2,780 research outputs found
PSR J1453+1902 and the radio luminosities of solitary versus binary millisecond pulsars
We present 3 yr of timing observations for PSR J1453+1902, a 5.79-ms pulsar
discovered during a 430-MHz drift-scan survey with the Arecibo telescope. Our
observations show that PSR J1453+1902 is solitary and has a proper motion of
8(2) mas/yr. At the nominal distance of 1.2 kpc estimated from the pulsar's
dispersion measure, this corresponds to a transverse speed of 46(11) km/s,
typical of the millisecond pulsar population. We analyse the current sample of
55 millisecond pulsars in the Galactic disk and revisit the question of whether
the luminosities of isolated millisecond pulsars are different from their
binary counterparts. We demonstrate that the apparent differences in the
luminosity distributions seen in samples selected from 430-MHz surveys can be
explained by small-number statistics and observational selection biases. An
examination of the sample from 1400-MHz surveys shows no differences in the
distributions. The simplest conclusion from the current data is that the spin,
kinematic, spatial and luminosity distributions of isolated and binary
millisecond pulsars are consistent with a single homogeneous population.Comment: 8 pages, 5 figures and 3 tables, accepted for publication by MNRA
Transient Analysis of X-34 Pressurization System
Two transient operational modes of the X-34 pressurization system were analyzed using the ROCket Engine Transition Simulation (ROCETS) program. The first operational mode considers the normal operation. For the engine burn period, the required helium mass and pressure of each propellant tank were calculated. In the second case, the possibility of failure of the pressurization system solenoid valves, its consequence on the over-pressurization, and simultaneous operation of pressurization and vent/relief systems were evaluated
On spin-rotation contribution to nuclear spin conversion in C_{3v}-symmetry molecules. Application to CH_3F
The symmetrized contribution of E-type spin-rotation interaction to
conversion between spin modifications of E- and A_1-types in molecules with
C_{3v}-symmetry is considered. Using the high-J descending of collisional
broadening for accidental rotational resonances between these spin
modifications, it was possible to co-ordinate the theoretical description of
the conversion with (updated) experimental data for two carbon-substituted
isotopes of fluoromethane. As a result, both E-type spin-rotation constants are
obtained. They are roughly one and a half times more than the corresponding
constants for (deutero)methane.Comment: 13 pages with single-spacing, REVTeX, no figures, accepted for
publication in <J. Phys. B
Managing action research: the PEArL framework
The difficulty of managing and validating Action Research field studies has been widely discussed. Several different approaches to Action Research have emerged, and one of the most widely used models is Checkland’s FMA model, where a framework is provided to facilitate interested individuals in ‘recovering’ the route of the inquiry. In this paper, I argue that the FMA model is a valuable tool for planning the application of theoretical ideas in a practical situation, but that, as a guide to Action Research, it still fails to provide a sense of the manner in which an inquiry is undertaken. The PEArL mnemonic has been previously offered as a guide to facilitate researchers, participants, and those interested in gaining an appreciation of the manner in which an inquiry is conducted. In this paper, it is argued that applying the PEArL elements does not provide insight into the dynamic nature of collaborative inquiry. In order to gain a sense of the manner in which an inquiry was undertaken it is necessary to apply the PEArL mnemonic alongside a framework that facilitates the flow of the action research cycle. To illustrate the framework, an Action Research field study is described that was undertaken with residents and key workers in a shelter for the homeless, where the aim was to create a shared understanding of complex needs and support requirements
An overview of adakite, tonalite-trondhjemite-granodiorite (TTG), and sanukitoid; relationships and some implications for crustal evolution.
Abstract Examination of an extensive adakite geochemical database identifies two distinct compositional groups. One consists of high-SiO 2 adakites (HSA) which is considered to represent subducted basaltic slab-melts that have reacted with peridotite during ascent through mantle wedge. The second group consists of low-SiO 2 adakites (LSA) which we interpret to have formed by melting of a peridotitic mantle wedge whose composition has been modified by reaction with felsic slab-melts. The chemical composition of less differentiated (primitive) Archaean tonalite-trondhjemite-granodiorite (TTG) magmas evolved from 4.0 to 2.5 Ga. Mg# (molecular Mg/(Mg+Fe 2+ ), Ni, and Cr contents increased over this period of time and we interpret these changes in terms of changes in the degree to which the TTG magmas interacted with mantle peridotite. Over the same period, concentrations of (CaO+Na 2 O) and Sr also increased, as the amount of plagioclase, residual from basalt melting, decreased in response to increased pressures at the site of slab-melting. In the Early Archaean, it appears that these interactions were very rare or absent thus leading to the conclusion that subduction was typically flat and lacked the development of a mantle wedge. In contrast, the relatively lower heat production by~2.5 Ga meant that slab-melting occurred at greater depth, where plagioclase was no longer stable, and where the development of a thick mantle wedge ensured interaction between the slab-melts and mantle peridotite. Close compositional similarities between HSA and Late Archaean TTG (Tb~3.0 Ga) strongly suggest a petrogenetic analogy. However, an analogy between the older Archaean TTG and HSA is not complete because evidence for mantle wedge interaction is missing in most Early Archaean TTGs. Late Archaean sanukitoids and the compositionally similar Closepet-type granites have compositions significantly different from TTG of all ages. However, they show some affinity with LSA which could be considered as their possible analogue. These magmas are all thought to result from melting of a mantle peridotite whose composition has been modified by reaction with slab-melts. We propose that all these magmas are directly linked to slab-melting. Archaean TTG and HSA represent slab-melts that have interacted with peridotite to varying extent, whereas sanukitoids, Closepet-type granites, and LSA correspond to melts of peridotite previously metasomatised by slab-melt. The changes observed from Early Archaean TTG to Late Archaean TTG and to sanukitoids reflect change in both the nature and efficiency of interaction between slab-melt and mantle wedge peridotite. Comparisons between all of these rocks suggest that ancient styles of subduction that have operated since at least~3.3 Ga persist in a limited way today. The secular changes in the degree and style of these interactions is a direct consequence of the cooling of Earth that modified the thermal and dynamic parameters at the subducted slab-mantle wedge interface.
RE-EDS Using GAFF Topologies: Application to Relative Hydration Free-Energy Calculations for Large Sets of Molecules
Free-energy differences between pairs of end-states can be estimated based on
molecular dynamics (MD) simulations using standard pathway-dependent methods
such as thermodynamic integration (TI), free-energy perturbation, or Bennett's
acceptance ratio. Replica-exchange enveloping distribution sampling (RE-EDS),
on the other hand, allows for the sampling of multiple end-states in a single
simulation without the specification of any pathways. In this work, we use the
RE-EDS method as implemented in GROMOS together with generalized AMBER force
field (GAFF) topologies, converted to a GROMOS-compatible format with a newly
developed GROMOS++ program amber2gromos, to compute relative hydration free
energies for a series of benzene derivatives. The results obtained with RE-EDS
are compared to the experimental data as well as calculated values from the
literature. In addition, the estimated free-energy differences in water and in
vacuum are compared to values from TI calculations carried out with GROMACS.
The hydration free energies obtained using RE-EDS for multiple molecules are
found to be in good agreement with both the experimental data and the results
calculated using other free-energy methods. While all considered free-energy
methods delivered accurate results, the RE-EDS calculations required the least
amount of total simulation time. This work serves as a validation for the use
of GAFF topologies with the GROMOS simulation package and the RE-EDS approach.
Furthermore, the performance of RE-EDS for a large set of 28 end-states is
assessed with promising results
Metal-Insulator Transition and Magnetic Order in the Pyrochlore Oxide Hg2Ru2O7
We report results of NMR experiments on the ruthenium oxide Hg2Ru2O7 with the
pyrochlore structure, which exhibits a metal-insulator transition at TMI = 107
K. In the metallic phase above TMI, the nuclear spin-lattice relaxation rate
1/T1 and the Knight shift at the Hg sites follow the Korringa relation,
indicating the absence of substantial spatial spin correlation. At low
temperatures in the insulating phase, 99,101Ru-NMR signals are observed at zero
magnetic field, providing evidence for a commensurate antiferromagnetic order.
The estimated ordered moment is about 1 muB per Ru, much smaller than 3 muB
expected for the ionic (4d)3plus configuration of Ru5plus. Thus the localized
spin models are not appropriate for the insulating phase of Hg2Ru2O7. We also
discuss possible antiferromagnetic spin structures.Comment: 10 pages, 7 figure
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