2,594 research outputs found
The Connection between Gamma-Ray Bursts and Extremely Metal-Poor Stars as Nucleosynthetic Probes of the Early Universe
The connection between the long GRBs and Type Ic Supernovae (SNe) has
revealed the interesting diversity: (i) GRB-SNe, (ii) Non-GRB Hypernovae (HNe),
(iii) X-Ray Flash (XRF)-SNe, and (iv) Non-SN GRBs (or dark HNe). We show that
nucleosynthetic properties found in the above diversity are connected to the
variation of the abundance patterns of extremely-metal-poor (EMP) stars, such
as the excess of C, Co, Zn relative to Fe. We explain such a connection in a
unified manner as nucleosynthesis of hyper-aspherical (jet-induced) explosions
Pop III core-collapse SNe. We show that (1) the explosions with large energy
deposition rate, , are observed as GRB-HNe and their yields
can explain the abundances of normal EMP stars, and (2) the explosions with
small are observed as GRBs without bright SNe and can be
responsible for the formation of the C-rich EMP (CEMP) and the hyper metal-poor
(HMP) stars. We thus propose that GRB-HNe and the Non-SN GRBs (dark HNe) belong
to a continuous series of BH-forming stellar deaths with the relativistic jets
of different .Comment: 8 pages, 6 figures. To appear in "Massive Stars as Cosmic Engines",
Proceedings of IAU Symposium 250 (December 2007, Kauai), eds. F. Bresolin,
P.A. Crowther, & J. Puls (Cambridge Univ. Press
Nucleosynthesis in Type II Supernovae
Presupernova evolution and explosive nucleosynthesis in massive stars for
main-sequence masses from 13 to 70 are calculated. We
examine the dependence of the supernova yields on the stellar mass,
^{12}C(\alpha, \gamma) ^{16}O} rate, and explosion energy. The supernova
yields integrated over the initial mass function are compared with the solar
abundances.Comment: 1 Page Latex source, 10 PostScript figures, to appear in Nuclear
Physics A, Vol. A616 (1997
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Midbrain Dopamine Neurons Signal Belief in Choice Accuracy during a Perceptual Decision
Central to the organization of behavior is the ability to predict the values of outcomes to guide choices. The accuracy of such predictions is honed by a teaching signal that indicates how incorrect a prediction was (âreward prediction error,â RPE). In several reinforcement learning contexts, such as Pavlovian conditioning and decisions guided by reward history, this RPE signal is provided by midbrain dopamine neurons. In many situations, however, the stimuli predictive of outcomes are perceptually ambiguous. Perceptual uncertainty is known to influence choices, but it has been unclear whether or how dopamine neurons factor it into their teaching signal. To cope with uncertainty, we extended a reinforcement learning model with a belief state about the perceptually ambiguous stimulus; this model generates an estimate of the probability of choice correctness, termed decision confidence. We show that dopamine responses in monkeys performing a perceptually ambiguous decision task comply with the modelâs predictions. Consequently, dopamine responses did not simply reflect a stimulusâ average expected reward value but were predictive of the trial-to-trial fluctuations in perceptual accuracy. These confidence-dependent dopamine responses emerged prior to monkeysâ choice initiation, raising the possibility that dopamine impacts impending decisions, in addition to encoding a post-decision teaching signal. Finally, by manipulating reward size, we found that dopamine neurons reflect both the upcoming reward size and the confidence in achieving it. Together, our results show that dopamine responses convey teaching signals that are also appropriate for perceptual decisions
Optical Spectropolarimetry and Asphericity of Type Ic SN 2007gr
We present optical spectropolarimetric observations of Type Ic supernova (SN)
2007gr with Subaru telescope at 21 days after the maximum brightness (~37 days
after the explosion). Non-zero polarization as high as ~3% is observed at the
absorption feature of Ca II IR triplet. The polarization of the continuum light
is ~0.5% if we estimate the interstellar polarization (ISP) component assuming
that the continuum polarization has a single polarization angle. This suggests
that the axis ratio of the SN photosphere projected to the sky is different
from unity by ~10%. The polarization angle at the Ca II absorption is almost
aligned to that of the continuum light. These features may be understood by the
model where a bipolar explosion with an oblate photosphere is viewed from the
slightly off-axis direction and explosively synthesized Ca near the polar
region obscures the light originated around the minor axis of the SN
photosphere. Given the uncertainty of the ISP, however, the polarization data
could also be interpreted by the model with an almost spherically symmetric
photosphere and a clumpy Ca II distribution.Comment: 9 pages, 8 figures, Accepted for publication in the Astrophysical
Journa
Multipole expansion for magnetic structures: A generation scheme for symmetry-adapted orthonormal basis set in crystallographic point group
We propose a systematic method to generate a complete orthonormal basis set
of multipole expansion for magnetic structures in arbitrary crystal structure.
The key idea is the introduction of a virtual atomic cluster of a target
crystal, on which we can clearly define the magnetic configurations
corresponding to symmetry-adapted multipole moments. The magnetic
configurations are then mapped onto the crystal so as to preserve the magnetic
point group of the multipole moments, leading to the magnetic structures
classified according to the irreducible representations of crystallographic
point group. We apply the present scheme to pyrhochlore and hexagonal ABO3
crystal structures, and demonstrate that the multipole expansion is useful to
investigate the macroscopic responses of antiferromagnets
Nucleosynthesis in Core-Collapse Supernovae and GRB--Metal-Poor Star Connection
We review the nucleosynthesis yields of core-collapse supernovae (SNe) for
various stellar masses, explosion energies, and metallicities. Comparison with
the abundance patterns of metal-poor stars provides excellent opportunities to
test the explosion models and their nucleosynthesis. We show that the abundance
patterns of extremely metal-poor (EMP) stars, e.g., the excess of C, Co, Zn
relative to Fe, are in better agreement with the yields of hyper-energetic
explosions (Hypernovae, HNe) rather than normal supernovae.
We note that the variation of the abundance patterns of EMP stars are related
to the diversity of the Supernova-GRB connection. We summarize the diverse
properties of (1) GRB-SNe, (2) Non-GRB HNe/SNe, (3) XRF-SN, and (4) Non-SN GRB.
In particular, the Non-SN GRBs (dark hypernovae) have been predicted in order
to explain the origin of C-rich EMP stars. We show that these variations and
the connection can be modeled in a unified manner with the explosions induced
by relativistic jets. Finally, we examine whether the most luminous supernova
2006gy can be consistently explained with the pair-instability supernova model.Comment: 15 pages, 9 figures. To appear in "Supernova 1987A: 20 Years After:
Supernovae and Gamma-Ray Bursters", eds. S. Immler, K. Weiler, & R. McCray
(American Institute of Physics) (2007
Influence of graft perfusion pressure on graft viability after small bowel preservation and transplantation
Nucleosynthesis Basics and Applications to Supernovae
This review concentrates on nucleosynthesis processes in general and their
applications to massive stars and supernovae. A brief initial introduction is
given to the physics in astrophysical plasmas which governs composition
changes. We present the basic equations for thermonuclear reaction rates and
nuclear reaction networks. The required nuclear physics input for reaction
rates is discussed, i.e. cross sections for nuclear reactions,
photodisintegrations, electron and positron captures, neutrino captures,
inelastic neutrino scattering, and beta-decay half-lives. We examine especially
the present state of uncertainties in predicting thermonuclear reaction rates,
while the status of experiments is discussed by others in this volume (see M.
Wiescher). It follows a brief review of hydrostatic burning stages in stellar
evolution before discussing the fate of massive stars, i.e. the nucleosynthesis
in type II supernova explosions (SNe II). Except for SNe Ia, which are
explained by exploding white dwarfs in binary stellar systems (which will not
be discussed here), all other supernova types seem to be linked to the
gravitational collapse of massive stars (M8M) at the end of their
hydrostatic evolution. SN1987A, the first type II supernova for which the
progenitor star was known, is used as an example for nucleosynthesis
calculations. Finally, we discuss the production of heavy elements in the
r-process up to Th and U and its possible connection to supernovae.Comment: 52 pages, 20 figures, uses cupconf.sty (included); to appear in
"Nuclear and Particle Astrophysics", eds. J. Hirsch., D. Page, Cambridge
University Pres
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