47,256 research outputs found
A novel corrective-source term approach to modeling unknown physics in aluminum extraction process
With the ever-increasing availability of data, there has been an explosion of
interest in applying modern machine learning methods to fields such as modeling
and control. However, despite the flexibility and surprising accuracy of such
black-box models, it remains difficult to trust them. Recent efforts to combine
the two approaches aim to develop flexible models that nonetheless generalize
well; a paradigm we call Hybrid Analysis and modeling (HAM). In this work we
investigate the Corrective Source Term Approach (CoSTA), which uses a
data-driven model to correct a misspecified physics-based model. This enables
us to develop models that make accurate predictions even when the underlying
physics of the problem is not well understood. We apply CoSTA to model the
Hall-H\'eroult process in an aluminum electrolysis cell. We demonstrate that
the method improves both accuracy and predictive stability, yielding an overall
more trustworthy model
A two-parameter criterion for classifying the explodability of massive stars by the neutrino-driven mechanism
Thus far, judging the fate of a massive star (either a neutron star (NS) or a
black hole) solely by its structure prior to core collapse has been ambiguous.
Our work and previous attempts find a non-monotonic variation of successful and
failed supernovae with zero-age main-sequence mass, for which no single
structural parameter can serve as a good predictive measure. However, we
identify two parameters computed from the pre-collapse structure of the
progenitor, which in combination allow for a clear separation of exploding and
non-exploding cases with only few exceptions (~1-2.5%) in our set of 621
investigated stellar models. One parameter is M4, defining the normalized
enclosed mass for a dimensionless entropy per nucleon of s=4, and the other is
mu4 = d(m/M_sun)/d(r/1000 km) at s=4, being the normalized mass-derivative at
this location. The two parameters mu4 and M4*mu4 can be directly linked to the
mass-infall rate, Mdot, of the collapsing star and the electron-type neutrino
luminosity of the accreting proto-NS, L_nue ~ M_ns*Mdot, which play a crucial
role in the "critical luminosity" concept for the theoretical description of
neutrino-driven explosions as runaway phenomenon of the stalled accretion
shock. All models were evolved employing the approach of Ugliano et al. for
simulating neutrino-driven explosions in spherical symmetry. The neutrino
emission of the accretion layer is approximated by a gray transport solver,
while the uncertain neutrino emission of the 1.1 M_sun proto-NS core is
parametrized by an analytic model. The free parameters connected to the
core-boundary prescription are calibrated to reproduce the observables of
Supernova 1987A for five different progenitor models.Comment: 23 pages, 12 figures; accepted by ApJ; revised version considerably
enlarged (Fig. 7 and Sect.3.6 added
Three-dimensional modeling of Type Ia supernovae - The power of late time spectra
Late time synthetic spectra of Type Ia supernovae, based on three-dimensional
deflagration models, are presented. We mainly focus on one
model,"c3_3d_256_10s", for which the hydrodynamics (Roepke 2005) and
nucleosynthesis (Travaglio et al. 2004) was calculated up to the homologous
phase of the explosion. Other models with different ignition conditions and
different resolution are also briefly discussed. The synthetic spectra are
compared to observed late time spectra. We find that while the model spectra
after 300 to 500 days show a good agreement with the observed Fe II-III
features, they also show too strong O I and C I lines compared to the observed
late time spectra. The oxygen and carbon emission originates from the
low-velocity unburned material in the central regions of these models. To get
agreement between the models and observations we find that only a small mass of
unburned material may be left in the center after the explosion. This may be a
problem for pure deflagration models, although improved initial conditions, as
well as higher resolution decrease the discrepancy. The relative intensity from
the different ionization stages of iron is sensitive to the density of the
emitting iron-rich material. We find that clumping, with the presence of low
density regions, is needed to reproduce the observed iron emission, especially
in the range between 4000 and 6000 AA. Both temperature and ionization depend
sensitively on density, abundances and radioactive content. This work therefore
illustrates the importance of including the inhomogeneous nature of realistic
three-dimensional explosion models. We briefly discuss the implications of the
spectral modeling for the nature of the explosion.Comment: 20 pages, 9 figures, resolution of Fig 1 is reduced to meet astro-ph
file size restriction, submitted to A&
Neutrino-driven Explosions
The question why and how core-collapse supernovae (SNe) explode is one of the
central and most long-standing riddles of stellar astrophysics. A solution is
crucial for deciphering the SN phenomenon, for predicting observable signals
such as light curves and spectra, nucleosynthesis, neutrinos, and gravitational
waves, for defining the role of SNe in the evolution of galaxies, and for
explaining the birth conditions and properties of neutron stars (NSs) and
stellar-mass black holes. Since the formation of such compact remnants releases
over hundred times more energy in neutrinos than the SN in the explosion,
neutrinos can be the decisive agents for powering the SN outburst. According to
the standard paradigm of the neutrino-driven mechanism, the energy transfer by
the intense neutrino flux to the medium behind the stagnating core-bounce
shock, assisted by violent hydrodynamic mass motions (sometimes subsumed by the
term "turbulence"), revives the outward shock motion and thus initiates the SN
blast. Because of the weak coupling of neutrinos in the region of this energy
deposition, detailed, multidimensional hydrodynamic models including neutrino
transport and a wide variety of physics are needed to assess the viability of
the mechanism. Owing to advanced numerical codes and increasing supercomputer
power, considerable progress has been achieved in our understanding of the
physical processes that have to act in concert for the success of
neutrino-driven explosions. First studies begin to reveal observational
implications and avenues to test the theoretical picture by data from
individual SNe and SN remnants but also from population-integrated observables.
While models will be further refined, a real breakthrough is expected through
the next Galactic core-collapse SN, when neutrinos and gravitational waves can
be used to probe the conditions deep inside the dying star. (abridged)Comment: Author version of chapter for 'Handbook of Supernovae,' edited by A.
Alsabti and P. Murdin, Springer. 54 pages, 13 figure
Type Ia Supernova Explosion Models: Homogeneity versus Diversity
Type Ia supernovae (SN Ia) are generally believed to be the result of the
thermonuclear disruption of Chandrasekhar-mass carbon-oxygen white dwarfs,
mainly because such thermonuclear explosions can account for the right amount
of Ni-56, which is needed to explain the light curves and the late-time
spectra, and the abundances of intermediate-mass nuclei which dominate the
spectra near maximum light. Because of their enormous brightness and apparent
homogeneity SN Ia have become an important tool to measure cosmological
parameters. In this article the present understanding of the physics of
thermonuclear explosions is reviewed. In particular, we focus our attention on
subsonic (``deflagration'') fronts, i.e. we investigate fronts propagating by
heat diffusion and convection rather than by compression. Models based upon
this mode of nuclear burning have been applied very successfully to the SN Ia
problem, and are able to reproduce many of their observed features remarkably
well. However, the models also indicate that SN Ia may differ considerably from
each other, which is of importance if they are to be used as standard candles.Comment: 11 pages, 4 figures. To appear in Proc. 10th Ann. Astrophys. Conf.
"Cosmic Explosions", Univ. of Maryland 1999, eds. S.S. Holt and W.W. Zhan
- …