16,758 research outputs found
Chemical (in)homogeneity and atomic diffusion in the open cluster M67
Context. The benchmark open cluster M67 is known to have solar metallicity
and similar age as the Sun. It thus provides us a great opportunity to study
the properties of solar twins, as well as the evolution of Sun-like stars.
Aims. Previous spectroscopic studies reported to detect possible subtle changes
in stellar surface abundances throughout the stellar evolutionary phase, namely
the effect of atomic diffusion, in M67. In this study we attempt to confirm and
quantify more precisely the effect of atomic diffusion, as well as to explore
the level of chemical (in)homogeneity in M67. Methods. We presented a strictly
line-by-line differential chemical abundance analysis of two groups of stars in
M67: three turn-off stars and three sub-giants. Stellar atmospheric parameters
and elemental abundances were obtained with very high precision using the
Keck/HIRES spectra. Results. The sub-giants in our sample show negligible
abundance variations ( 0.02 dex), which implies that M67 was born
chemically homogeneous. We note there is a significant abundance difference
( 0.1 - 0.2 dex) between sub-giants and turn-off stars, which can be
interpreted as the signature of atomic diffusion. Qualitatively stellar models
with diffusion agree with the observed abundance results. Some turn-off stars
do not follow the general pattern, which suggests that in some cases diffusion
can be inhibited, or they might suffered some sort of mixing event related to
planets. Conclusions. Our results pose additional challenges for chemical
tagging when using turn-off stars. In particular, the effects of atomic
diffusion, which could be as large as 0.1 - 0.2 dex, must be taken into account
in order for chemical tagging to be successfully applied.Comment: 19 pages, 21 figures; submitted to A&A on February, 2019, accepted
for publication in A&A on June, 201
Simulation of Coherent Non-Linear Neutrino Flavor Transformation in the Supernova Environment I: Correlated Neutrino Trajectories
We present results of large-scale numerical simulations of the evolution of
neutrino and antineutrino flavors in the region above the late-time
post-supernova-explosion proto-neutron star. Our calculations are the first to
allow explicit flavor evolution histories on different neutrino trajectories
and to self-consistently couple flavor development on these trajectories
through forward scattering-induced quantum entanglement. Employing the
atmospheric-scale neutrino mass-squared difference and values of theta_13
allowed by current bounds, we find transformation of neutrino and antineutrino
flavors over broad ranges of energy and luminosity in roughly the ``bi-polar''
collective mode. We find that this large-scale flavor conversion, largely
driven by the flavor off-diagonal neutrino-neutrino forward scattering
potential, sets in much closer to the proto-neutron star than simple estimates
based on flavor-diagonal potentials and Mikeheyev-Smirnov-Wolfenstein evolution
would indicate. In turn, this suggests that models of r-process nucleosynthesis
sited in the neutrino-driven wind could be affected substantially by
active-active neutrino flavor mixing, even with the small measured neutrino
mass-squared differences.Comment: 23 pages, 12 figures, revtex4 format. Version accepted by PR
Local Operations in qubit arrays via global but periodic Manipulation
We provide a scheme for quantum computation in lattice systems via global but
periodic manipulation, in which only effective periodic magnetic fields and
global nearest neighbor interaction are required. All operations in our scheme
are attainable in optical lattice or solid state systems. We also investigate
universal quantum operations and quantum simulation in 2 dimensional lattice.
We find global manipulations are superior in simulating some nontrivial many
body Hamiltonians.Comment: 5 pages, 2 figures, to appear in Phys. Rev.
Coherent Development of Neutrino Flavor in the Supernova Environment
We calculate coherent neutrino and antineutrino flavor transformation in the
supernova environment, for the first time including a self-consistent treatment
of forward scattering-induced coupling and entanglement of intersecting
neutrino/antineutrino trajectories. For the atmospheric neutrino mass-squared
difference we find that in the normal (inverted) mass hierarchy the more
tangentially-propagating (radially-propagating) neutrinos and antineutrinos can
initiate collective, simultaneous medium-enhanced flavor conversion of these
particles across broad ranges of energy and propagation direction. Accompanying
alterations in neutrino/antineutrino energy spectra and/or fluxes could affect
supernova nucleosynthesis and the expected neutrino signal.Comment: 4 pages, 3 figure
Deterministic Generation of Entangled Photons in Superconducting Resonator Arrays
We present a scheme for the deterministic generation of entangled photon
pairs in a superconducting resonator array. The resonators form a
Jaynes-Cummings lattice via the coupling to superconducting qubits, and the
Kerr-like nonlinearity arises due to the coupling.We show that entangled
photons can be generated on demand by applying spectroscopic techniques and
exploiting the nonlinearity and symmetry in the resonators. The scheme is
robust against small parameter spreads due to fabrication errors. Our findings
can be used as a key element for quantum information processing in
superconducting quantum circuits.Comment: 4 pages, 3 figure
The effects of the die half angle of taper die on plane strain extrusion
In this research, a study of the effect of a die half angle on the extrusion process has been performed. The experiments were conducted at room temperature around 27 °C. Two types of taper die with different die half angles (45° and 60°) were prepared. The test lubricants used were paraffinic mineral oil VG460 and Refined, Bleached and Deodorized (RBD) Palm stearin. The material of the workpiece (billet) was annealed with A1100 aluminium. The experimental results were focused on the extrusion load, tool and workpiece surface roughness and plastic deformation of the workpiece. The resultant relative velocity was calculated using a visioplasticity method. The results shows that a taper angle of 60° recorded higher resultant relative velocity with a lower extrusion load and surface roughness compare to the taper angle of 45°. The comparison study between Paraffinic Mineral Oil and RBD Palm Stearin shows no significant effect in both taper angles tested
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