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Register a new userA Theoretical Investigation of the Possible Detection of C24 in Space
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Astronomical infrared spectral features at ~6.6, 9.8 and 20 micronm have recently been suggested as being due to the planar graphene form of C24 carbon cluster. Here we report density functional theory and coupled cluster calculations on wavefunctions stability, relative energies, and infrared spectra of four different types of C24 isomers, including the graphene and fullerene forms. The types of vibrational motions under these bands are also discussed. Among the four isomers, we find that the astronomical data are best approximated by the graphene form of C24.
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Observations of early-type dwarf galaxies in clusters often show that cluster dwarf members have significantly higher velocities and less symmetric distributions than cluster giant ellipticals, suggesting that these dwarfs are recently accreted galaxies, possibly from an infalling group. We use a series of $N$-body simulations, exploring a parameter space of groups falling into clusters, to study the observed velocity distributions of the infall components along various lines of sight. We show that, as viewed along a line of sight parallel to the groups infall direction, there is a significant peculiar velocity boost during the pericentric passage of the group, and an increase in velocity dispersion that persists for many Gyr after the merger. The remnants of the infalling group, however, do not form a spatially distinct system -- consistent with recent observations of dwarf galaxies in the Virgo and Fornax clusters. This velocity signature is completely absent when viewed along a direction perpendicular to the merger. Additionally, the phase-space distribution of radial velocity along the infall direction versus cluster-centric radius reveals the separate dynamical evolution of the groups central core and outer halo, including the presence of infalling remnants outside the escape velocity envelope of the system. The distinct signature in velocity space of an infalling groups galaxies can therefore prove important in understanding the dynamical history of clusters and their dwarfs. Our results suggest that dwarf galaxies, being insensitive to dynamical friction, are excellent probes of their host clusters dynamical histories.
Large holes in graphene membranes were recently shown to heal, either at room temperature during a low energy STEM experiment, or by annealing at high temperatures. However, the details of the healing mechanism remain unclear. We carried out fully atomistic reactive molecular dynamics simulations in order to address these mechanisms under different experimental conditions. Our results show that, if a carbon atom source is present, high temperatures can provide enough energy for the carbon atoms to overcome the potential energy barrier and to produce perfect reconstruction of the graphene hexagonal structure. At room temperature, this perfect healing is only possible if the heat effects of the electron beam from STEM experiment are explicitly taken into account. The reconstruction process of a perfect or near perfect graphene structure involves the formation of linear carbon chains, as well as rings containing 5, 6, 7 and 8 atoms with planar (Stone-Wales) and non-planar (lump like) structures. These results shed light on the healing mechanism of graphene when subjected to different experimental conditions. Additionally, the methodology presented here can be useful for investigating the tailoring and manipulations of other nano-structures.
The vibrationally resolved spectra of the pyrene cation and doubly-dehydrogenated pyrene cation (C$_{16}$H$_{10}$$^{.+}$; Py$^+$ and C$_{16}$H$_{8}$$^{.+}$; ddPy$^+$) are presented. Infrared predissociation spectroscopy is employed to measure the vibrational spectrum of both species using a cryogenically cooled 22-pole ion trap. The spectrum of Py$^+$ allows a detailed comparison with harmonic and anharmonic density functional theory (DFT) calculated normal mode frequencies. The spectrum of ddPy$^+$ is dominated by absorption features from two isomers (4,5-ddPy$^+$ and 1,2-ddPy$^+$) with, at most, minor contributions from other isomers. These findings can be extended to explore the release of hydrogen from interstellar PAH species. Our results suggest that this process favours the loss of adjacent hydrogen atoms.
We report a theoretical equation of state (EOS) table for boron across a wide range of temperatures (5.1$times$10$^4$-5.2$times$10$^8$ K) and densities (0.25-49 g/cm$^3$), and experimental shock Hugoniot data at unprecedented high pressures (5608$pm$118 GPa). The calculations are performed with full, first-principles methods combining path integral Monte Carlo (PIMC) at high temperatures and density functional theory molecular dynamics (DFT-MD) methods at lower temperatures. PIMC and DFT-MD cross-validate each other by providing coherent EOS (difference $<$1.5 Hartree/boron in energy and $<$5% in pressure) at 5.1$times$10$^5$ K. The Hugoniot measurement is conducted at the National Ignition Facility using a planar shock platform. The pressure-density relation found in our shock experiment is on top of the shock Hugoniot profile predicted with our first-principles EOS and a semi-empirical EOS table (LEOS 50). We investigate the self diffusivity and the effect of thermal and pressure-driven ionization on the EOS and shock compression behavior in high pressure and temperature conditions We study the performance sensitivity of a polar direct-drive exploding pusher platform to pressure variations based on comparison of the first-principles calculations with LEOS 50 via 1D hydrodynamic simulations. The results are valuable for future theoretical and experimental studies and engineering design in high energy density research. (LLNL-JRNL-748227)
We present timing and broad-band spectral studies of the high mass X-ray binary pulsar 4U 1909+07 using data from Suzaku observation during 2010 November 2-3. The pulse period of the pulsar is estimated to be 604.11+/-0.14 s. Pulsations are seen in the X-ray light curve up to ~70 keV. The pulse profile is found to be strongly energy-dependent: a complex, multi-peaked structure at low energy that becomes a simple single peak at higher energy. We found that the 1-70 keV pulse averaged continuum can be fitted by the sum of a black body and a partial covering Negative and Positive power-law with EXponential cutoff (NPEX) model. A weak iron fluorescence emission line at 6.4 keV was detected in the spectrum. An absorption like feature at ~44 keV was clearly seen in the residue of the spectral fitting, independent of the continuum model adopted. To check the possible presence of a CRSF in the spectrum, we normalized the pulsar spectrum with the spectrum of the Crab Nebula. The resulting Crab ratio also showed a clear dip centered at ~44 keV. We performed statistical tests on the residue of the spectral fitting and also on the Crab spectral ratio to determine the significance of the absorption like feature and identified it as a CRSF of the pulsar. We estimated the corresponding surface magnetic field of the pulsar to be 3.8 x 10^12 Gauss.
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