2,712 research outputs found
Formation and accretion history of terrestrial planets from runaway growth through to late time: implications for orbital eccentricity
Remnant planetesimals might have played an important role in reducing the
orbital eccentricities of the terrestrial planets after their formation via
giant impacts. However, the population and the size distribution of remnant
planetesimals during and after the giant impact stage are unknown, because
simulations of planetary accretion in the runaway growth and giant impact
stages have been conducted independently. Here we report results of direct
N-body simulations of the formation of terrestrial planets beginning with a
compact planetesimal disk. The initial planetesimal disk has a total mass and
angular momentum as observed for the terrestrial planets, and we vary the width
(0.3 and 0.5AU) and the number of planetesimals (1000-5000). This initial
configuration generally gives rise to three final planets of similar size, and
sometimes a fourth small planet forms near the location of Mars. Since a
sufficient number of planetesimals remains, even after the giant impact phase,
the final orbital eccentricities are as small as those of the Earth and Venus.Comment: 36 pages, 9 figures, 1 table, Accepted in Ap
DATA CONSTRUCTORS: ON THE INTEGRATION OF RULES AND RELATIONS
Although the goals and means of rule-based and data-based systems are
too different to be fully integrated at the present time, it seems appropriate to
investigate a closer integration of language constructs and a better cooperation
of execution models for both kinds of approaches.
In this paper, we propose a new language construct called constructor that â when applied to a base relation â causes relation membership to become true
for all tuples constructable through the predicates provided by the constructor
definition. The approach is shown to provide expressive power at least
equivalent to PROLOG's declarative semantics while blending well both with a
strongly typed modular programming language and with a relational calculus
query formalism. A three-step compilation, optimization, and evaluation methodology
for expressions with constructed relations is described that integrates
constructors with the surrounding database programming environment. In particular,
many recursive queries can be evaluated more efficiently within the
set-construction framework of database systems than with proof-oriented
methods typical for a rule-based approach.Information Systems Working Papers Serie
Total synthesis of the cyclic monoterpenoid pyrano[3,2-a]carbazole alkaloids derived from 2-hydroxy-6-methylcarbazole
The synthesis of seven pyrano[3,2-a]carbazole alkaloids has been achieved using their putative biogenetic precursor 2-hydroxy-6-methylcarbazole as key intermediate
Non-equilibrium sedimentation of colloids on the particle scale
We investigate sedimentation of model hard sphere-like colloidal dispersions
confined in horizontal capillaries using laser scanning confocal microscopy,
dynamical density functional theory, and Brownian dynamics computer
simulations. For homogenized initial states we obtain quantitative agreement of
the results from the respective approaches for the time evolution of the
one-body density distribution and the osmotic pressure on the walls. We
demonstrate that single particle information can be obtained experimentally in
systems that were initialized further out-of-equilibrium such that complex
lateral patterns form.Comment: to be published in Phys. Rev. Let
Where do we stand On Organ Printing
Abstract Attitude towards organ donation and the risks associated with organ transplantation drive the search for alternatives. One such alternative, albeit a conceptual level, could be the generation of an organ replacement in a controlled setting. For instance, growing suitable cells onto a printed matrix under appropriate conditions would then lead to the formation of a functional organ. How about the practical issues surrounding either duplication or de novo generation of an organ with, say, a device to print a suitable matrix and grow and differentiate cells on it? Here, we wish to outline selected safety-related questions arising from the ex vivo growth, differentiation and maintenance of cells or cell systems
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