55,552 research outputs found
Modelling ion populations in astrophysical plasmas: carbon in the solar transition region
The aim of this work is to improve the modelling of ion populations in higher
density, lower temperature astrophysical plasmas, of the type commonly found in
lower solar and stellar atmospheres. Ion population models for these regions
frequently employ the coronal approximation, which assumes conditions more
suitable to the upper solar atmosphere, where high temperatures and lower
densities prevail. Using the coronal approximation for modelling the solar
transition region gives theoretical lines intensities for the Li-like and
Na-like isoelectronic sequences which can be factors of 2-5 times lower than
observed. The works of Burgess & Summers (1969) and Nussbaumer & Storey (1975)
showed the important part ions in excited levels play when included in the
modelling. Their models, however, used approximations for the atomic rates to
determine the ion balance. Presented here is the first stage in updating these
earlier models of carbon by using rates from up-to-date atomic calculations and
more recent photo-ionising radiances for the quiet Sun. Where such atomic rates
are not readily available, in the case of electron-impact direct ionisation and
excitation--auto-ionisation, new calculations have been made and compared to
theoretical and experimental studies. The effects each atomic process has on
the ion populations as density changes is demonstrated, and final results from
the modelling are compared to the earlier works. Lastly, the new results for
ion populations are used to predict line intensities for the solar transition
region in the quiet Sun, and these are compared with predictions from
coronal-approximation modelling and with observations. Significant improvements
in the predicted line intensities are seen in comparison to those obtained from
zero-density modelling of carbon.Comment: Draft accepted by A&A, 13 pages, 15 figure
On the Average Comoving Number Density of Halos
I compare the numerical multiplicity function given in Yahagi, Nagashima &
Yoshii (2004) with the theoretical multiplicity function obtained by means of
the excursion set model and an improved version of the barrier shape obtained
in Del Popolo & Gambera (1998), which implicitly takes account of total angular
momentum acquired by the proto-structure during evolution and of a non-zero
cosmological constant. I show that the multiplicity function obtained in the
present paper, is in better agreement with Yahagi, Nagashima & Yoshii (2004)
simulations than other previous models (Sheth & Tormen 1999; Sheth, Mo & Tormen
2001; Sheth & Tormen 2002; Jenkins et al. 2001) and that differently from some
previous multiplicity function models (Jenkins et al. 2001; Yahagi, Nagashima &
Yoshii 2004) it was obtained from a sound theoretical background
Lorentzian compact manifolds: isometries and geodesics
In this work we investigate families of compact Lorentzian manifolds in
dimension four. We show that every lightlike geodesic on such spaces is
periodic, while there are closed and non-closed spacelike and timelike
geodesics. Their isometry groups are computed. We also show that there is a non
trivial action by isometries of \Heis_3(\RR) on the nilmanifold S^1\times
(\Gamma_k \bsh \Heis_3(\RR)) for a lattice of \Heis_3(\RR).Comment: 17 page
A Hamiltonian functional for the linearized Einstein vacuum field equations
By considering the Einstein vacuum field equations linearized about the
Minkowski metric, the evolution equations for the gauge-invariant quantities
characterizing the gravitational field are written in a Hamiltonian form by
using a conserved functional as Hamiltonian; this Hamiltonian is not the analog
of the energy of the field. A Poisson bracket between functionals of the field,
compatible with the constraints satisfied by the field variables, is obtained.
The generator of spatial translations associated with such bracket is also
obtained.Comment: 5 pages, accepted in J. Phys.: Conf. Serie
Thermal structure of hot non-flaring corona from Hinode/EIS
In previous studies a very hot plasma component has been diagnosed in solar
active regions through the images in three different narrow-band channels of
SDO/AIA. This diagnostic from EUV imaging data has also been supported by the
matching morphology of the emission in the hot Ca XVII line, as observed with
Hinode/EIS. This evidence is debated because of unknown distribution of the
emission measure along the line of sight. Here we investigate in detail the
thermal distribution of one of such regions using EUV spectroscopic data. In an
active region observed with SDO/AIA, Hinode/EIS and XRT, we select a subregion
with a very hot plasma component and another cooler one for comparison. The
average spectrum is extracted for both, and 14 intense lines are selected for
analysis, that probe the 5.5 < log T < 7 temperature range uniformly. From
these lines the emission measure distributions are reconstructed with the MCMC
method. Results are cross-checked with comparison of the two subregions, with a
different inversion method, with the morphology of the images, and with the
addition of fluxes measured with from narrow and broad-band imagers. We find
that, whereas the cool region has a flat and featureless distribution that
drops at temperature log T >= 6.3, the distribution of the hot region shows a
well-defined peak at log T = 6.6 and gradually decreasing trends on both sides,
thus supporting the very hot nature of the hot component diagnosed with
imagers. The other cross-checks are consistent with this result. This study
provides a completion of the analysis of active region components, and the
resulting scenario supports the presence of a minor very hot plasma component
in the core, with temperatures log T > 6.6.Comment: 12 pages, 8 figures, accepted for publicatio
Mesophases in Nearly 2D Room-Temperature Ionic Liquids
Computer simulations of (i) a [C12mim][Tf2N] film of nanometric thickness
squeezed at kbar pressure by a piecewise parabolic confining potential reveal a
mesoscopic in-plane density and composition modulation reminiscent of
mesophases seen in 3D samples of the same room-temperature ionic liquid (RTIL).
Near 2D confinement, enforced by a high normal load, relatively long aliphatic
chains are strictly required for the mesophase formation, as confirmed by
computations for two related systems made of (ii) the same [C12mim][Tf2N]
adsorbed at a neutral solid surface and (iii) a shorter-chain RTIL
([C4mim][Tf2N]) trapped in the potential well of part i. No in-plane modulation
is seen for ii and iii. In case ii, the optimal arrangement of charge and
neutral tails is achieved by layering parallel to the surface, while, in case
iii, weaker dispersion and packing interactions are unable to bring aliphatic
tails together into mesoscopic islands, against overwhelming entropy and
Coulomb forces. The onset of in-plane mesophases could greatly affect the
properties of long-chain RTILs used as lubricants.Comment: 24 pages 10 figure
Parametric thermal analysis for the optimization of Double Walled Tubes layout in the Water Cooled Lithium Lead inboard blanket of DEMO fusion reactor
Within the roadmap that will lead to the nuclear fusion exploitation for electric energy generation, the construction of a DEMOnstration (DEMO) reactor is, probably, the most important milestone to be reached since it will demonstrate the technological feasibility and economic competitiveness of an industrial-scale nuclear fusion reactor. In order to reach this goal, several European universities and research centres have joined their efforts in the EUROfusion action, funded by HORIZON 2020 UE programme. Within the framework of EUROfusion research activities, ENEA and University of Palermo are involved in the design of the Water-Cooled Lithium Lead Breeding Blanket (WCLL BB), that is one of the two BB concepts under consideration to be adopted in the DEMO reactor. It is mainly characterized by a liquid lithium-lead eutectic alloy acting as breeder (lithium) and neutron multiplier (lead), as well as by subcooled pressurized water as coolant. Two separate circuits, both characterized by a pressure of 15.5 MPa and inlet/outlet temperatures of 295 °C/328 °C, are deputed to cool down the First Wall (FW) and the Breeder Zone (BZ). The former consists in a system of radial-toroidal-radial C-shaped squared channels where countercurrent water flow occurs while the latter relies in the use of bundles of poloidal-radial Double Walled Tubes (DWTs) housed within the breeder. A parametric thermal study has been carried out in order to assess the best DWTs' layout assuring that the structural material maximum temperature does not overcome the allowable limit of 550 °C and that the overall coolant thermal rise fulfils the design target value of 33 °C. The study has been performed following a theoretical-numerical approach based on the Finite Element Method (FEM) and adopting the quoted Abaqus FEM code. Main assumptions and models together with results obtained are herewith reported and critically discussed
On azimuthal spin correlations in Higgs plus jet events at LHC
We consider the recent proposal that the distribution of the difference
between azimuthal angles of the two accompanying jets in gluon-fusion induced
Higgs-plus-two-jet events at LHC reflects the CP of the Higgs boson produced.
We point out that the hierarchy between the Higgs boson mass and the jet
transverse energy makes this observable vulnerable to logarithmically enhanced
higher-order perturbative corrections. We present an evolution equation that
describes the scale variation of the azimuthal angular correlation for the two
jets. The emission of extra partons leads to a significant suppression of the
correlation. Using the HERWIG Monte Carlo event generator, we carry out a
parton-shower analysis to confirm the findings.Comment: Published version. 11 pages, 4 figures, uses JHEP3.cl
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