Showing papers by "Brian D. Fields published in 2000"

Primordial Lithium and Big Bang Nucleosynthesis.

Abstract: Recent determinations of the abundance of the light-element Li in very metal-poor stars show that its intrinsic dispersion is essentially zero and that the random error in the estimated mean Li abundance is negligible. However, a decreasing trend in the Li abundance toward lower metallicity indicates that the primordial abundance of Li can be inferred only after allowing for nucleosynthesis processes that must have been in operation in the early history of the Galaxy. We show that the observed Li versus Fe trend provides a strong discriminant between alternative models for Galactic chemical evolution of the light elements at early epochs. We critically assess current systematic uncertainties and determine the primordial Li abundance within new, much tighter limits: (Li/H)p = 1.23 × 10-10. We show that the Li constraint on ΩB is now limited as much by uncertainties in the nuclear cross sections used in big bang nucleosynthesis (BBN) calculations as by the observed abundance itself. A clearer understanding of systematics allows us to sharpen the comparison with 4He and deuterium and the resulting test of BBN.

Testing Spallation Processes with Beryllium and Boron

Abstract: 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 ∝ O and are well motivated by the data if O/Fe is constant at low metallicity. In contrast, "secondary" mechanisms predict BeB ∝ O2 and are consistent with the data if O/Fe increases toward 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 Population II O abundances (and O/Fe trends), which have recently seen renewed interest. We explore the implications of these results phenomenologically, using a systematic and consistent compilation and fitting of BeBOFe data. Two-component Be-O fits indicate 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 do not show any evidence for primary production. On the other hand, the IRFM data do 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.

Chemical abundance constraints on white dwarfs as halo dark matter

Abstract: We examine the chemical abundance constraints on a population of white dwarfs in the halo of our Galaxy. We are motivated by microlensing experiments that have reported evidence for massive compact halo objects (MACHOs) in the halo of our Galaxy, with an estimated mass of 0.1-1 M☉; the only conventional dark astrophysical candidates for objects in this mass range are white dwarfs. However, our work constrains white dwarfs in the halo regardless of what the MACHOs are. Further motivation for our work comes from the recent claimed possible detection of a large population of white dwarfs in the Hubble Deep Field. We focus on the composition of the material that would be ejected as the white dwarfs are formed. This material would bear the signatures of nucleosynthesis processing and contain abundance patterns that can be used to constrain white dwarf production scenarios. Using both analytical and numerical chemical evolution models, we confirm previous work that very strong constraints come from Galactic Population II and extragalactic carbon abundances. We also point out that in some cases, depending on the stellar model, significant nitrogen is produced rather than carbon. The combined constraints from carbon and nitrogen give ΩWDh 2 × 10-4 from comparison with the low abundances of these elements measured in the Lyα forest. We note, however, that these results are subject to uncertainties regarding the nucleosynthetic yields of low-metallicity stars. We thus investigate additional constraints from the light elements D and 4He, the nucleosynthesis of which is less uncertain. We find that these elements can be kept within observational limits only for ΩWD 0.003 and for a white dwarf progenitor initial mass function sharply peaked at low mass (2 M☉). 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 find that these supernovae also lead to unacceptably large abundances of iron. The only ways we know of to avoid these constraints are that (1) the ejecta from low-metallicity MACHO progenitors are absent or completely unprocessed or (2) the processed ejecta remain as hot (0.3 keV) gas that is segregated from all observable neutral material to a precision of 99%. Aside from these loopholes, we conclude that abundance constraints exclude white dwarfs as MACHOs.

Standard Cosmic Ray Energetics and Light Element Production

Abstract: 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)

Death of stellar baryonic dark matter candidates

Abstract: The nature of the dark matter in the Universe is one of the outstanding questions in astrophysics. In this talk, I address possible stellar baryonic contributions to the 50-90% of our Galaxy that is made of unknown dark matter. First I show that faint stars and brown dwarfs constitute only a few percent of the mass of the Galaxy. Next, I show that stellar remnants, including white dwarfs and neutron stars, are also insufficient in abundance to explain all the dark matter of the Galaxy. High energy gamma-rays observed in HEGRA data place the most robust constraints, ΩWD < 3× 10h, where h is the Hubble constant in units of 100 km s Mpc. Overproduction of chemical abundances (carbon, nitrogen, and helium) provide the most stringent constraints, ΩWD < 2 × 10 h. Comparison with recent updates of microlensing data are also made. According to the gamma-ray limit, all Massive Compact Halo Objects seen by the experiments (Machos) can be white dwarfs if one takes the extreme numbers; however, from chemical overproduction limits, NOT all Machos can be white dwarfs. Comments on recent observations of the infrared background and of white dwarfs are also made. In conclusion, a nonbaryonic component in the Halo seems to be

Death of Stellar Baryonic Dark Matter Candidates

Abstract: The nature of the dark matter in the Universe is one of the outstanding questions in astrophysics. In this talk, I address possible stellar baryonic contributions to the 50-90% of our Galaxy that is made of unknown dark matter. First I show that faint stars and brown dwarfs constitute only a few percent of the mass of the Galaxy. Next, I show that stellar remnants, including white dwarfs and neutron stars, are also insufficient in abundance to explain all the dark matter of the Galaxy. High energy gamma-rays observed in HEGRA data place the most robust constraints, $\Omega_{WD} < 3 \times 10^{-3} h^{-1}$, where $h$ is the Hubble constant in units of 100 km s$^{-1}$ Mpc$^{-1}$. Overproduction of chemical abundances (carbon, nitrogen, and helium) provide the most stringent constraints, $\Omega_{WD} < 2 \times 10^{-4} h^{-1}$. Comparison with recent updates of microlensing data are also made. According to the gamma-ray limit, all Massive Compact Halo Objects seen by the experiments (Machos) can be white dwarfs if one takes the extreme numbers; however, from chemical overproduction limits, NOT all Machos can be white dwarfs. Comments on recent observations of the infrared background and of white dwarfs are also made. In conclusion, a nonbaryonic component in the Halo seems to be required.

Death of Stellar Baryonic Dark Matter

Abstract: The nature of the dark matter in the Universe is one of the outstanding questions in astrophysics. In this talk, I address possible stellar baryonic contributions to the 50–90% of our Galaxy that is made of unknown dark matter. First I show that faint stars and brown dwarfs constitute only a few percent of the mass of the Galaxy. Next, I show that stellar remnants, including white dwarfs and neutron stars, are also insufficient in abundance to explain all the dark matter of the Galaxy. High energy gamma-rays observed in HEGRA data place the most robust constraints, Ω WD < 3 × 10−3 h −1, where h is the Hubble constant in units of 100 km s−1 Mpc−1. Overproduction of chemical abundances (carbon, nitrogen, and helimn) provide the most stringent constraints, Ω WD < 2 × 10−4 h(−1). Comparison with recent updates of microlensing data are also made. According to the gamma-ray limit, all Massive Compact Halo Objects seen by the experiments (Machos) can be white dwarfs if one takes the extreme numbers; however, from chemical overproduction limits, NOT all Machos can be white dwarfs. Comments on recent observations of the infrared background and of white dwarfs are also made. In conclusion, a nonbaryonic component in the Halo seems to be required.