15,476 research outputs found
Modelling Accretion in Transitional Disks
Transitional disks are protoplanetary disk around young stars that display
inner holes in the dust distribution within a few AU, which is accompanied
nevertheless by some gas accretion onto the central star. These cavities could
possibly be created by the presence of one or more massive planets. If the gap
is created by planets and gas is still present in it, then there should be a
flow of gas past the planet into the inner region. It is our goal to study the
mass accretion rate into the gap and in particular the dependency on the
planet's mass and the thermodynamic properties of the disk. We performed 2D
hydro simulations for disks with embedded planets. We added radiative cooling
from the disk surfaces, radiative diffusion in the disk midplane, and stellar
irradiation to the energy equation to have more realistic models. The mass flow
rate into the gap region depends, for given disk thermodynamics,
non-monotonically on the mass of the planet. Generally, more massive planets
open wider and deeper gaps which would tend to reduce the mass accretion into
the inner cavity. However, for larger mass planets the outer disk becomes
eccentric and the mass flow rate is enhanced over the low mass cases. As a
result, for the isothermal disks the mass flow is always comparable to the
expected mass flow of unperturbed disks M_d, while for more realistic radiative
disks the mass flow is very small for low mass planets (<= 4 M_jup) and about
50% for larger planet masses. For the radiative disks that critical planet mass
for the disk to become eccentric is much larger that in the isothermal case.
Massive embedded planets can reduce the mass flow across the gap considerably,
to values of about an order of magnitude smaller than the standard disk
accretion rate, and can be responsible for opening large cavities. The
remaining mass flow into the central cavity is in good agreement with the
observations.Comment: 10 pages, 29 figures, accepted for publication in Astronomy &
Astrophysic
Urbanization Sans Development?
From a theoretical standpoint, the question has to be asked whether or not the census should retain its present definition of urban areas. There is a need for a more meaningful integration of the structural, institutional and demographic criteria, if the term is to hold substantive meaning. In this light, this paper proposes that a two-tier urban definition be employed. The old definition should be retained for trend analysis purposes while a new definition that incorporates socioeconomic purposes should be instituted.urban management, population and family relation, urbanization, data and statistics
Urbanization Sans Development?
From a theoretical standpoint, the question has to be asked whether or not the census should retain its present definition of urban areas. There is a need for a more meaningful integration of the structural, institutional and demographic criteria, if the term is to hold substantive meaning. In this light, this paper proposes that a two-tier urban definition be employed. The old definition should be retained for trend analysis purposes while a new definition that incorporates socioeconomic purposes should be instituted.urban management, population and family relation, urbanization, data and statistics
Stationary and transient leakage current in the Pauli spin blockade
We study the effects of cotunneling and a non-uniform Zeeman splitting on the
stationary and transient leakage current through a double quantum dot in the
Pauli spin blockade regime. We find that the stationary current due to
cotunneling vanishes at low temperature and large applied magnetic field,
allowing for the dynamical preparation of a pure spin ground state, even at
large voltage bias. Additionally, we analyze current that flows between
blocking events, characterized, in general, by a fractional effective charge
. This charge can be used as a sensitive probe of spin relaxation
mechanisms and can be used to determine the visibility of Rabi oscillations.Comment: v1: 4 pages; v2: 4 pages+ additional supplementary material, version
to appear in PR
Computing generators of the unit group of an integral abelian group ring
We describe an algorithm for obtaining generators of the unit group of the
integral group ring ZG of a finite abelian group G. We used our implementation
in Magma of this algorithm to compute the unit groups of ZG for G of order up
to 110. In particular for those cases we obtained the index of the group of
Hoechsmann units in the full unit group. At the end of the paper we describe an
algorithm for the more general problem of finding generators of an arithmetic
group corresponding to a diagonalizable algebraic group
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