6,884 research outputs found
Method to determine the optimal parameters of the Arecibo 46.8-MHz antenna system
The spherical reflector at the Arecibo Observatory (AO) offers great advantages for the design of simple and inexpensive high performance steerable antennas at VHF. Light and small feeds have the added benefit that they can be quickly installed in the Arecibo platform. It is important to evaluate the performance of any given feed including the effects of the spherical reflector. The optimization is emphasized of two parameters, namely, the distance below the focal point of the reflector and the beam width of a point feed. For the design of the feed at 46.8 MHz at the AO there were other requirements independent of MST (mesosphere stratosphere troposphere) work. The design of the primary array is discussed along with its performance with the AO spherical reflector
Performance and loads data from an outdoor hover test of a Lynx tail rotor
A Lynx tail rotor was tested in hover at the Outdoor Aerodynamic Research Facility at NASA Ames Research Center. The test objectives were to measure the isolated rotor performance to provide a baseline for subsequent testing, and to operate the rotor throughout the speed and collective envelope before testing in the NFAC 40- by 80-Foot Wind Tunnel. Rotor forces and blade bending moments were measured at ambient wind conditions from zero to 6.23 m/sec. The test envelope was limited to rotor speeds of 1550 to 1850 rpm and minus 13 deg to plus 20 deg of blade collective pitch. The isolated rotor performance and blade loads data are presented
Ab-initio computation of neutron-rich oxygen isotopes
We compute the binding energy of neutron-rich oxygen isotopes and employ the
coupled-cluster method and chiral nucleon-nucleon interactions at
next-to-next-to-next-to-leading order with two different cutoffs. We obtain
rather well-converged results in model spaces consisting of up to 21 oscillator
shells. For interactions with a momentum cutoff of 500 MeV, we find that 28O is
stable with respect to 24O, while calculations with a momentum cutoff of 600
MeV result in a slightly unbound 28O. The theoretical error estimates due to
the omission of the three-nucleon forces and the truncation of excitations
beyond three-particle-three-hole clusters indicate that the stability of 28O
cannot be ruled out from ab-initio calculations, and that three-nucleon forces
and continuum effects play the dominant role in deciding this question.Comment: 5 pages + eps, 3 figure
Ab-initio approach to effective single-particle energies in doubly closed shell nuclei
The present work discusses, from an ab-initio standpoint, the definition, the
meaning, and the usefulness of effective single-particle energies (ESPEs) in
doubly closed shell nuclei. We perform coupled-cluster calculations to quantify
to what extent selected closed-shell nuclei in the oxygen and calcium isotopic
chains can effectively be mapped onto an effective independent-particle
picture. To do so, we revisit in detail the notion of ESPEs in the context of
strongly correlated many-nucleon systems and illustrate the necessity to
extract ESPEs through the diagonalization of the centroid {\it matrix}, as
originally argued by Baranger. For the purpose of illustration, we analyse the
impact of correlations on observable one-nucleon separation energies and
non-observable ESPEs in selected closed-shell oxygen and calcium isotopes. We
then state and illustrate the non-observability of ESPEs. Similarly to
spectroscopic factors, ESPEs can indeed be modified by a redefinition of
inaccessible quantities while leaving actual observables unchanged. This leads
to the absolute necessity to employ consistent structure and reaction models
based on the same nuclear Hamiltonian to extract the shell structure in a
meaningful fashion from experimental data.Comment: 14 pages, 10 figures, published in Physical Review
Measured Spectra of the Hygroscopic Fraction of Atmospheric Aerosol Particles
The relation between dry diameter (X0) and critical supersaturation (Sc) for atmospheric submicron aerosol particles is investigated using a long term air sampling program at Rolla, Missouri. The particles are passed through an electrostatic aerosol size classifier, and then through an isothermal haze chamber. Results are reported in terms of an apparent volume fraction of soluble material, εv defined such that for particles composed only of ammonium sulfate and water insoluble compounds, εv is the actual volume fraction of soluble material. The probability distribution of εv is found to be approximately Gaussian in the εv range 0.2 to 1.3. The mean εv is 0.5, for electrostatic aerosol classifier settings of 0.2, 0.3, and 0.4 μm diameter
YidC and SecY mediate membrane insertion of a type I transmembrane domain
YidC has been identified recently as an evolutionary conserved factor that is involved in the integration of inner membrane proteins (IMPs) in Escherichia coli. The discovery of YidC has inspired the reevaluation of membrane protein assembly pathways in E. coli. In this study, we have analyzed the role of YidC in membrane integration of a widely used model IMP, leader peptidase (Lep). Site-directed photocross-linking experiments demonstrate that both YidC and SecY contact nascent Lep very early during biogenesis, at only 50-amino acid nascent chain length. At this length the first transmembrane domain (TM), which acquires a type I topology, is not even fully exposed outside the ribosome. The pattern of interactions appears dependent on the position of the cross-linking probe in the nascent chain. Upon elongation, nascent Lep remains close to YidC and comes into contact with lipids as well. Our results suggest a role for YidC in both the reception and lipid partitioning of type I TMs
HS 1857+5144 : a hot and young pre-cataclysmic variable
Aims. We report the discovery of a new white dwarf/M dwarf binary, HS 1857+5144, identified in the Hamburg Quasar Survey (HQS).
Methods. Time-resolved optical spectroscopy and photometry were carried out to determine the properties of this new cataclysmic variable progenitor (pre-CV).
Results. The light curves of HS 1857+5144 display a sinusoidal variation with a period of Porb = 383.52 min and peak-to-peak amplitudes of 0.7 mag and 1.1mag in the B-band and R-band, respectively. The large amplitude of the brightness variation results from a reflection effect on the heated inner hemisphere of the companion star, suggesting a very high temperature of the white
dwarf. Our radial velocity study confirms the photometric period as the orbital period of the system. A model atmosphere fit to the spectrum of the white dwarf obtained at minimum light provides limits to its mass and temperature of Mwd 0.6−1.0 M and Twd 70 000−100 000 K, respectively. The detection of He II λ4686 absorption classifies the primary star of HS 1857+5144 as a
DAO white dwarf. Combining the results from our spectroscopy and photometry, we estimate the mass of the companion star and the binary inclination to be Msec 0.15−0.30 M and i 45◦−55◦, respectively.
Conclusions. We classify HS 1857+5144 as one of the youngest pre-CV known to date. The cooling age of the white dwarf suggests that the present system has just emerged from a common envelope phase ∼105 yr ago. HS 1857+5144 will start mass transfer within or below the 2–3 h period gap
Adjusting the melting point of a model system via Gibbs-Duhem integration: application to a model of Aluminum
Model interaction potentials for real materials are generally optimized with
respect to only those experimental properties that are easily evaluated as
mechanical averages (e.g., elastic constants (at T=0 K), static lattice
energies and liquid structure). For such potentials, agreement with experiment
for the non-mechanical properties, such as the melting point, is not guaranteed
and such values can deviate significantly from experiment. We present a method
for re-parameterizing any model interaction potential of a real material to
adjust its melting temperature to a value that is closer to its experimental
melting temperature. This is done without significantly affecting the
mechanical properties for which the potential was modeled. This method is an
application of Gibbs-Duhem integration [D. Kofke, Mol. Phys.78, 1331 (1993)].
As a test we apply the method to an embedded atom model of aluminum [J. Mei and
J.W. Davenport, Phys. Rev. B 46, 21 (1992)] for which the melting temperature
for the thermodynamic limit is 826.4 +/- 1.3K - somewhat below the experimental
value of 933K. After re-parameterization, the melting temperature of the
modified potential is found to be 931.5K +/- 1.5K.Comment: 9 pages, 5 figures, 4 table
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