9,141 research outputs found
Magnetic Fields in Core Collapse Supernovae: Possibilities and Gaps
Spectropolarimetry of core collapse supernovae has shown that they are
asymmetric and often, but not universally, bi-polar. The Type IIb SN1993J and
similar events showed large scatter in the Stokes parameter plane.
Observational programs clearly have much more to teach us about the complexity
of asymmetric supernovae and the physics involved in the asymmetry. Jet-induced
supernova models give a typical jet/torus structure that is reminiscent of some
objects like the Crab nebula, SN1987A and perhaps Cas A. Jets, in turn, may
arise from the intrinsic rotation and magnetic fields that are expected to
accompany core collapse. We summarize the potential importance of the
magneto-rotational instability for the core collapse problem and sketch some of
the effects that large magnetic fields, ~10^{15} G, may have on the physics of
the supernova explosion. Open issues in the problem of multi-dimensional
magnetic core collapse are summarized and a critique is given of some recent
MHD collapse calculations.Comment: 20 pages, 2 figures, to appear in the proceedings of the INT workshop
"Open Issues in Understanding Core Collapse Supernovae," Seattle, 2004, ed.
T. Mezzacappa (World Scientific
The Non-Monotonic Dependence of Supernova and Remnant Formation on Progenitor Rotation
Traditional models of core collapse suggest the issue of successful versus
failed supernova explosions and neutron star versus black hole formation
depends monotonically on the mass (and metallicity) of the progenitor star.
Here we argue that the issue of success or failure of the explosion or other
possible outcomes may depend non--monotonically on the rotation of the
progenitor star even at fixed progenitor mass and composition. We have computed
"shellular" models of core collapse for a star of 15 M_solar with initial
central angular velocity, Omega_0, in the range 0.1 -- 8 rad/s until a few
hundred ms after bounce to explore qualitative trends. The non--monotonic
behavior will be manifested in the rotation of the proto--neutron star and
hence in the strength of the associated magnetic field that will be generated
by shear in that rotating environment. We estimate that our maximally rotating
and shearing models generate toroidal fields approaching or exceeding 10^17G,
strengths nearing dynamical significance.Comment: 20 pages, 6 figures, accepted for publication in The Astrophysical
Journal. Few typos are correcte
Supernova Asymmetries
All core collapse supernovae are strongly aspherical. The "Bochum event,"
with velocity components displaced symmetrically about the principal H
line, strongly suggests that SN 1987A was a bi-polar rather than a uni-polar
explosion. While there is a general tendency to display a single prominant axis
in images and spectropolarimetry, there is also growing evidence for frequent
departures from axisymmetry. There are various mechanisms that might contribute
to large scale departures from spherical symmetry: jet-induced processes, the
spherical shock accretion instability (SASI) and associated phenomena, and
non-axisymmetric instabilities (NAXI). The MRI gives inevitable production of
large toroidal magnetic fields. In sum: no without B. The role of
magnetic fields, non-axisymmetric instabilities, and of the de-leptonization
phase are discussed.Comment: 8 pages, 3 figures, to be published in the proceedings of the
conference "Supernova 1987A: 20 Years After" Aspen, 200
Inapproximability of maximal strip recovery
In comparative genomic, the first step of sequence analysis is usually to
decompose two or more genomes into syntenic blocks that are segments of
homologous chromosomes. For the reliable recovery of syntenic blocks, noise and
ambiguities in the genomic maps need to be removed first. Maximal Strip
Recovery (MSR) is an optimization problem proposed by Zheng, Zhu, and Sankoff
for reliably recovering syntenic blocks from genomic maps in the midst of noise
and ambiguities. Given genomic maps as sequences of gene markers, the
objective of \msr{d} is to find subsequences, one subsequence of each
genomic map, such that the total length of syntenic blocks in these
subsequences is maximized. For any constant , a polynomial-time
2d-approximation for \msr{d} was previously known. In this paper, we show that
for any , \msr{d} is APX-hard, even for the most basic version of the
problem in which all gene markers are distinct and appear in positive
orientation in each genomic map. Moreover, we provide the first explicit lower
bounds on approximating \msr{d} for all . In particular, we show that
\msr{d} is NP-hard to approximate within . From the other
direction, we show that the previous 2d-approximation for \msr{d} can be
optimized into a polynomial-time algorithm even if is not a constant but is
part of the input. We then extend our inapproximability results to several
related problems including \cmsr{d}, \gapmsr{\delta}{d}, and
\gapcmsr{\delta}{d}.Comment: A preliminary version of this paper appeared in two parts in the
Proceedings of the 20th International Symposium on Algorithms and Computation
(ISAAC 2009) and the Proceedings of the 4th International Frontiers of
Algorithmics Workshop (FAW 2010
The First Ground Level Enhancement Event of Solar Cycle 24: Direct Observation of Shock Formation and Particle Release Heights
We report on the 2012 May 17 Ground Level Enhancement (GLE) event, which is
the first of its kind in Solar Cycle 24. This is the first GLE event to be
fully observed close to the surface by the Solar Terrestrial Relations
Observatory (STEREO) mission. We determine the coronal mass ejection (CME)
height at the start of the associated metric type II radio burst (i.e., shock
formation height) as 1.38 Rs (from the Sun center). The CME height at the time
of GLE particle release was directly measured from a STEREO image as 2.32 Rs,
which agrees well with the estimation from CME kinematics. These heights are
consistent with those obtained for cycle-23 GLEs using back-extrapolation. By
contrasting the 2012 May 17 GLE with six other non-GLE eruptions from
well-connected regions with similar or larger flare size and CME speed, we find
that the latitudinal distance from the ecliptic is rather large for the non-GLE
events due to a combination of non-radial CME motion and unfavorable solar B0
angle, making the connectivity to Earth poorer. We also find that the coronal
environment may play a role in deciding the shock strength.Comment: 16 pages, 4 figures, 1 tabl
Determining All Universal Tilers
A universal tiler is a convex polyhedron whose every cross-section tiles the
plane. In this paper, we introduce a certain slight-rotating operation for
cross-sections of pentahedra. Based on a selected initial cross-section and by
applying the slight-rotating operation suitably, we prove that a convex
polyhedron is a universal tiler if and only if it is a tetrahedron or a
triangular prism.Comment: 18 pages, 12 figure
Ground Level Enhancement in the 2014 January 6 Solar Energetic Particle Event
We present a study of the 2014 January 6 solar energetic particle (SEP)
event, which produced a small ground level enhancement (GLE), making it the
second GLE of this unusual solar cycle 24. This event was primarily observed by
the South Pole neutron monitors (increase of ~2.5%) whereas a few other neutron
monitors recorded smaller increases. The associated coronal mass ejection (CME)
originated behind the western limb and had the speed of 1960 km/s. The height
of the CME at the start of the associated metric type II radio burst, which
indicates the formation of a strong shock, was measured to be 1.61 Rs using a
direct image from STEREO-A/EUVI. The CME height at the time of GLE particle
release (determined using the South Pole neutron monitor data) was directly
measured as 2.96 Rs, from the STEREO-A/COR1 white-light observations. These CME
heights are consistent with those obtained for the GLE71, the only other GLE of
the current cycle as well as cycle-23 GLEs derived using back-extrapolation.
GLE72 is of special interest because it is one of the only two GLEs of cycle
24, one of the two behind-the-limb GLEs and one of the two smallest GLEs of
cycles 23 and 24
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