Relationships between synoptic-scale transport and interannual variability of inorganic cations in surface snow at Summit, Greenland: 1992-1996

To fully utilize the long-term chemical records retrieved from central Greenland ice cores, specific relationships between atmospheric circulation and the variability of chemical species in the records need to be better understood. This research examines associations between the variability of surface snow inorganic cation chemistry at Summit, Greenland (collected during 1992–1996 summer field seasons) and changes in air mass transport pathways and source regions, as well as variations in aerosol source strength. Transport patterns and source regions are determined through 10-day isentropic backward air mass trajectories during a 1 month (late May to late June) common season over the 5 years. Changes in the extent of exposed continental surfaces in source regions are evaluated to estimate aerosol-associated calcium and magnesium ion source strength, while forest fire activity in the circumpolar north is investigated to estimate aerosol ammonium ion source strength. During the 1995 common season, 3 times more calcium and magnesium accumulated in the snowpack than the other study years. Also, an increasing trend of ammonium concentration was noted throughout the 5 years. Anomalous transport pathways and velocities were observed during 1995, which likely contributed to the high levels of calcium and magnesium. Increased forest fire activity in North America was concurrent with increased levels of ammonium and potassium, except for 1996, when ion levels were above average and forest fire activity was below average. Because of the ubiquitous nature of soluble ions, we conclude that it is very difficult to establish a quantitative link between the ion content of snow and firn at Summit and changes in aerosol source regions and source strength

Why do ultrasoft repulsive particles cluster and crystallize? Analytical results from density functional theory

We demonstrate the accuracy of the hypernetted chain closure and of the mean-field approximation for the calculation of the fluid-state properties of systems interacting by means of bounded and positive-definite pair potentials with oscillating Fourier transforms. Subsequently, we prove the validity of a bilinear, random-phase density functional for arbitrary inhomogeneous phases of the same systems. On the basis of this functional, we calculate analytically the freezing parameters of the latter. We demonstrate explicitly that the stable crystals feature a lattice constant that is independent of density and whose value is dictated by the position of the negative minimum of the Fourier transform of the pair potential. This property is equivalent with the existence of clusters, whose population scales proportionally to the density. We establish that regardless of the form of the interaction potential and of the location on the freezing line, all cluster crystals have a universal Lindemann ratio L = 0.189 at freezing. We further make an explicit link between the aforementioned density functional and the harmonic theory of crystals. This allows us to establish an equivalence between the emergence of clusters and the existence of negative Fourier components of the interaction potential. Finally, we make a connection between the class of models at hand and the system of infinite-dimensional hard spheres, when the limits of interaction steepness and space dimension are both taken to infinity in a particularly described fashion.Comment: 19 pages, 5 figures, submitted to J. Chem. Phys; new version: minor changes in structure of pape

The structure of fluid trifluoromethane and methylfluoride

We present hard X-ray and neutron diffraction measurements on the polar fluorocarbons HCF3 and H3CF under supercritical conditions and for a range of molecular densities spanning about a factor of ten. The Levesque-Weiss-Reatto inversion scheme has been used to deduce the site-site potentials underlying the measured partial pair distribution functions. The orientational correlations between adjacent fluorocarbon molecules -- which are characterized by quite large dipole moments but no tendency to form hydrogen bonds -- are small compared to a highly polar system like fluid hydrogen chloride. In fact, the orientational correlations in HCF3 and H3CF are found to be nearly as small as those of fluid CF4, a fluorocarbon with no dipole moment.Comment: 11 pages, 9 figure

Structure Factor and Electronic Structure of Compressed Liquid Rubidium

We have applied the quantal hypernetted-chain equations in combination with the Rosenfeld bridge-functional to calculate the atomic and the electronic structure of compressed liquid-rubidium under high pressure (0.2, 2.5, 3.9, and 6.1 GPa); the calculated structure factors are in good agreement with experimental results measured by Tsuji et al. along the melting curve. We found that the Rb-pseudoatom remains under these high pressures almost unchanged with respect to the pseudoatom at room pressure; thus, the effective ion-ion interaction is practically the same for all pressure-values. We observe that all structure factors calculated for this pressure-variation coincide almost into a single curve if wavenumbers are scaled in units of the Wigner-Seitz radius $a$ although no corresponding scaling feature is observed in the effective ion-ion interaction.This scaling property of the structure factors signifies that the compression in liquid-rubidium is uniform with increasing pressure; in absolute Q-values this means that the first peak-position ($Q_1$) of the structure factor increases proportionally to $V^{-1/3}$ ($V$ being the specific volume per ion), as was experimentally observed by Tsuji et al.Comment: 18 pages, 11 figure

Determination of astrophysical 12N(p,g)13O reaction rate from the 2H(12N, 13O)n reaction and its astrophysical implications

The evolution of massive stars with very low-metallicities depends critically on the amount of CNO nuclides which they produce. The $^{12}$N($p$,\,$\gamma$)$^{13}$O reaction is an important branching point in the rap-processes, which are believed to be alternative paths to the slow 3$\alpha$ process for producing CNO seed nuclei and thus could change the fate of massive stars. In the present work, the angular distribution of the $^2$H($^{12}$N,\,$^{13}$O)$n$ proton transfer reaction at $E_{\mathrm{c.m.}}$ = 8.4 MeV has been measured for the first time. Based on the Johnson-Soper approach, the square of the asymptotic normalization coefficient (ANC) for the virtual decay of $^{13}$O$_\mathrm{g.s.}$ $\rightarrow$ $^{12}$N + $p$ was extracted to be 3.92 $\pm$ 1.47 fm$^{-1}$ from the measured angular distribution and utilized to compute the direct component in the $^{12}$N($p$,\,$\gamma$)$^{13}$O reaction. The direct astrophysical S-factor at zero energy was then found to be 0.39 $\pm$ 0.15 keV b. By considering the direct capture into the ground state of $^{13}$O, the resonant capture via the first excited state of $^{13}$O and their interference, we determined the total astrophysical S-factors and rates of the $^{12}$N($p$,\,$\gamma$)$^{13}$O reaction. The new rate is two orders of magnitude slower than that from the REACLIB compilation. Our reaction network calculations with the present rate imply that $^{12}$N($p,\,\gamma$)$^{13}$O will only compete successfully with the $\beta^+$ decay of $^{12}$N at higher ($\sim$two orders of magnitude) densities than initially predicted.Comment: 8 figures, 2 tables, Submitted to Physical Review

Modeling a social brain for interactive agents: integrating mirroring and mentalizing

Kahl S, Kopp S. Modeling a social brain for interactive agents: integrating mirroring and mentalizing. In: Brinkman WP, Broekens J, Heylen DKJ, eds. Intelligent Virtual Agents. Springer; 2015: 77-86

Nuclear structure of 30S and its implications for nucleosynthesis in classical novae

The uncertainty in the 29P(p,gamma)30S reaction rate over the temperature range of 0.1 - 1.3 GK was previously determined to span ~4 orders of magnitude due to the uncertain location of two previously unobserved 3+ and 2+ resonances in the 4.7 - 4.8 MeV excitation region in 30S. Therefore, the abundances of silicon isotopes synthesized in novae, which are relevant for the identification of presolar grains of putative nova origin, were uncertain by a factor of 3. To investigate the level structure of 30S above the proton threshold (4394.9(7) keV), a charged-particle spectroscopy and an in-beam gamma-ray spectroscopy experiments were performed. Differential cross sections of the 32S(p,t)30S reaction were measured at 34.5 MeV. Distorted wave Born approximation calculations were performed to constrain the spin-parity assignments of the observed levels. An energy level scheme was deduced from gamma-gamma coincidence measurements using the 28Si(3He,n-gamma)30S reaction. Spin-parity assignments based on measurements of gamma-ray angular distributions and gamma-gamma directional correlation from oriented nuclei were made for most of the observed levels of 30S. As a result, the resonance energies corresponding to the excited states in 4.5 MeV - 6 MeV region, including the two astrophysically important states predicted previously, are measured with significantly better precision than before. The uncertainty in the rate of the 29P(p,gamma)30S reaction is substantially reduced over the temperature range of interest. Finally, the influence of this rate on the abundance ratios of silicon isotopes synthesized in novae are obtained via 1D hydrodynamic nova simulations.Comment: 22 pages, 12 figure

Density fluctuations and single-particle dynamics in liquid lithium

The single-particle and collective dynamical properties of liquid lithium have been evaluated at several thermodynamic states near the triple point. This is performed within the framework of mode-coupling theory, using a self-consistent scheme which, starting from the known static structure of the liquid, allows the theoretical calculation of several dynamical properties. Special attention is devoted to several aspects of the single-particle dynamics, which are discussed as a function of the thermodynamic state. The results are compared with those of Molecular Dynamics simulations and other theoretical approaches.Comment: 31 pages (in preprint format), 14 figures. Submitted to Phys. Rev.