460 research outputs found
Core Collapse and Then? The Route to Massive Star Explosions
The rapidly growing base of observational data for supernova explosions of
massive stars demands theoretical explanations. Central of these is a
self-consistent model for the physical mechanism that provides the energy to
start and drive the disruption of the star. We give arguments why the delayed
neutrino-heating mechanism should still be regarded as the standard paradigm to
explain most explosions of massive stars and show how large-scale and even
global asymmetries can result as a natural consequence of convective overturn
in the neutrino-heating region behind the supernova shock. Since the explosion
is a threshold phenomenon and depends sensitively on the efficiency of the
energy transfer by neutrinos, even relatively minor differences in numerical
simulations can matter on the secular timescale of the delayed mechanism. To
enhance this point, we present some results of recent one- and two-dimensional
computations, which we have performed with a Boltzmann solver for the neutrino
transport and a state-of-the-art description of neutrino-matter interactions.
Although our most complete models fail to explode, the simulations demonstrate
that one is encouragingly close to the critical threshold because a modest
variation of the neutrino transport in combination with postshock convection
leads to a weak neutrino-driven explosion with properties that fulfill
important requirements from observations.Comment: 14 pages; 3 figures. Invited Review, in: ``From Twilight to
Highlight: The Physics of Supernovae'', Eds. W. Hillebrandt and B.
Leibundgut, Springer Series ``ESO Astrophysics Symposia'', Berli
Unrecognized Astrometric Confusion in the Galactic Centre
The Galactic Centre is a highly crowded stellar field and frequent
unrecognized events of source confusion, which involve undetected faint stars,
are expected to introduce astrometric noise on a sub-mas level. This confusion
noise is the main non-instrumental effect limiting the astrometric accuracy and
precision of current near-infrared imaging observations and the long-term
monitoring of individual stellar orbits in the vicinity of the central
supermassive black hole. We self-consistently simulate the motions of the known
and the yet unidentified stars to characterize this noise component and show
that a likely consequence of source confusion is a bias in estimates of the
stellar orbital elements, as well as the inferred mass and distance of the
black hole, in particular if stars are being observed at small projected
separations from it, such as the star S2 during pericentre passage.
Furthermore, we investigate modeling the effect of source confusion as an
additional noise component that is time-correlated, demonstrating a need for
improved noise models to obtain trustworthy estimates of the parameters of
interest (and their uncertainties) in future astrometric studies.Comment: accepted for publication in MNRA
Two-Dimensional Hydrodynamic Models of Super Star Clusters with a Positive Star Formation Feedback
Using the hydrodynamic code ZEUS, we perform 2D simulations to determine the
fate of the gas ejected by massive stars within super star clusters. It turns
out that the outcome depends mainly on the mass and radius of the cluster. In
the case of less massive clusters, a hot high velocity ( km
s) stationary wind develops and the metals injected by supernovae are
dispersed to large distances from the cluster. On the other hand, the density
of the thermalized ejecta within massive and compact clusters is sufficiently
large as to immediately provoke the onset of thermal instabilities. These
deplete, particularly in the central densest regions, the pressure and the
pressure gradient required to establish a stationary wind, and instead the
thermally unstable parcels of gas are rapidly compressed, by a plethora of
re-pressurizing shocks, into compact high density condensations. Most of these
are unable to leave the cluster volume and thus accumulate to eventually feed
further generations of star formation.
The simulations cover an important fraction of the parameter-space, which
allows us to estimate the fraction of the reinserted gas which accumulates
within the cluster and the fraction that leaves the cluster as a function of
the cluster mechanical luminosity, the cluster size and heating efficiency.Comment: Accepted for publication in ApJ; 27 pages, 9 figures, 1 tabl
Initiation of the detonation in the gravitationally confined detonation model of Type Ia supernovae
We study the initiation of the detonation in the gravitationally confined
detonation (GCD) model of Type Ia supernovae (SNe Ia). Initiation of the
detonation occurs spontaneously in a region where the length scale of the
temperature gradient extending from a flow (in which carbon burning is already
occurring) into unburned fuel is commensurate to the range of critical length
scales which have been derived from 1D simulations that resolve the initiation
of a detonation. By increasing the maximum resolution in a truncated cone that
encompasses this region, beginning somewhat before initiation of the detonation
occurs, we successfully simulate in situ the first gradient-initiated
detonation in a whole-star simulation. The detonation emerges when a
compression wave overruns a pocket of fuel situated in a Kelvin-Helmholtz cusp
at the leading edge of the inwardly directed jet of burning carbon. The
compression wave pre-conditions the temperature in the fuel in such a way that
the Zel'dovich gradient mechanism can operate and a detonation ensues. We
explore the dependence of the length scale of the temperature gradient on
spatial resolution and discuss the implications for the robustness of this
detonation mechanism. We find that the time and the location at which
initiation of the detonation occurs varies with resolution. In particular,
initiation of a detonation had not yet occurred in our highest resolution
simulation by the time we ended the simulation because of the computational
demand it required. We suggest that the turbulent shear layer surrounding the
inwardly directed jet provides the most favorable physical conditions, and
therefore the most likely location, for initiation of a detonation in the GCD
model.Comment: 28 pages, 12 figures, 1 table, accepted to Ap
The Dynamics of Radiative Shock Waves: Linear and Nonlinear Evolution
The stability properties of one-dimensional radiative shocks with a power-law
cooling function of the form are the main
subject of this work. The linear analysis originally presented by Chevalier &
Imamura, is thoroughfully reviewed for several values of the cooling index
and higher overtone modes. Consistently with previous results, it is
shown that the spectrum of the linear operator consists in a series of modes
with increasing oscillation frequency. For each mode a critical value of the
cooling index, , can be defined so that modes with are unstable, while modes with
are stable. The perturbative analysis is complemented by several numerical
simulations to follow the time-dependent evolution of the system for different
values of . Particular attention is given to the comparison between
numerical and analytical results (during the early phases of the evolution) and
to the role played by different boundary conditions. It is shown that an
appropriate treatment of the lower boundary yields results that closely follow
the predicted linear behavior. During the nonlinear regime, the shock
oscillations saturate at a finite amplitude and tend to a quasi-periodic cycle.
The modes of oscillations during this phase do not necessarily coincide with
those predicted by linear theory, but may be accounted for by mode-mode
coupling.Comment: 33 pages, 12 figures, accepted for publication on the Astrophysical
Journa
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