884 research outputs found
Kelvin-Helmholtz instability of a coronal streamer
The shear-flow-driven instability can play an important role in energy
transfer processes in coronal plasma. We present for the first time the
observation of a kink-like oscillation of a streamer probably caused by the
streaming kinkmode Kelvin-Helmholtz instability. The wave-like behavior of the
streamer was observed by Large Angle and Spectrometric Coronagraph Experiment
(LASCO) C2 and C3 aboard SOlar and Heliospheric Observatory (SOHO). The
observed wave had a period of about 70 to 80 minutes, and its wavelength
increased from 2 Rsun to 3 Rsun in about 1.5 hours. The phase speeds of its
crests and troughs decreased from 406 \pm 20 to 356 \pm 31kms^{-1} during the
event. Within the same heliocentric range, the wave amplitude also appeared to
increase with time. We attribute the phenomena to the MHD Kelvin-Helmholtz
instability which occur at a neutral sheet in a fluid wake. The free energy
driving the instability is supplied by the sheared flow and sheared magnetic
field across the streamer plane. The plasma properties of the local environment
of the streamer were estimated from the phase speed and instability threshold
criteria.Comment: ApJ, accepte
Fallback and Black Hole Production in Massive Stars
The compact remnants of core collapse supernovae - neutron stars and black
holes - have properties that reflect both the structure of their stellar
progenitors and the physics of the explosion. In particular, the masses of
these remnants are sensitive to the density structure of the presupernova star
and to the explosion energy. To a considerable extent, the final mass is
determined by the ``fallback'', during the explosion, of matter that initially
moves outwards, yet ultimately fails to escape. We consider here the simulated
explosion of a large number of massive stars (10 to 100 \Msun) of Population I
(solar metallicity) and III (zero metallicity), and find systematic differences
in the remnant mass distributions. As pointed out by Chevalier(1989),
supernovae in more compact progenitor stars have stronger reverse shocks and
experience more fallback. For Population III stars above about 25 \Msun and
explosion energies less than erg, black holes are a common
outcome, with masses that increase monotonically with increasing main sequence
mass up to a maximum hole mass of about 35 \Msun. If such stars produce primary
nitrogen, however, their black holes are systematically smaller. For modern
supernovae with nearly solar metallicity, black hole production is much less
frequent and the typical masses, which depend sensitively on explosion energy,
are smaller. We explore the neutron star initial mass function for both
populations and, for reasonable assumptions about the initial mass cut of the
explosion, find good agreement with the average of observed masses of neutron
stars in binaries. We also find evidence for a bimodal distribution of neutron
star masses with a spike around 1.2 \Msun (gravitational mass) and a broader
distribution peaked around 1.4 \Msun.Comment: Accepted for publication in Ap
Two-Dimensional Core-Collapse Supernova Models with Multi-Dimensional Transport
We present new two-dimensional (2D) axisymmetric neutrino
radiation/hydrodynamic models of core-collapse supernova (CCSN) cores. We use
the CASTRO code, which incorporates truly multi-dimensional, multi-group,
flux-limited diffusion (MGFLD) neutrino transport, including all relevant
terms. Our main motivation for carrying out this study is to
compare with recent 2D models produced by other groups who have obtained
explosions for some progenitor stars and with recent 2D VULCAN results that did
not incorporate terms. We follow the evolution of 12, 15,
20, and 25 solar-mass progenitors to approximately 600 milliseconds after
bounce and do not obtain an explosion in any of these models. Though the reason
for the qualitative disagreement among the groups engaged in CCSN modeling
remains unclear, we speculate that the simplifying ``ray-by-ray' approach
employed by all other groups may be compromising their results. We show that
``ray-by-ray' calculations greatly exaggerate the angular and temporal
variations of the neutrino fluxes, which we argue are better captured by our
multi-dimensional MGFLD approach. On the other hand, our 2D models also make
approximations, making it difficult to draw definitive conclusions concerning
the root of the differences between groups. We discuss some of the diagnostics
often employed in the analyses of CCSN simulations and highlight the intimate
relationship between the various explosion conditions that have been proposed.
Finally, we explore the ingredients that may be missing in current calculations
that may be important in reproducing the properties of the average CCSNe,
should the delayed neutrino-heating mechanism be the correct mechanism of
explosion.Comment: ApJ accepted version. Minor changes from origina
Very Low Energy Supernovae: Light Curves and Spectra of Shock Breakout
The brief transient emitted as a shock wave erupts through the surface of a
presupernova star carries information about the stellar radius and explosion
energy. Here the CASTRO code, which treats radiation transport using multigroup
flux-limited diffusion, is used to simulate the light curves and spectra of
shock breakout in very low-energy supernovae (VLE SNe), explosions in giant
stars with final kinetic energy much less than 10 erg. VLE SNe light
curves, computed here with the KEPLER code, are distinctively faint, red, and
long-lived, making them challenging to find with transient surveys. The
accompanying shock breakouts are brighter, though briefer, and potentially
easier to detect. Previous analytic work provides general guidance, but
numerical simulations are challenging due to the range of conditions and lack
of equilibration between color and effective temperatures. We consider previous
analytic work and extend discussions of color temperature and opacity to the
lower energy range explored by these events. Since this is the first
application of the CASTRO code to shock breakout, test simulations of normal
energy shock breakout of SN1987A are carried out and compared with the
literature. A set of breakout light curves and spectra are then calculated for
VLE SNe with final kinetic energies in the range ergs for
red supergiants with main sequence masses 15 Msun and 25 Msun. The importance
of uncertainties in stellar atmosphere model, opacity, and ambient medium is
discussed, as are observational prospects with current and forthcoming
missions.Comment: 19 pages; submitted to Astrophysical Journa
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