5,689 research outputs found
Vertical shear instability in accretion disc models with radiation transport
The origin of turbulence in accretion discs is still not fully understood.
While the magneto-rotational instability is considered to operate in
sufficiently ionized discs, its role in the poorly ionized protoplanetary disc
is questionable. Recently, the vertical shear instability (VSI) has been
suggested as a possible alternative. Our goal is to study the characteristics
of this instability and the efficiency of angular momentum transport, in
extended discs, under the influence of radiative transport and irradiation from
the central star. We use multi-dimensional hydrodynamic simulations to model a
larger section of an accretion disc. First we study inviscid and weakly viscous
discs using a fixed radial temperature profile in two and three spatial
dimensions. The simulations are then extended to include radiative transport
and irradiation from the central star. In agreement with previous studies we
find for the isothermal disc a sustained unstable state with a weak positive
angular momentum transport of the order of . Under the
inclusion of radiative transport the disc cools off and the turbulence
terminates. For discs irradiated from the central star we find again a
persistent instability with a similar value as for the isothermal
case. We find that the VSI can indeed generate sustained turbulence in discs
albeit at a relatively low level with about few times Comment: 12 pages, 24 figures, accepted for publication in Astronomy &
Astrophysic
Optical waveguiding in proton-implanted GaAs
We have produced optical waveguides in n-type GaAs by implantation with 300-keV protons. The guiding is shown to be due to the elimination of charge carriers from the implanted region. Annealing of the waveguide leads to very large reductions in the 1.15-” guided-wave absorption
Within Cities and Suburbs: Racial Residential Concentration and the Spatial Distribution of Employment Opportunities across Submetropolitan Areas
In this paper, we examine and compare the spatial distributions of jobs and people across submetropolitan areas using data on firms from the Multi-City Study of Urban Inequality and data on people from the U.S. Bureau of the Census. The results indicate that less-educated people and those on public assistance mostly reside in areas with high minority populations. Low-skill jobs are quite scarce in these areas, while the availability of such jobs relative to less-educated people in heavily white suburban areas is high. Large fractions of the low-skill jobs in these metropolitan areas are not accessible by public transit. Furthermore, there is significant variation within both central cities and suburbs in the ethnic composition of residents and in the availability of low-skill jobs. The ability of various minority groups to gain employment in each area depends heavily on the ethnic composition of the particular area.
Correlation effects in ionic crystals: I. The cohesive energy of MgO
High-level quantum-chemical calculations, using the coupled-cluster approach
and extended one-particle basis sets, have been performed for (Mg2+)n (O2-)m
clusters embedded in a Madelung potential. The results of these calculations
are used for setting up an incremental expansion for the correlation energy of
bulk MgO. This way, 96% of the experimental cohesive energy of the MgO crystal
is recovered. It is shown that only 60% of the correlation contribution to the
cohesive energy is of intra-ionic origin, the remaining part being caused by
van der Waals-like inter-ionic excitations.Comment: LaTeX, 20 pages, no figure
Pseudopotential Approaches to Ca, Sr and Ba Hybrides. Why are some Alkaline Earth MX Compounds Bent?
Quasirelativistic and nonrelativistic lo-valence-electronp seudopotentialsf or Ca, Sr, and Ba are presented. Results of calculations with 6s6p5d basis sets for MH, MH , and MH, are compared with all-electron and 2-valence-electron pseudopotential calculations with and , without core-polarization potentials. The lo-valence-electron pseudopotential approach agrees well with all-electron calculations. It circumvents problems for the 2-valence-electron pseudopotentials arising from an incomplete separation of valence and subvalence shells in polar molecular systems due to strongly contracted occupied (n - 1 )-d orbitals. All higherlevel calculations show SrH and BaII, to be bent with angles of - 140° and 120°, respectively, while CaH is linear with a flat potential-energy surface for the bending motion. The use of a core-polarization potential together with the 2-valence-electronp seudopotentiala pproach allows an investigation of the relative importance of core-polarization vs direct d-orbital bonding participation as reasons for the bent structures. The calculations strongly suggest that both contribute to the bending in SrH and BaII. Even at the Hartree-Fock level of theory lovalence- electronp seudopotentialc alculations given reasonablea nglesw hen the potentialenergy surface is not exceedingly flat, and only moderately contracted basis sets including both compact d functions and diffuse p functions are used. The effect of core-valence correlation and the importance off functions also are discussed
Thermal sensitivity of grapevine leaves affected by Plasmopara viticola and water stress
Research Note
Particle accretion onto planets in discs with hydrodynamic turbulence
The growth process of proto-planets can be sped-up by accreting a large
number of solid, pebble-sized objects that are still present in the
protoplanetary disc. It is still an open question on how efficient this process
works in realistic turbulent discs. Here, we investigate the accretion of
pebbles in turbulent discs that are driven by the purely hydrodynamical
vertical shear instability (VSI). For this purpose, we perform global
three-dimensional simulations of locally isothermal, VSI turbulent discs with
embedded protoplanetary cores from 5 to 100 that are placed at 5.2
au distance from the star. In addition, we follow the evolution of a swarm of
embedded pebbles of different size under the action of drag forces between gas
and particles in this turbulent flow. Simultaneously, we perform a set of
comparison simulations for laminar viscous discs where the particles experience
stochastic kicks. For both cases, we measure the accretion rate onto the cores
as a function of core mass and Stokes number () of the particles and
compare it to recent MRI turbulence simulations. Overall the dynamic is very
similar for the particles in the VSI turbulent disc and the laminar case with
stochastic kicks. For the small mass planets (i.e. 5 and 10 ),
well-coupled particles with , which have a size of about one meter
at this location, we find an accretion efficiency (rate of particles accreted
over drifting inward) of about 1.6-3%. For smaller and larger particles this
efficiency is higher. However, the fast inward drift for particles
makes them the most effective for rapid growth, leading to mass doubling times
of about 20,000 yr. For masses between 10 and 30 the core reaches
the pebble isolation mass and the particles are trapped at the pressure maximum
just outside of the planet, shutting off further particle accretion.Comment: 18 pages, accepted to A&
Electron correlations for ground state properties of group IV semiconductors
Valence energies for crystalline C, Si, Ge, and Sn with diamond structure
have been determined using an ab-initio approach based on information from
cluster calculations. Correlation contributions, in particular, have been
evaluated in the coupled electron pair approximation (CEPA), by means of
increments obtained for localized bond orbitals and for pairs and triples of
such bonds. Combining these results with corresponding Hartree-Fock (HF) data,
we recover about 95 % of the experimental cohesive energies. Lattice constants
are overestimated at the HF level by about 1.5 %; correlation effects reduce
these deviations to values which are within the error bounds of this method. A
similar behavior is found for the bulk modulus: the HF values which are
significantly too high are reduced by correlation effects to about 97 % of the
experimental values.Comment: 22 pages, latex, 2 figure
Ab initio wavefunction based methods for excited states in solids: correlation corrections to the band structure of ionic oxides
Ab initio wavefunction based methods are applied to the study of electron
correlation effects on the band structure of oxide systems. We choose MgO as a
prototype closed-shell ionic oxide. Our analysis is based on a local
Hamiltonian approach and performed on finite fragments cut from the infinite
solid. Localized Wannier functions and embedding potentials are obtained from
prior periodic Hartree-Fock (HF) calculations. We investigate the role of
various electron correlation effects in reducing the HF band gap and modifying
the band widths. On-site and nearest-neighbor charge relaxation as well as
long-range polarization effects are calculated. Whereas correlation effects are
essential for computing accurate band gaps, we found that they produce smaller
changes on the HF band widths, at least for this material. Surprisingly, a
broadening effect is obtained for the O 2p valence bands. The ab initio data
are in good agreement with the energy gap and band width derived from
thermoreflectance and x-ray photoemission experiments. The results show that
the wavefunction based approach applied here allows for well controlled
approximations and a transparent identification of the microscopic processes
which determine the electronic band structure
Correlation effects in MgO and CaO: Cohesive energies and lattice constants
A recently proposed computational scheme based on local increments has been
applied to the calculation of correlation contributions to the cohesive energy
of the CaO crystal. Using ab-initio quantum chemical methods for evaluating
individual increments, we obtain 80% of the difference between the experimental
and Hartree-Fock cohesive energies. Lattice constants corrected for correlation
effects deviate by less than 1% from experimental values, in the case of MgO
and CaO.Comment: LaTeX, 4 figure
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