Terry P. Walker

Primordial nucleosynthesis redux

The abundances of D, He-3, He-4, and Li-7, are presently recalculated within the framework of primordial nucleosynthesis in the standard hot big band model, in order to estimate the primordial abundances of the light elements. A comparison between theory and experiment demonstrates the consistency of standard model predictions; the baryon density parameter is constrained on the basis of a nucleon-to-photon ratio of 2.8-4.0. These bounds imply that the bulk of the baryons in the universe are dark, requiring that the universe be dominated by nonbaryonic matter. 140 refs.

Primordial nucleosynthesis: Theory and observations

We review the cosmology and physics underlying Primordial Nucleosynthesis and survey current observational data in order to compare the predictions of Big Bang Nucleosynthesis with the inferred primordial abundances. From this comparison we report on the status of the consistency of the standard hot big bang model, we constrain the universal density of baryons (nucleons), and we set limits to the numbers and/or e!ective interactions of hypothetical new ‘lighta particles (equivalent massless neutrinos). ( 2000 Elsevier Science B.V. All rights reserved.

Big-bang nucleosynthesis revisited

Abstract We compute the homogeneous big-bang nucleosynthesis yields of D, 3 He, 4 He, and 7 Li, taking into account recent measurements of the neutron mean-life as well as updates of several nuclear reactions rates which primarily affect the production of 7 Li. We discuss the extraction of primordial abundances from observation and the likelihood that the primordial mass fraction of 4 He, Y p , is less than or equal to 0.24. Using the primordial abundances of D+ 3 He and 7 Li we limit the baryon-to-photon ratio ( η in units of 10 −10 ): 2.6 ⩽ η 10 ⩽ 4.3 (or, in terms of the present mass density in baryons, 1.8×10 −31 ⩽ ϱ B ⩽ 3.0×10 −31 g/cm 3 , for a microwave background temperature of 2.75°) which we use to argue that baryons contribute between 0.02 and 0.11 to the critical energy density of the universe. An upper limit to Y p of 0.24 constrains the number of light neutrinos to N ν ⩽ 3.4, in excellent agreement with the LEP and SLC collider results. We turn this argument around to show that the collider limit of 3 neutrino species can be used to bound the primordial abundance of 4 He: 0.235 ⩽ Y p ⩽ 0.245.

Halo Star Lithium Depletion

The depletion of lithium during the pre-main-sequence and main-sequence phases of stellar evolution plays a crucial role in the comparison of the predictions of big bang nucleosynthesis with the abundances observed in halo stars. Previous work has indicated a wide range of possible depletion factors, ranging from minimal in standard (nonrotating) stellar models to as much as an order of magnitude in models that include rotational mixing. Recent progress in the study of the angular momentum evolution of low-mass stars permits the construction of theoretical models capable of reproducing the angular momentum evolution of low-mass open cluster stars. The distribution of initial angular momenta can be inferred from stellar rotation data in young open clusters. In this paper we report on the application of these models to the study of lithium depletion in main-sequence halo stars. A range of initial angular momenta produces a range of lithium depletion factors on the main sequence. Using the distribution of initial conditions inferred from young open clusters leads to a well-defined halo lithium plateau with modest scatter and a small population of outliers. The mass-dependent angular momentum loss law inferred from open cluster studies produces a nearly flat plateau, unlike previous models that exhibited a downward curvature for hotter temperatures in the 7Li-Teff plane. The overall depletion factor for the plateau stars is sensitive primarily to the solar initial angular momentum used in the calibration for the mixing diffusion coefficients. Uncertainties remain in the treatment of the internal angular momentum transport in the models, and the potential impact of these uncertainties on our results is discussed. The 6Li/7Li depletion ratio is also examined. We find that the dispersion in the plateau and the 6Li/7Li depletion ratio scale with the absolute 7Li depletion in the plateau, and we use observational data to set bounds on the 7Li depletion in main-sequence halo stars. A maximum of 0.4 dex depletion is set by the observed dispersion and 6Li/7Li depletion ratio, and a minimum of 0.2 dex depletion is required by both the presence of highly overdepleted halo stars and consistency with the solar and open cluster 7Li data. The cosmological implications of these bounds on the primordial abundance of 7Li are discussed.

Big Bang nucleosynthesis in crisis

A new evaluation of the constraint on the number of light neutrino species ({ital N}{sub {nu}}) from big bang nucleosynthesis suggests a discrepancy between the predicted light element abundances and those inferred from observations, unless the inferred primordial {sup 4}He abundance has been underestimated by 0.014{plus_minus}0.004 (1{sigma}) or less than 10% (95% C.L.) of {sup 3}He survives stellar processing. With the quoted systematic errors in the observed abundances and a conservative chemical evolution parametrization, the best fit to the combined data is {ital N}{sub {nu}}=2.1{plus_minus}0.3 (1{sigma}) and the upper limit is {ital N}{sub {nu}}{lt}2.6 (95% C.L.). The data are inconsistent with the standard model ({ital N}{sub {nu}}=3) at the 98.6% C.L. {copyright} {ital 1995} {ital The} {ital American} {ital Physical} {ital Society}.

Direct X-ray Constraints on Sterile Neutrino Warm Dark Matter

Warm dark matter (WDM) might more easily account for small scale clustering measurements than the heavier particles typically invoked incold dark matter (�CDM) cosmologies. In this paper, we consider aWDM cosmology in which sterile neutrinoss, with a mass ms of roughly 1-100 keV, are the dark matter. We use the diffuse X-ray spectrum (total minus resolved point source emission) of the Andromeda galaxy to constrain the rate of sterile neutrino radiative decay: �s → �e,µ,� + . Our findings demand that ms < 3.5 keV (95% C.L.) which is a significant improvement over the previous (95% C.L.) limits inferred from the X-ray emission of nearby clusters, ms < 8.2 keV (Virgo A) and ms < 6.3 keV (Virgo A + Coma).

Stellar Mixing and the Primordial Lithium Abundance

We compare the properties of recent samples of the lithium abundances in halo stars to one another and to the predictions of theoretical models including rotational mixing, and we examine the data for trends with metal abundance. We apply two statistical tests to the data: a Kolomorgorov-Smirnov (K-S) test sensitive to the behavior around the sample median, and Monte Carlo tests of the probability to draw the observed number of outliers from the theoretical distributions. We find from a K-S test that in the absence of any correction for chemical evolution, the Ryan, Norris, & Beers (RNB) sample is fully consistent with mild rotational mixing induced depletion and, therefore, with an initial lithium abundance higher than the observed value. Tests for outliers depend sensitively on the threshold for defining their presence, but we find a 10%-45% probability that the RNB sample is drawn from the rotationally mixed models with a 0.2 dex median depletion with lower probabilities corresponding to higher depletion factors. Including or excluding the one upper limit in the sample changes the absolute probabilities but does not affect the overall conclusions. When chemical evolution trends (Li/H vs. Fe/H) are included in our analysis we find that the dispersion in the RNB sample is not explained by chemical evolution; the inferred bounds on lithium depletion from rotational mixing are similar to those derived from models without chemical evolution. Finally, we explore the differences between the RNB sample and other halo star data sets. We find that differences in the equivalent width measurements are primarily responsible for different observational conclusions concerning the lithium dispersion in halo stars. The different data sets are all consistent with mild stellar depletion, but the systematic errors arising from different observational data sets are a major component of the error budget and need to be addressed. The implications for cosmology are discussed. We find that the standard big bang nucleosynthesis predicted lithium abundance that corresponds to the deuterium abundance inferred from observations of high-redshift, low-metallicity QSO absorbers requires halo star lithium depletion in an amount consistent with that from our models of rotational mixing but inconsistent with no depletion.

Is Cygnus X-3 strange?

Abstract We discuss the recently reported measurements of the properties of high energy cosmic rays arriving from the direction of the compact binary X-ray source Cygnus X-3. We argue that the source of these events may be a strange quark star, and that the primary which directly produces them is a low baryon number neutral hadron with multiple strangeness which is stable up to (at least) simultaneous double strangeness changing weak decays.

Production of Li, Be, and B in the early Galaxy

The production of lithium, beryllium, and boron via collisions between cosmic rays and interstellar gas nuclei during the early (Population II) evolution of the Galaxy is considered. There is a qualitative as well as quantitative difference from the usual (Population I) spallation production of these intermediate-mass nuclei. In particular, since 6 Li and 7 Li can be produced via α + α fusion reactions whereas 9 Be, 10 B, and 11 B require CNO nuclei as spallation targets, the ration of Li to Be or B is vastly different from that predicted in previous spallation calculations

Cosmic microwave background constraint on residual annihilations of relic particles

Energy injected into the cosmic microwave background at redshifts z{approx} (equivalent to){sigma}{sub 0}, the bound is f(m{sub X}/MeV){sup -1}[({sigma}{sub 0}/6x10{sup -27} cm{sup 3}s{sup -1})({Omega}{sub X{bar X}}h{sup 2}){sup 2}]<0.2, where m{sub X} is the particle mass, {Omega}{sub X{bar X}} is the fraction of the critical density the particle and its antiparticle contribute if they survive to the present time, h=H{sub 0}/100 kms{sup -1}Mpc{sup -1}, H{sub 0} is the Hubble constant, and f is the fraction of the annihilation energy that interacts electromagnetically. We also compute the less stringent limits for p-wave annihilation. We update other bounds on residual annihilations and compare them to our CMB bound.