1,168 research outputs found
Global modeling of comets: Nucleus, neutral and ionized coma of comets 67P/Churyumov-Gerasimenko and 46P/Wirtanen. Preparations for the Rosetta Radio Science Investigations.
Models of the thermal behavior of a cometary nucleus, the evolution of the neutral gas coma, the ionized cometary coma and of the interaction of the cometary plasma with the solar wind are studied in this work. The general aim is to develop a global model of the comet and its environment in order to characterize the physical conditions around comets 67P/Churyumov-Gerasimenko and 46P/Wirtanen with respect to the heliocentric distance. The results also provide estimates of the effects of the cometary environment on the radio science investigations experiment (RSI) aboard the spacecraft Rosetta. After the launch that is scheduled for February 2004, the Rosetta mission is planned to encounter comet 67P/Churyumov-Gerasimenko and accompany it on its orbit. Comet 46P/Wirtanen has been the original target comet, but serves now as back-up target due to the postponement of the Rosetta launch in January 2003. The model of the heat diffusion within the cometary nucleus is one-dimensional. A grid of one-dimensional models is distributed over the nucleus in order to determine the temperature distribution and the sublimation characteristics of the comet on the whole surface of the comet. A heat balance equation is applied as boundary condition on the surface. Many parameters that have to be accounted for in a heat diffusion model are not precisely known to date. The variation of these parameters within reasonable limits yields a wide range of possible results. The heat diffusion within the cometary nucleus is derived from an energy conservation equation that includes heat conduction through the porous cometary material and heat convection due to the transport of latent heat by the gas phase within the nucleus. Model results are evaluated by a comparison of modeled and observed global gas production rates. Exemplary maps of the local temperature distribution and local sublimation rates at particular heliocentric distances are also provided. The neutral gas coma of the comet is modeled with a hydrodynamic approximation. This method is justified within a collision dominated regime. Due to the expected weak gas production of a comet at large heliocentric distances, this hydrodynamic regime might be small and might not enclose the whole nucleus. When the comet approaches the sun and the gas production increases, the hydrodynamic regime extends to cometocentric distances of several hundred or thousand kilometer. The gas mass flux within the coma perturbs an orbiting spacecraft. The acceleration of the spacecraft due to the gas mass flux is evaluated with the model results. The resulting change in velocity can be measured as a Doppler shift of the recorded frequency of the carrier signal. Case studies at several heliocentric distances are carried out. It turns out that even at heliocentric distances of ~3 AU the drag force of the gas can become strong enough to perturb the measurements of the second order gravity coefficients, which is a primary science objective of RSI. The ionized coma of a comet can also have an effect on the carrier signal. Changes of the electron content in the line of sight between spacecraft and observer at earth are in principle observable. A one-dimensional model of the plasma density at the comet-sun axis is developed. The assumption of photochemical equilibrium is not necessarily justified within the coma of weak outgassing comets. The continuity equation of the plasma density has to be solved without this simplifying assumption. A model of the electron temperature profile is also generated. The transition from a regime where electrons are effectively cooled to a region with temperatures of the electron fluid similar to solar wind levels is assumed to set in at the position of the thermal electron collisionopause. The plasma densities obtained from the ionospheric model indicate only minor effects on the carrier signal. The interaction of the cometary plasma with the solar wind is also studied. The respective standoff distances of the bow shock, the cavity surface and the collisionopause of comets 67P/Churyumov-Gerasimenko and 46P/Wirtanen are determined with respect to the heliocentric distance. The variation of the solar wind parameters with heliocentric distance is accounted for. Effects of transient solar events, such as solar flares or coronal mass ejections, are discussed. It can be concluded that the plasma environment of comets 67P/Churyumov-Gerasimenko and 46P/Wirtanen and their interaction with the solar wind will have only a minor effect on the carrier signal. Special scenarios might be needed in order to locate plasma boundaries within the cometary environment with RSI
Search for three alpha states around an O core in Si
We investigate the existence of weakly coupled gas-like states comprised of
three particles around an O core in Si. We calculate the
excited states in Si using the multi-configuration mixing method based
on the O + 3 cluster model. We also include the O +
C and Mg + basis wave functions prepared by the
generator coordinate method. To identify the gas-like states, we calculate the
isoscalar monopole transition strengths and the overlap of the obtained states
with the geometrical cluster wave function and the
Tohsaki-Horiuchi-Schuck-R\"{o}pke (THSR) wave function. The results show that
the obtained fourth and twelfth states significantly overlap with the THSR wave
function. These two states clearly coexist with the O + C cluster
states, emerging at similar energies. The calculated isoscalar monopole
strengths between those two states are significantly large, indicating that the
states are members of the excitation mode. Furthermore, the calculated
root-mean-squared (RMS) radii for these states also suggest that a layer of
gas-like three particles could exist around the surface of the
O core, which can be described as a "two-dimensional gas" in the
intermediate state before the Hoyle-like three states emerge.Comment: 5 pages, 3 figure
Within-person structures of daily cognitive performance cannot be inferred from between-person structures of cognitive abilities
First published online: 09 June 2020Over a century of research on between-person differences has resulted in the consensus that human cognitive abilities are hierarchically organized, with a general factor, termed general intelligence or "g," uppermost. Surprisingly, it is unknown whether this body of evidence is informative about how cognition is structured within individuals. Using data from 101 young adults performing nine cognitive tasks on 100 occasions distributed over six months, we find that the structures of individuals' cognitive abilities vary among each other, and deviate greatly from the modal between-person structure. Working memory contributes the largest share of common variance to both between- and within-person structures, but the g factor is much less prominent within than between persons. We conclude that between-person structures of cognitive abilities cannot serve as a surrogate for within-person structures. To reveal the development and organization of human intelligence, individuals need to be studied over time
Di-neutron elastic transfer in the 4He(6He,6He)4He reaction
Elastic He+He data measured at 15.9, and 60.3
MeV have been analyzed within the coupled reaction channels (CRC) formalism,
with the elastic-scattering and two-neutron () transfer amplitudes
coherently included. Contributions from the direct (one-step) and sequential
(two-step) -transfers were treated explicitly based on a realistic
assumption for the -transfer form factor. The oscillatory pattern observed
in He(He,He)He angular distribution at low energies was found
to be due to an interference between the elastic scattering and -transfer
amplitudes. Our CRC analysis shows consistently that the direct -transfer
strongly dominates over the sequential transfer and thus confirms the dominance
of 2He configuration over the He one in the He wave function.
This result suggests a strong clusterization of the two valence neutrons and
allows, therefore, a reliable estimate for the \emph{di-neutron} spectroscopic
amplitude.Comment: Accepted for publication in Phys. Lett.
Enhanced excitation of Giant Pairing Vibrations in heavy-ion reactions induced by weakly-bound projectiles
The use of radioactive ion beams is shown to offer the possibility to study
collective pairing states at high excitation energy, which are not usually
accessible with stable projectiles because of large energy mismatch. In the
case of two-neutron stripping reactions induced by 6He, we predict a population
of the Giant Pairing Vibration in 208Pb or 116Sn with cross sections of the
order of a millibarn, dominating over the mismatched transition to the ground
state.Comment: 6 pages, 4 figure
Alpha-cluster Condensations in Nuclei and Experimental Approaches for their Studies
The formation of alpha-clusters in nuclei close to the decay thresholds is
discussed. These states can be considered to be boson-condensates, which are
formed in a second order phase transition in a mixture of nucleons and
alpha-particles. The de Broglie wavelength of the alpha-particles is larger
than the nuclear diameter, therefore the coherent properties of the
alpha-particles give particular effects for the study of such states. The
states are above the thresholds thus the enhanced emission of multiple-alphas
into the same direction is observed. The probability for the emission of
multiple-alphas is not described by Hauser-Feshbach theory for compound nucleus
decay.Comment: 21 pages, 12 figures
Helium-cluster decay widths of molecular states in beryllium and carbon isotopes
The particle and He emissions from possible molecular states in
beryllium and carbon isotopes have been studied using a mean-field-type cluster
potential. Calculations can reproduce well the -decay widths of excited
states in Be, C and Ne. For the nucleus Be, we
discussed the -decay widths with different shapes or decay modes, in
order to understand the very different decay widths of two excited states. The
widths of He decay from Be and decays from C
are predicted, which could be useful for future experiments.Comment: 12 pgaes, 1 figur
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