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 16^{16}O core in 28^{28}Si

We investigate the existence of weakly coupled gas-like states comprised of three $\alpha$ particles around an $^{16}$O core in $^{28}$Si. We calculate the excited states in $^{28}$Si using the multi-configuration mixing method based on the $^{16}$O + 3$\alpha$ cluster model. We also include the $^{16}$O + $^{12}$C and $^{24}$Mg + $\alpha$ 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 $^{16}$O + $^{12}$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 $\alpha$ particles could exist around the surface of the $^{16}$O core, which can be described as a "two-dimensional gas" in the intermediate state before the Hoyle-like three $\alpha$ 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

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

Di-neutron elastic transfer in the 4He(6He,6He)4He reaction

Elastic $^{6}$He+$^4$He data measured at $E_{\rm c.m.}=11.6,$ 15.9, and 60.3 MeV have been analyzed within the coupled reaction channels (CRC) formalism, with the elastic-scattering and two-neutron ($2n$) transfer amplitudes coherently included. Contributions from the direct (one-step) and sequential (two-step) $2n$-transfers were treated explicitly based on a realistic assumption for the $2n$-transfer form factor. The oscillatory pattern observed in $^4$He($^6$He,$^6$He)$^4$He angular distribution at low energies was found to be due to an interference between the elastic scattering and $2n$-transfer amplitudes. Our CRC analysis shows consistently that the direct $2n$-transfer strongly dominates over the sequential transfer and thus confirms the dominance of 2$n-^4$He configuration over the $n-^5$He one in the $^6$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.

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

Potraga za spljoštenim i trokutnim konfiguracijama ugljikovih izotopa

A systematic search through experimental data for the nuclei 12−14C has been made to identify states which have triangular shape. These states are oblate and must have strong α-clustering. They are expected to form rotational bands with high moments of inertia. The properties of these states are discussed on the basis of α-clusters and neutron orbitals.Proveli smo sustavnu potragu kroz postojeće eksperimentalne rezultate za jezgre 12−14C da bismo raspoznali stanja trokutnog oblika. Ta su stanja spljoštena, pokazuju izrazitu nakupinsku strukturu i grupiraju se u rotacijske vrpce velikog momenta inercije. Njihova se svojstva raspravljaju na temelju α-nakupina i neutronskih orbitala