475 research outputs found
Galactic Evolution Of D And 3He Including Stellar Production Of 3He
New stellar models which track the production and destruction of He (and
D) have been evolved for a range of stellar masses , metallicities and initial (main
sequence) He mass fractions . Armed
with the He yields from these stellar models we have followed the evolution
of D and He using a variety of chemical evolution models with and without
infall of primordial or processed material. Production of new He by the
lower mass stars overwhelms any reasonable primordial contributions and leads
to predicted abundances in the presolar nebula and/or the present interstellar
medium in excess of the observationally inferred values. This result, which
obtains even for zero primordial D and He, and was anticipated by Rood,
Steigman \& Tinsley (1976), is insensitive to the choice of chemical evolution
model; it is driven by the large He yields from low mass stars. In an
attempt to ameliorate this problem we have considered a number of non-standard
models in which the yields from low mass stars have been modified. Although
several of these non-standard models may be consistent with the He data,
they may be inconsistent with observations of C/C, O and,
most seriously, the super-He rich planetary nebulae (Rood, Bania \& Wilson
1992). Even using the most extreme of these non-standard models (Hogan 1995),
we obtain a generous upper bound to pre-galactic He: X which, nonetheless, leads to a stringent lower bound to the
universal density of nucleons.Comment: 21 pages, plus 10 figures, accepted by Ap
Axions, SN 1987A, and one pion exchange
Nucleon-nucleon, axion bremsstrahlung is the primary mechanism for axion emission from the nascent neutron star associated with SN 1987A, and the rate for this process has been calculated in the one pion exchange approximation (OPE). The axion mass limit which follows from SN 1987A, m sub a less than or approx equal to 10 to the -3 eV, is the most stringent astrophysical bound, and has received much scrutiny. It has been suggested that by using OPE to calculate the cross section for the analog process, pp yields pp + pi sup o, and comparing the result of the experimental data one can test the validity of this approximation, and further, that such a comparison indicates that OPE leads to a value for this cross section which is a factor of 30 to 40 too large. If true, this would suggest that the axion mass limit should be revised upward by a factor of approximately 6. The cross section for pp yields pp + pi sup o using OPE is carefully evaluated, and excellent agreement found (to better than a factor of 2) with the experimental data
Non-BBN Constraints On The Key Cosmological Parameters
Since the baryon-to-photon ratio "eta" is in some doubt at present, we ignore
the constraints on eta from big bang nucleosynthesis (BBN) and fit the three
key cosmological parameters (h, Omega_M, eta) to four other observational
constraints: Hubble parameter, age of the universe, cluster gas (baryon)
fraction, and effective shape parameter "Gamma". We consider open and flat CDM
models and flat "Lambda"-CDM models, testing goodness of fit and drawing
confidence regions by the Delta-chi^2 method. CDM models with Omega_M = 1 (SCDM
models) are accepted only because we allow a large error on h, permitting h <
0.5. Open CDM models are accepted only for Omega_M \gsim 0.4. Lambda-CDM models
give similar results. In all of these models, large eta (\gsim 6) is favored
strongly over small eta, supporting reports of low deuterium abundances on some
QSO lines of sight, and suggesting that observational determinations of
primordial 4He may be contaminated by systematic errors. Only if we drop the
crucial Gamma constraint are much lower values of Omega_M and eta permitted.Comment: 12 pages, Kluwer Latex, 2 Postscript figures, to appear in the
proceedings of the ISSI Workshop, "The Primordial Nuclei and Their Galactic
Evolution" (Bern, May 6-10, 1997), ed. N. Prantzos, M. Tosi, and R. von
Steiger (Kluwer, Dordrecht
BBN For Pedestrians
The simplest, `standard' model of Big Bang Nucleosynthesis (SBBN) assumes
three light neutrinos (N_nu = 3) and no significant electron neutrino
asymmetry, leaving only one adjustable parameter: the baryon to photon ratio
eta. The primordial abundance of any one nuclide can, therefore, be used to
measure the baryon abundance and the value derived from the observationally
inferred primordial abundance of deuterium closely matches that from current,
non-BBN data, primarily from the WMAP survey. However, using this same estimate
there is a tension between the SBBN-predicted 4He and 7Li abundances and their
current, observationally inferred primordial abundances, suggesting that N_nu
may differ from the standard model value of three and/or that there may be a
non-zero neutral lepton asymmetry (or, that systematic errors in the abundance
determinations have been underestimated or overlooked). The differences are not
large and the allowed ranges of the BBN parameters permitted by the data are
quite small. Within these ranges, the BBN-predicted abundances of D, 3He, 4He,
and 7Li are very smooth, monotonic functions of eta, N_nu, and the lepton
asymmetry. It is possible to describe the dependencies of these abundances (or
powers of them) upon the three parameters by simple, linear fits which, over
their ranges of applicability, are accurate to a few percent or better. The
fits presented here have not been maximized for their accuracy but, for their
simplicity. To identify the ranges of applicability and relative accuracies,
they are compared to detailed BBN calculations; their utility is illustrated
with several examples. Given the tension within BBN, these fits should prove
useful in facilitating studies of the viability of proposals for non-standard
physics and cosmology, prior to undertaking detailed BBN calculations.Comment: Submitted to a Focus Issue on Neutrino Physics in New Journal of
Physics (www.njp.org
Neutron Diffusion and Nucleosynthesis in an Inhomogeneous Big Bang Model
This article presents an original code for Big Bang Nucleosynthesis in a
baryon inhomogeneous model of the universe. In this code neutron diffusion
between high and low baryon density regions is calculated simultaneously with
the nuclear reactions and weak decays that compose the nucleosynthesis process.
The size of the model determines the time when neutron diffusion becomes
significant. This article describes in detail how the time of neutron diffusion
relative to the time of nucleosynthesis affects the final abundances of He4,
deuterium and Li7. These results will be compared with the most recent
observational constraints of He4, deuterium and Li7. This inhomogeneous model
has He4 and deuterium constraints in concordance for baryon to photon ratio eta
= (4.3 - 12.3) X 10^{-10} Li7 constraints are brought into concordance with the
other isotope constraints by including a depletion factor as high as 5.9. These
ranges for the baryon to photon ratio and for the depletion factor are larger
than the ranges from a Standard Big Bang Nucleosynthesis model.Comment: 7/15, added reference
Neutrinos And Big Bang Nucleosynthesis
The early universe provides a unique laboratory for probing the frontiers of
particle physics in general and neutrino physics in particular. The primordial
abundances of the relic nuclei produced during the first few minutes of the
evolution of the Universe depend on the electron neutrinos through the
charged-current weak interactions among neutrons and protons (and electrons and
positrons and neutrinos), and on all flavors of neutrinos through their
contributions to the total energy density which regulates the universal
expansion rate. The latter contribution also plays a role in determining the
spectrum of the temperature fluctuations imprinted on the Cosmic Background
Radiation (CBR) some 400 thousand years later. Using deuterium as a baryometer
and helium-4 as a chronometer, the predictions of BBN and the CBR are compared
to observations. The successes of, as well as challenges to the standard models
of particle physics and cosmology are identified. While systematic
uncertainties may be the source of some of the current tensions, it could be
that the data are pointing the way to new physics. In particular, BBN and the
CBR are used to address the questions of whether or not the relic neutrinos
were fully populated in the early universe and, to limit the magnitude of any
lepton asymmetry which may be concealed in the neutrinos.Comment: Accepted for publication in the Proceedings of Nobel Symposium 129,
"Neutrino Physics"; to appear in Physics Scripta, eds., L Bergstrom, O.
Botner, P. Carlson, P. O. Hulth, and T. Ohlsso
BBN and the Primordial Abundances
The relic abundances of the light elements synthesized during the first few
minutes of the evolution of the Universe provide unique probes of cosmology and
the building blocks for stellar and galactic chemical evolution, while also
enabling constraints on the baryon (nucleon) density and on models of particle
physics beyond the standard model. Recent WMAP analyses of the CBR temperature
fluctuation spectrum, combined with other, relevant, observational data, has
yielded very tight constraints on the baryon density, permitting a detailed,
quantitative confrontation of the predictions of Big Bang Nucleosynthesis with
the post-BBN abundances inferred from observational data. The current status of
this comparison is presented, with an emphasis on the challenges to astronomy,
astrophysics, particle physics, and cosmology it identifies.Comment: To appear in the Proceedings of the ESO/Arcetri Workshop on "Chemical
Abundances and Mixing in Stars in the Milky Way and its Satellites", eds., L.
Pasquini and S. Randich (Springer-Verlag Series, "ESO Astrophysics Symposia"
The Evolution of Helium and Hydrogen Ionization Corrections as HII Regions Age
Helium and hydrogen recombination lines observed in low-metallicity,
extragalactic, HII regions provide the data used to infer the primordial helium
mass fraction, Y_P. In deriving abundances from observations, the correction
for unseen neutral helium or hydrogen is usually assumed to be absent; i.e.,
the ionization correction factor is taken to be unity (icf = 1). In a previous
paper (VGS), we revisited the question of the icf, confirming a "reverse"
ionization correction: icf < 1. In VGS the icf was calculated using more nearly
realistic models of inhomogeneous HII regions, suggesting that the published
values of Y_P needed to be reduced by an amount of order 0.003. As star
clusters age, their stellar spectra evolve and so, too, will their icfs. Here
the evolution of the icf is studied, along with that of two, alternate,
measures of the "hardness" of the radiation spectrum. The differences between
the icf for radiation-bounded and matter-bounded models are also explored,
along with the effect on the icf of the He/H ratio (since He and H compete for
some of the same ionizing photons). Particular attention is paid to the amount
of doubly-ionized helium predicted, leading us to suggest that observations of,
or bounds to, He++ may help to discriminate among models of HII regions ionized
by starbursts of different ages and spectra. We apply our analysis to the
Izotov & Thuan (IT) data set utilizing the radiation softness parameter, the
[OIII]/[OI] ratio, and the presence or absence of He++ to find 0.95 < icf <
0.99. This suggests that the IT estimate of the primordial helium abundance
should be reduced by Delta-Y = 0.006 +- 0.002, from 0.244 +- 0.002 to 0.238 +-
0.003.Comment: 27 double-spaced pages, 11 figures, 5 equations; revised to match the
version accepted for publication in the Ap
Neutrino statistics and big bang nucleosynthesis
Neutrinos may possibly violate the spin-statistics theorem, and hence obey
Bose statistics or mixed statistics despite having spin half. We find the
generalized equilibrium distribution function of neutrinos which depends on a
single fermi-bose parameter, \kappa, and interpolates continuously between the
bosonic and fermionic distributions when \kappa changes from -1 to +1. We
consider modification of the Big Bang Nucleosynthesis (BBN) in the presence of
bosonic or partly bosonic neutrinos. For pure bosonic neutrinos the abundances
change (in comparison with the usual Fermi-Dirac case) by -3.2% for 4He (which
is equivalent to a decrease of the effective number of neutrinos by \Delta
N_\nu = - 0.6), +2.6% for 2H and -7% for 7Li. These changes provide a better
fit to the BBN data. Future BBN studies will be able to constrain the
fermi-bose parameter to \kappa > 0.5, if no deviation from fermionic nature of
neutrinos is found. We also evaluate the sensitivity of future CMB and LSS
observations to the fermi-bose parameter.Comment: 11 pages, 3 figures, matches version in JCAP, discussion and
references extended slightl
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