583 research outputs found
A Parametric Study of the Acoustic Mechanism for Core-Collapse Supernovae
We investigate the criterion for the acoustic mechanism to work successfully
in core-collapse supernovae. The acoustic mechanism is an alternative to the
neutrino-heating mechanism. It was proposed by Burrows et al., who claimed that
acoustic waves emitted by -mode oscillations in proto-neutron stars (PNS)
energize a stalled shock wave and eventually induce an explosion. Previous
works mainly studied to which extent the -modes are excited in the PNS. In
this paper, on the other hand, we investigate how strong the acoustic wave
needs to be if it were to revive a stalled shock wave. By adding the acoustic
power as a new axis, we draw a critical surface, an extension of the critical
curve commonly employed in the context of neutrino heating. We perform both 1D
and 2D parametrized simulations, in which we inject acoustic waves from the
inner boundary. In order to quantify the power of acoustic waves, we use the
extended Myers theory to take neutrino reactions into proper account. We find
for the 1D simulations that rather large acoustic powers are required to
relaunch the shock wave, since the additional heating provided by the secondary
shocks developed from acoustic waves is partially canceled by the neutrino
cooling that is also enhanced. In 2D, the required acoustic powers are
consistent with those of Burrows et al. Our results seem to imply, however,
that it is the sum of neutrino heating and acoustic powers that matters for
shock revival.Comment: 20 pages, 19 figures, accepted by Ap
Nonequilibrium electron spin polarization in a double quantum dot. Lande mechanism
In moderately strong magnetic fields, the difference in Lande g-factors in
each of the dots of a coupled double quantum dot device may induce oscillations
between singlet and triplet states of the entangled electron pair and lead to a
nonequilibrium electron spin polarization. We will show that this polarization
may partially survive the rapid inhomogeneous decoherence due to random nuclear
magnetic fields.Comment: New version contains figures. New title better reflects the content
of the pape
S-adenosyl-L-methionine: (S)-scoulerine 9-O-methyltransferase, a highly stereo- and regio-specific enzyme in tetrahydroprotoberberine biosynthesis
Suspension cultures of Berberis species are useful sources for the detection and isolation of a new enzyme which transfers the methyl group from S-adenosyl-L-methionine specifically to the 9-position of the (S)-enantiomer of scoulerine, producing (S)-tetrahydrocolumbamine. The enzyme was enriched 27-fold; it is not particle bound, has a pH optimum of 8.9, a molecular weight of 63 000 and shows a high degree of substrate specificity
String amplitudes in arbitrary dimensions
We calculate gravitational dressed tachyon correlators in non critcal
dimensions. The 2D gravity part of our theory is constrained to constant
curvature. Then scaling dimensions of gravitational dressed vertex operators
are equal to their bare conformal dimensions. Considering the model as d+2
dimensional critical string we calculate poles of generalized Shapiro-Virasoro
amplitudes.Comment: 14 page
Failed Gamma-Ray Bursts: Thermal UV/Soft X-ray Emission Accompanied by Peculiar Afterglows
We show that the photospheres of "failed" Gamma-Ray Bursts (GRBs), whose bulk
Lorentz factors are much lower than 100, can be outside of internal shocks. The
resulting radiation from the photospheres is thermal and bright in UV/Soft
X-ray band. The photospheric emission lasts for about one thousand seconds with
luminosity about several times 10^46 erg/s. These events can be observed by
current and future satellites. It is also shown that the afterglows of failed
GRBs are peculiar at the early stage, which makes it possible to distinguish
failed GRBs from ordinary GRBs and beaming-induced orphan afterglows.Comment: 19 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Development of General Relativistic Magnetohydrodynamic Code and its Application to Central Engine of Long Gamma-Ray Bursts
In order to investigate formation of relativistic jets at the center of a
progenitor of a long gamma-ray burst (GRB), we develop a two-dimensional
general relativistic magnetohydrodynamic (GRMHD) code. We show the code passes
many, well-known test calculations, by which the reliability of the code is
confirmed. Then we perform a numerical simulation of a collapsar using a
realistic progenitor model. It is shown that a jet is launched from the center
of the progenitor. We also find that the mass accretion rate after the launch
of the jet shows rapid time variability that resembles to a typical time
profile of a GRB. The structure of the jet is similar to the previous study: a
poynting flux jet is surrounded by a funnel-wall jet. Even at the final stage
of the simulation, bulk Lorentz factor of the jet is still low, and total
energy of the jet is still as small as 10^48 erg. However, we find that the
energy flux per unit rest-mass flux is as high as 10^2 at the bottom of the
jet. Thus we conclude that the bulk Lorentz factor of the jet can be
potentially high when it propagates outward. It is shown that the outgoing
poynting flux exists at the horizon around the polar region, which proves that
the Blandford-Znajek mechanism is working. However, we conclude that the jet is
launched mainly by the magnetic field amplified by the gravitational collapse
and differential rotation around the black hole, rather than the
Blandford-Znajek mechanism.Comment: 14 pages, 17 figures, Submitted to The Astrophysical Journal, High
resolution version is available at
http://www2.yukawa.kyoto-u.ac.jp/~nagataki/GRMHD.pd
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