1,617 research outputs found
LiBeB: High and low energy cosmic ray production and comparison with neutrino induced nucleosynthesis in SNII
We present new calculations of LiBeB production by accelerated particles with
various compositions and energy spectra ranging from low energies to
relativistic energies, and various ambient medium metallicities ().
The observed, essentially constant, Be/Fe ratio as a function of
can be best understood if the metallicity of the accelerated particles (high
energy or low energy) does not vary much with . This could be
achieved if the particles are accelerated directly from the ejecta of Type II
supernovae (SNII) and not from the interstellar medium. Using the observed
Be/Fe and the fact that most of the Fe at low is from SNII, we
derive the energy content in accelerated particles per SNII (a few times
10 erg). We show that additional B production by neutrinos is
consistent with the available data, allowing a neutrino yield from SNII less
than or equal to the nominal published yields. We further show that the
observed Li at low suggests that the accelerated particles
responsible for the LiBeB at such are confined to low energies and
have a relatively high He/O abundance.Comment: 9 pages with 4 emdedded figures, uuencoded, compressed, tared, style
files included. In press Nuclei In the Cosmos, Nuclear Physics, Conference
Proceeding
Different Values Common Goal
The military promotion system tends to value highly and to promote those individuals who are competent operators. You [military officers] are thus products of a system that rewards you for becoming skilled in organizing for combat, for readily subordinating your own will to that of a superior, and for being able to obtain the loyalty and dedicated service of your own subordinates. Your training is focused on learning how to apply force against an enemy\u27s weak point, on analyzing problems from a conservative, some would say a worst case, perspective in order to ensure that you always have enough to do the job
Nuclear Aspects of Nucleosynthesis in Massive Stars
Preliminary results of a new set of stellar evolution and nucleosynthesis
calculations for massive stars are presented. These results were obtained with
an extended reaction network up to Bi. The discussion focuses on the importance
of nuclear rates in pre- and post-explosive nucleosynthesis. The need for
further experiments to study specific reactions and nuclear properties (optical
alpha+nucleus potentials) is emphasized.Comment: 6 pages, 2 figures; invited talk, to appear in the Proceedings of the
Int. Conf. "Structure of the Nucleus at the Dawn of the Century", May 2000,
Bologna, Ital
Nucleosynthesis in massive stars revisited
We have performed the first calculations to follow the evolution of all
stable nuclei and their radioactive progenitors in a finely-zoned stellar model
computed from the onset of central hydrogen burning through explosion as a Type
II supernova. Calculations were done for 15, 20, and 25 solar masses Pop I
stars using the most recently available set of experimental and theoretical
nuclear data, revised opacity tables, and taking into account mass loss due to
stellar winds. Here results are presented for one 15 solar masses model.Comment: 4 pages, 1 figure; needs espcrc1.sty; talk at "Nuclei in the Cosmos
2000", Aarhus, Denmark, June 2000; will appear in Nucl. Phys.
Carbon Ignition in Type Ia Supernovae: An Analytic Model
The observable properties of a Type Ia supernova are sensitive to how the
nuclear runaway ignites in a Chandrasekhar mass white dwarf - at a single point
at its center, off-center, or at multiple points and times. We present a simple
analytic model for the runaway based upon a combination of stellar
mixing-length theory and recent advances in understanding Rayleigh-Benard
convection. The convective flow just prior to runaway is likely to have a
strong dipolar component, though higher multipoles may contribute appreciably
at the very high Rayleigh number (10) appropriate to the white dwarf
core. A likely outcome is multi-point ignition with an exponentially increasing
number of ignition points during the few tenths of a second that it takes the
runaway to develop. The first sparks ignite approximately 150 - 200 km off
center, followed by ignition at smaller radii. Rotation may be important to
break the dipole asymmetry of the ignition and give a healthy explosion.Comment: 14 pages, 0 figures, submitted to ApJ, corrected typo in first
author's nam
The Production of Ti44 and Co60 in Supernova
The production of the radioactive isotopes Ti and Co in all
types of supernovae is examined and compared to observational constraints
including Galactic --ray surveys, measurements of the diffuse 511 keV
radiation, --ray observations of Cas A, the late time light curve of SN
1987A, and isotopic anomalies found in silicon carbide grains in meteorites.
The (revised) line flux from Ti decay in the Cas A supernova remnant
reported by COMPTEL on the Compton Gamma-Ray Observatory is near the upper
bound expected from our models. The necessary concurrent ejection of Ni
would also imply that Cas A was a brighter supernova than previously thought
unless extinction in the intervening matter was very large. Thus, if confirmed,
the reported amount of Ti in Cas A provides very interesting constraints
on both the supernova environment and its mechanism. The abundances of
Ti and Co ejected by Type II supernovae are such that
gamma-radiation from Ti decay SN 1987A could be detected by a future
generation of gamma-ray telescopes and that the decay of Co might
provide an interesting contribution to the late time light curve of SN 1987A
and other Type II supernovae. To produce the solar Ca abundance and
satisfy all the observational constraints, nature may prefer at least the
occasional explosion of sub-Chandrasekhar mass white dwarfs as Type Ia
supernovae. Depending on the escape fraction of positrons due to Co made
in all kinds of Type Ia supernovae, a significant fraction of the steady state
diffuse 511 keV emission may arise from the annihilation of positrons produced
during the decay of Ti to Ca. The Ca and Ti isotopic anomalies in
pre-solar grains confirm the production of Ti in supernovae and thatComment: 27 pages including 7 figures. uuencoded, compressed, postscript. in
press Ap
Integrated Nucleosynthesis in Neutrino Driven Winds
Although they are but a small fraction of the mass ejected in core-collapse
supernovae, neutrino-driven winds (NDWs) from nascent proto-neutron stars
(PNSs) have the potential to contribute significantly to supernova
nucleosynthesis. In previous works, the NDW has been implicated as a possible
source of r-process and light p-process isotopes. In this paper we present
time-dependent hydrodynamic calculations of nucleosynthesis in the NDW which
include accurate weak interaction physics coupled to a full nuclear reaction
network. Using two published models of PNS neutrino luminosities, we predict
the contribution of the NDW to the integrated nucleosynthetic yield of the
entire supernova. For the neutrino luminosity histories considered, no true
r-process occurs in the most basic scenario. The wind driven from an older model for a PNS is moderately neutron-rich at late times however, and
produces Rb, Sr, Y, and Zr in near solar
proportions relative to oxygen. The wind from a more recently studied PNS is proton-rich throughout its entire evolution and does not
contribute significantly to the abundance of any element. It thus seems very
unlikely that the simplest model of the NDW can produce the r-process. At most,
it contributes to the production of the N = 50 closed shell elements and some
light p-nuclei. In doing so, it may have left a distinctive signature on the
abundances in metal poor stars, but the results are sensitive to both uncertain
models for the explosion and the masses of the neutron stars involved.Comment: 18 Pages, 14 Figures, Astrophysical Journal (Submitted 4/16/10
An INTEGRAL/IBIS view of Young Galactic SNRs through the 44Ti gamma-ray lines
We present preliminary results of INTEGRAL/IBIS observations on Cas A, Tycho
and Vela Junior Supernova remnants in the line emission of 44Ti. This
radioactive nucleus is thought to be exclusively produced in supernovae during
the first stages of the explosion. It has a lifetime of about 87 y and is then
the best indicator of young SNRs, as exemplified by the detection of 44Ti in
the youngest known Galactic SNR Cas A with GRO/COMPTEL and latter with
BeppoSAX. In this paper, we will focus on this SNR for which we confirm the
detection of 44Ti and point out the importance to know the nature of the hard
X-ray continuum, the Tycho SNR, for which no indication of 44Ti was ever
reported, and Vela Junior, for which the claimed detection of 44Ti with COMPTEL
is still controversial. The INTEGRAL/IBIS observations bring new constraints on
the nature of these SNRs and on the nucleosynthesis which took place during the
explosions.Comment: Accepted for publication in New Astronomy Reviews ("Astronomy with
Radioactivites V" conference, Clemson Univ., Sept. 2005). 6 pages, 2 figure
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