3,898 research outputs found
Gas- and dust evolution in protoplanetary disks
Context. Current models of the size- and radial evolution of dust in
protoplanetary disks generally oversimplify either the radial evolution of the
disk (by focussing at one single radius or by using steady state disk models)
or they assume particle growth to proceed monodispersely or without
fragmentation. Further studies of protoplanetary disks - such as observations,
disk chemistry and structure calculations or planet population synthesis models
- depend on the distribution of dust as a function of grain size and radial
position in the disk.
Aims. We attempt to improve upon current models to be able to investigate how
the initial conditions, the build-up phase, and the evolution of the
protoplanetary disk influence growth and transport of dust.
Methods. We introduce a new version of the model of Brauer et al. (2008) in
which we now include the time-dependent viscous evolution of the gas disk, and
in which more advanced input physics and numerical integration methods are
implemented.
Results. We show that grain properties, the gas pressure gradient, and the
amount of turbulence are much more influencing the evolution of dust than the
initial conditions or the build-up phase of the protoplanetary disk. We
quantify which conditions or environments are favorable for growth beyond the
meter size barrier. High gas surface densities or zonal flows may help to
overcome the problem of radial drift, however already a small amount of
turbulence poses a much stronger obstacle for grain growth.Comment: accepted to A&
Trapped surfaces in spherical expanding open universes
Consider spherically symmetric initial data for a cosmology which, in the
large, approximates an open Friedmann-Lema{\^\i}tre
universe. Further assume that the data is chosen so that the trace of the
extrinsic curvature is a constant and that the matter field is at rest at this
instant of time. One expects that no trapped surfaces appear in the data if no
significant clump of excess matter is to be found. This letter confirms this
belief by displaying a necessary condition for the existence of trapped
surfaces.This necessary condition, simply stated, says that a relatively large
amount of excess matter must be concentrated in a small volume for trapped
surfaces to appear.Comment: 8 pages, Late
Dust retention in protoplanetary disks
Context: Protoplanetary disks are observed to remain dust-rich for up to
several million years. Theoretical modeling, on the other hand, raises several
questions. Firstly, dust coagulation occurs so rapidly, that if the small dust
grains are not replenished by collisional fragmentation of dust aggregates,
most disks should be observed to be dust poor, which is not the case. Secondly,
if dust aggregates grow to sizes of the order of centimeters to meters, they
drift so fast inwards, that they are quickly lost.
Aims: We attempt to verify if collisional fragmentation of dust aggregates is
effective enough to keep disks 'dusty' by replenishing the population of small
grains and by preventing excessive radial drift.
Methods: With a new and sophisticated implicitly integrated coagulation and
fragmentation modeling code, we solve the combined problem of coagulation,
fragmentation, turbulent mixing and radial drift and at the same time solve for
the 1-D viscous gas disk evolution.
Results: We find that for a critical collision velocity of 1 m/s, as
suggested by laboratory experiments, the fragmentation is so effective, that at
all times the dust is in the form of relatively small particles. This means
that radial drift is small and that large amounts of small dust particles
remain present for a few million years, as observed. For a critical velocity of
10 m/s, we find that particles grow about two orders of magnitude larger, which
leads again to significant dust loss since larger particles are more strongly
affected by radial drift.Comment: Letter accepted 3 July 2009, included comments of language edito
Anatomy of extraordinary rainfall and flash flood in a Dutch lowland catchment
On 26 August 2010 the eastern part of The Netherlands and the bordering part of Germany were struck by a series of rainfall events lasting for more than a day. Over an area of 740 km2 more than 120 mm of rainfall were observed in 24 h. This extreme event resulted in local flooding of city centres, highways and agricultural fields, and considerable financial loss. In this paper we report on the unprecedented flash flood triggered by this exceptionally heavy rainfall event in the 6.5 km2 Hupsel Brook catchment, which has been the experimental watershed employed by Wageningen University since the 1960s. This study aims to improve our understanding of the dynamics of such lowland flash floods. We present a detailed hydrometeorological analysis of this extreme event, focusing on its synoptic meteorological characteristics, its space-time rainfall dynamics as observed with rain gauges, weather radar and a microwave link, as well as the measured soil moisture, groundwater and discharge response of the catchment. At the Hupsel Brook catchment 160 mm of rainfall was observed in 24 h, corresponding to an estimated return period of well over 1000 years. As a result, discharge at the catchment outlet increased from 4.4 × 10-3 to nearly 5 m3 s-1. Within 7 h discharge rose from 5 × 10-2 to 4.5 m3 s-1. The catchment response can be divided into four phases: (1) soil moisture reservoir filling, (2) groundwater response, (3) surface depression filling and surface runoff and (4) backwater feedback. The first 35 mm of rainfall were stored in the soil without a significant increase in discharge. Relatively dry initial conditions (in comparison to those for past discharge extremes) prevented an even faster and more extreme hydrological response
Symmetric groups are determined by their character degrees
Let be a finite group. Let be the first column of the ordinary
character table of In this paper, we will show that if
then As a consequence, we show that is uniquely determined
by the structure of the complex group algebra \C S_n.Comment: 12 pages, to appear in Journal of Algebr
On the Definition of Averagely Trapped Surfaces
Previously suggested definitions of averagely trapped surfaces are not
well-defined properties of 2-surfaces, and can include surfaces in flat
space-time. A natural definition of averagely trapped surfaces is that the
product of the null expansions be positive on average. A surface is averagely
trapped in the latter sense if and only if its area and Hawking mass
satisfy the isoperimetric inequality , with similar inequalities
existing for other definitions of quasi-local energy.Comment: 4 page
Twisted Conjugacy Classes in Abelian Extensions of Certain Linear Groups
Given an automorphism , one has an action of
on itself by -twisted conjugacy, namely, . The orbits
of this action are called -twisted conjugacy classes. One says that
has the -property if there are infinitely many
-twisted conjugacy classes for every automorphism of . In
this paper we show that SL and its congruence subgroups have
the -property. Further we show that any (countable) abelian extension
of has the -property where is a torsion free
non-elementary hyperbolic group, or SL, Sp or
a principal congruence subgroup of SL or the fundamental group
of a complete Riemannian manifold of constant negative curvature
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