168 research outputs found
Problem Assessment for China
One of the main purposes of the Food and Agriculture Program at IIASA is to build a series of national agricultural models, covering the relevant parts of the world's food production and consumptions.
China, having 22% of the world's population and producing 13,22% of the agricultural production, cannot be neglected in this context. As the first step of investigations, a problem assessment has been prepared. One should stress that the agricultural development in China as discussed in this paper, can be drastically changed in the nearest future. According to the latest information, new agricultural price policies have been introduced, which may have decisive effects. The basic problem, however, of, whether China will be able to feed her population, is a more complex one and depends on a series of different factors. This is an attempt to give a short summary of them
The Evolution of Helium Star Plus Carbon-Oxygen White Dwarf Binary Systems and Implications for Diverse Stellar Transients and Hypervelocity Stars
Helium accretion induced explosions in CO white dwarfs (WDs) are considered
promising candidates for a number of observed types of stellar transients,
including supernovae (SNe) of Type Ia and Type Iax. However, a clear favorite
outcome has not yet emerged. We explore the conditions of helium ignition in
the white dwarf and the final fates of helium star-WD binaries as function of
their initial orbital periods and component masses. We compute 274 model binary
systems with the Binary Evolution Code (BEC), where both components are fully
resolved. Stellar and orbital evolution is computed simultaneously, including
mass and angular momentum transfer, tides, and gravitational wave emission, as
well as differential rotation and internal hydrodynamic and magnetic angular
momentum transport. We find that helium detonations are expected only in
systems with the shortest initial orbital periods, and for initially massive
white dwarfs (MWD > 1.0 MSun ) and lower mass donors (Mdonor < 0.8 MSun), with
accumulated helium layers mostly exceeding 0.1 MSun. Upon detonation, these
systems would release the donor as a hypervelocity pre-WD runaway star, for
which we predict the expected range of kinematic and stellar properties.
Systems with more massive donors or initial periods exceeding 1.5 h will likely
undergo helium deflagrations after accumulating 0.1 - 0.001 MSun of helium.
Helium ignition in the white dwarf is avoided in systems with helium donor
stars below - 0.6 MSun, and lead to three distinctly different groups of double
white dwarf systems. The size of the parameter space open to helium detonation
corresponds to only about 3 % of the galactic SN Ia rate, and to 10 % of the SN
Iax rate, while the predicted large amounts of helium (>0.1 MSun) in
progenitors cannot easily be reconciled with observations of archetypical SN
Ia. ...Comment: Accepted for publication in A&A, 28 pages, 16 figures, 6 table
Velocity limits in the thermonuclear supernova ejection scenario for hypervelocity stars and the origin of US 708
Hypervelocity stars (HVS) are a class of stars moving at high enough
velocities to be gravitationally unbound from the Galaxy. Ejection from a close
binary system in which one of the components undergoes a thermonuclear
supernova (SN) has emerged as a promising candidate production mechanism for
the least massive specimens of this class. This study presents a thorough
theoretical analysis of candidate progenitor systems of thermonuclear SNe in
the single degenerate helium donor scenario in the relevant parameter space
leading to the ejection of HVS. The primary goal is investigation of the,
previously unclear, characteristics of the velocity spectra of the ejected
component. Presented are the results of 390 binary model sequences computed
with the MESA framework, investigating the evolution of supernova progenitors
composed of a helium-rich hot subdwarf and a accreting white dwarf. Results are
then correlated with an idealized kinematic analysis of the observed object US
708. It is seen that the ejection velocity spectra reach a maximum in the range
. Depending on the local Galactic
potential, all donors below are expected to become HVS.
This channel is able to account for runaway velocities up to with a Chandrasekhar mass accretor, exceeding
if super-Chandrasekhar mass detonations are taken into account. It is found
that the previously assumed mass of for US 708, combined with
more recently obtained proper motions, favor a sub-Chandrasekhar mass explosion
with a terminal WD mass between and . The presence
of clear ejection velocity maxima provides constraints on the terminal state of
a supernova progenitor. It is possible to discern certain types of explosion
mechanisms from the inferred ejection velocities alone.Comment: 17 pages, 14 figures, accepted for publication by A&A, replacement
due to typo in Fig. 1
Helium Accreting White Dwarfs as Progenitors of Explosive Stellar Transients
Accretion induced transients, which are thought to include the important, yet little understood, supernovae (SNe) of the Type Ia, are among the most luminous events observable in the cosmos. However, SNe Ia are only one of a number of distinct classes of transients that may be the result of accretion induced ignition of white dwarfs stars (WD). Other types include the somewhat recently identified SNe Iax, theoretically predicted SNe .Ia, classical novae or fast, faint, calcium-rich SNe. Theoretical predictions allow for the possibility of producing thermonuclear detonations at masses less than the Chandrasekhar mass in systems with low accretion rates, but the proposed mechanism (the double-detonation mechanism) sensitively depends on the conditions at the point of nuclear ignition, and thus on the evolution of the WD during the accretion phase, as well as physical mechanisms included in the calculations. This work discusses the viability of He-accreting WDs as candidate progenitors of SNe Ia and Iax and other transients from a number of different perspectives, presenting results of numerical simulations, focussing on the effects of previously neglected mechanisms, such as angular momentum diffusion through magnetic dynamo action (Tayler-Spruit mechanism) and the time variability of the mass transfer rate expected in physical binary systems. The constraints placed on progenitor systems by the evolutionary behavior of the non-degenerate companion is remarked upon, as is the expected evolutionary properties of the progenitor systems. It is shown that the physics included in the numerical framework have a significant impact on the obtainable predictions, that the double detonation scenario is statistically unable to account for a large majority of observed SNe Ia, but might, depending on the included physics, account for some of the observed peculiar transients. It is further shown, that inclusion of magnetic dynamos increase the amount of helium necessary to induce detonations, which reduces the viability of these systems as possible progenitors of SNe Ia and Iax, but suggests a possible link to faint, fast, Ca-rich SNe
The Hellan-Herrmann-Johnson and TDNNS method for linear and nonlinear shells
In this paper we extend the recently introduced mixed Hellan-Herrmann-Johnson
(HHJ) method for nonlinear Koiter shells to nonlinear Naghdi shells by means of
a hierarchical approach. The additional shearing degrees of freedom are
discretized by H(curl)-conforming N\'ed\'elec finite elements entailing a shear
locking free method. By linearizing the models we obtain in the small strain
regime linear Kirchhoff-Love and Reissner-Mindlin shell formulations, which
reduce for plates to the originally proposed HHJ and TDNNS method for
Kirchhoff-Love and Reissner-Mindlin plates, respectively. By using the Regge
interpolation operator we obtain locking-free arbitrary order shell methods.
Additionally, the methods can be directly applied to structures with kinks and
branched shells. Several numerical examples and experiments are performed
validating the excellence performance of the proposed shell elements
On pressure robustness and independent determination of displacement and pressure in incompressible linear elasticity
We investigate the possibility to determine the divergence-free displacement
\emph{independently} from the pressure reaction for a class of
boundary value problems in incompressible linear elasticity. If not possible,
we investigate if it is possible to determine it \emph{pressure robustly}, i.e.
pollution free from the pressure reaction.
For convex domains there is but one variational boundary value problem among
the investigated that allows the independent determination. It is the one with
essential no-penetration conditions combined with homogeneous tangential
traction conditions.
Further, in most but not all investigated cases, the weakly divergence-free
displacement can be computed pressure robustly provided the total body force is
decomposed into its direct sum of divergence- and rotation-free components
using a Helmholtz decomposition. The elasticity problem is solved using these
components as separate right-hand sides. The total solution is obtained using
the superposition principle.
We employ a higher-order finite element formulation with
discontinuous pressure elements. It is \emph{inf-sup} stable for polynomial
degree but not pressure robust by itself.
We propose a three step procedure to solve the elasticity problem preceded by
the Helmholtz decomposition of the total body force. The extra cost for the
three-step procedure is essentially the cost for the Helmholtz decomposition of
the assembled total body force, and the small cost of solving the elasticity
problem with one extra right-hand side. The results are corroborated by
theoretical derivations as well as numerical results.Comment: 31 pages, 31 references and 48 figure
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