65,737 research outputs found

    In search of phylogenetic congruence between molecular and morphological data in bryozoans with extreme adult skeletal heteromorphy

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    peerreview_statement: The publishing and review policy for this title is described in its Aims & Scope. aims_and_scope_url: http://www.tandfonline.com/action/journalInformation?show=aimsScope&journalCode=tsab20© Crown Copyright 2015. This document is the author's final accepted/submitted version of the journal article. You are advised to consult the publisher's version if you wish to cite from it

    Energy benchmarks for water clusters and ice structures from an embedded many-body expansion

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    We show how an embedded many-body expansion (EMBE) can be used to calculate accurate \emph{ab initio} energies of water clusters and ice structures using wavefunction-based methods. We use the EMBE described recently by Bygrave \emph{et al.} (J. Chem. Phys. \textbf{137}, 164102 (2012)), in which the terms in the expansion are obtained from calculations on monomers, dimers, etc. acted on by an approximate representation of the embedding field due to all other molecules in the system, this field being a sum of Coulomb and exchange-repulsion fields. Our strategy is to separate the total energy of the system into Hartree-Fock and correlation parts, using the EMBE only for the correlation energy, with the Hartree-Fock energy calculated using standard molecular quantum chemistry for clusters and plane-wave methods for crystals. Our tests on a range of different water clusters up to the 16-mer show that for the second-order M\o{}ller-Plesset (MP2) method the EMBE truncated at 2-body level reproduces to better than 0.1 mEhE_{\rm h}/monomer the correlation energy from standard methods. The use of EMBE for computing coupled-cluster energies of clusters is also discussed. For the ice structures Ih, II and VIII, we find that MP2 energies near the complete basis-set limit reproduce very well the experimental values of the absolute and relative binding energies, but that the use of coupled-cluster methods for many-body correlation (non-additive dispersion) is essential for a full description. Possible future applications of the EMBE approach are suggested

    Mapping the 3-D Dark Matter potential with weak shear

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    We investigate the practical implementation of Taylor's (2002) 3-dimensional gravitational potential reconstruction method using weak gravitational lensing, together with the requisite reconstruction of the lensing potential. This methodology calculates the 3-D gravitational potential given a knowledge of shear estimates and redshifts for a set of galaxies. We analytically estimate the noise expected in the reconstructed gravitational field, taking into account the uncertainties associated with a finite survey, photometric redshift uncertainty, redshift-space distortions, and multiple scattering events. In order to implement this approach for future data analysis, we simulate the lensing distortion fields due to various mass distributions. We create catalogues of galaxies sampling this distortion in three dimensions, with realistic spatial distribution and intrinsic ellipticity for both ground-based and space-based surveys. Using the resulting catalogues of galaxy position and shear, we demonstrate that it is possible to reconstruct the lensing and gravitational potentials with our method. For example, we demonstrate that a typical ground-based shear survey with redshift limit z=1 and photometric redshifts with error Delta z=0.05 is directly able to measure the 3-D gravitational potential for mass concentrations >10^14 M_\odot between 0.1<z<0.5, and can statistically measure the potential at much lower mass limits. The intrinsic ellipticity of objects is found to be a serious source of noise for the gravitational potential, which can be overcome by Wiener filtering or examining the potential statistically over many fields. We examine the use of the 3-D lensing potential to measure mass and position of clusters in 3-D, and to detect clusters behind clusters.Comment: 21 pages, including 24 figures, submitted to MNRA

    The Spatial Distribution of Atomic Carbon Emission in the Giant Molecular Cloud NGC 604-2

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    We have mapped a giant molecular cloud in the giant HII region NGC 604 in M33 in the 492 GHz ^3P_1 -- ^3P_0 transition of neutral atomic carbon using the James Clerk Maxwell Telescope. We find the distribution of the [CI] emission to be asymmetric with respect to the CO J=1--0 emission, with the peak of the [CI] emission offset towards the direction of the center of the HII region. In addition, the line ratio I_{[CI]}/I_{CO} is highest (~ 0.2) facing the HII region and lowest (< 0.1) away from it. These asymmetries indicate an edge-on morphology where the [CI] emission is strongest on the side of the cloud facing the center of the HII region, and not detected at all on the opposite side This suggests that the sources of the incident flux creating C from the dissociation of CO are the massive stars of the HII region. The lowest line ratios are similar to what is observed in Galactic molecular clouds, while the highest are similar to starburst galaxies and other regions of intense star formation. The column density ratio, N(C)/N(H_2) is a few times 10^{-6}, in general agreement with models of photodissociation regions.Comment: Accepted for publication in ApJ. 8 pages, 5 figures, 3 table
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