurements of $γ$-ray and spallation production cross sections at energies near threshold.

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arXiv1995

Nucleosynthesis Limits on the Mass of Long Lived Tau and Muon Neutrinos

We compute the nucleosynthesis bounds on the masses of stable Dirac and Majorana neutrinos by solving an evolution equation network comprising of all neutrino species which in the Dirac case includes different helicity states as separate species. We will not commit ourselves to any particular value of the nucleosynthesis bound on effective number of light neutrino degrees of freedom $N_ν$, but present all our mass bounds as {\em functions} of $\dNu$. For example, we find that the excluded region in the mass of a Majorana $μ$- or $τ$- neutrino, $0.31$ MeV $< m_ν^M < 52$ MeV corresponding to a bound $\dNu < 0.3$ gets relaxed to $0.93$ MeV $< m_ν^M < 31$ MeV if $\dNu < 1.0$ is used instead. For the Dirac neutrinos this latter constraint gives the upper limits (for $T_{\rm QCD} = 100$ MeV): $\mnm < 0.31$ MeV and $\mnt < 0.37$ MeV. Also, the constraint $\dNu <1$ {\it allows} a stable Dirac neutrino with $m^D_{ν_τ} > 22$ MeV.

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arXiv1995

LiBeB Production by Nuclei and Neutrinos

The production of LiBeB isotopes by nuclear and neutrino spallation are compared in the framework of galactic evolutionary models. As motivated by $γ$-ray observations of Orion, different possible sources of low-energy C and O nuclei are considered, such as supernovae of various masses and WC stars. We confirm that the low energy nuclei (LEN), injected in molecular clouds by stellar winds and type II supernovae originating from the most massive progenitors, can very naturally reproduce the observed Be and B evolution in the early galaxy (halo phase). Assuming the global importance of the LEN component, we compute upper and lower bounds to the neutrino process contribution corresponding to limiting cases of LEN particle spectra. A consistent solution is found with a spectrum of the kind proposed by Ramaty \etal (1995a,b), e.g. flat up to $E_c=30$ MeV/n and decreasing abruptly above. This solution fulfills the challenge of explaining at the same time the general Be and B evolution, and their solar system abundances without overproducing \li7 at very low metallicities, and the meteoritic \b11/\b10 ratio. In this case, neutrino spallation is constrained to play a limited role in the genesis of the solar system \b11. Galactic cosmic rays (GCR) become operative late in the evolution of the disk ([Fe/H]$>$-1), but their contribution to the solar abundances of \be9, \b10 and \b11 is not dominant (35\%, 30\% and 20\% respectively). Thus, with this LEN spectrum, GCR are {\it not}\ the main source of \be9 and B in the Galaxy. The most favorable case for neutrinos, (adopting the same kind of spectrum) has $E_c=20$ MeV/n. Even in this case, the neutrino yields of Woosley and Weaver (1995) must to be reduced by a factor of 5 to avoid \b11 overproduction. Furthermore, this solution leads to a high B/Be

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arXiv1995

Model-Independent Predictions of Big Bang Nucleosynthesis

We derive constraints from standard (with $N_ν= 3$) BBN arising solely from two cosmologically produced nuclides, \he4 and \li7, from which the extrapolation to primordial abundances is straightforward. The abundances of D and \he3 are at present only inferred from their solar and local interstellar medium values using models of galactic chemical evolution. However, our knowledge of chemical evolution suffers from large uncertainties, and so it is of use to take an approach which minimizes the reliance on chemical evolution in determining the consistency of BBN. Using only the data on \he4 and \li7, and reasonable estimates of possible systematic errors in their abundance determinations, concordance is found if the baryon-to-photon ratio lies in the range $1.4 < 10^{10} η< 3.8$ (95\% CL). The \he4 and \li7 abundances are also used to predict the primordial abundances D and \he3, which provides an initial condition for their chemical evolution.

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