35,724 research outputs found
Circumstellar Disks Around Binary Stars in Taurus
We have conducted a survey of 17 wide (> 100 AU) young binary systems in
Taurus with the Atacama Large Millimeter Array (ALMA) at two wavelengths. The
observations were designed to measure the masses of circumstellar disks in
these systems as an aid to understanding the role of multiplicity in star and
planet formation. The ALMA observations had sufficient resolution to localize
emission within the binary system. Disk emission was detected around all
primaries and ten secondaries, with disk masses as low as .
We compare the properties of our sample to the population of known disks in
Taurus and find that the disks from this binary sample match the scaling
between stellar mass and millimeter flux of
to within the scatter found in previous studies. We also compare the properties
of the primaries to those of the secondaries and find that the
secondary/primary stellar and disk mass ratios are not correlated; in three
systems, the circumsecondary disk is more massive than the circumprimary disk,
counter to some theoretical predictions.Comment: To appear in the Astrophysical Journal, 12 page
Can Post T Tauri Stars Be Found? Yes!
I review the observational challenges of finding post T Tauri stars (PTTS), defined here as low-mass, pre-main-sequence stars with ages of 10^7-10^8 yr. Such stars are difficult to find because they are less active than younger T Tauri stars, and they may not be associated with molecular gas. They are useful for studying the evolution of circumstellar disks and stellar activity between the 10^6-yr ages of nearby star-forming regions and the main sequence. However, care must be taken in the search process so that the selection criteria used to locate such stars do not bias the sample used for subsequent evolutionary studies
Investigation of resonant and transient phenomena in Josephson junction flux qubits
We present an analytical and computational study of resonances and transient
responses in a classical Josephson junction system. A theoretical basis for
resonances in a superconducting loop with three junctions is presented,
outlining both the direct relationship between the dynamics of single- and
multi-junction systems, and the direct relationships between observations of
the classical counterparts to Rabi oscillations, Ramsey fringes, and spin echo
oscillations in this class of systems. We show simulations data along with
analytical analyses of the classical model, and the results are related to
previously reported experiments conducted on three junction loops. We further
investigate the effect of off-resonant microwave perturbations to, e.g., the
Rabi-type response of the Josephson system, and we relate this response back to
the nonlinear and multi-valued resonance behavior previously reported for a
single Josephson junction. The close relationships between single and
multi-junction behavior demonstrates the underlying dynamical mechanism for a
whole class of classical counterparts to expected quantum mechanical
observations in a variety of systems; namely the resonant and transient
behavior of a particle in an anharmonic potential well with subsequent escape.Comment: 11 pages, seven figure
First-principles study of the energetics of charge and cation mixing in U_{1-x} Ce_x O_2
The formalism of electronic density-functional-theory, with Hubbard-U
corrections (DFT+U), is employed in a computational study of the energetics of
U_{1-x} Ce_x O_2 mixtures. The computational approach makes use of a procedure
which facilitates convergence of the calculations to multiple self-consistent
DFT+U solutions for a given cation arrangement, corresponding to different
charge states for the U and Ce ions in several prototypical cation
arrangements. Results indicate a significant dependence of the structural and
energetic properties on the nature of both charge and cation ordering. With the
effective Hubbard-U parameters that reproduce well the measured
oxidation-reduction energies for urania and ceria, we find that charge transfer
between U(IV) and Ce(IV) ions, leading to the formation of U(V) and Ce(III),
gives rise to an increase in the mixing energy in the range of 4-14 kJ/mol of
formula unit, depending on the nature of the cation ordering. The results
suggest that although charge transfer between uranium and cerium ions is
disfavored energetically, it is likely to be entropically stabilized at the
high temperatures relevant to the processing and service of urania-based solid
solutions.Comment: 8 pages, 6 figure
Three-body Thomas-Ehrman shifts of analog states of Ne and N
The lowest-lying states of the Borromean nucleus Ne (O+ +
) and its mirror nucleus N (N+ + ) are compared by using
the hyperspheric adiabatic expansion. Three-body resonances are computed by use
of the complex scaling method. The measured size of O and the low-lying
resonances of F (O+) are first used as constraints to
determine both central and spin-dependent two-body interactions. The
interaction obtained reproduces relatively accurately both experimental
three-body spectra. The Thomas-Ehrman shifts, involving excitation energy
differences, are computed and found to be less than 3% of the total Coulomb
energy shift for all states.Comment: 9 pages, 3 postscript figures, revtex style. To be published in Phys.
Rev.
Honeycomb lattice polygons and walks as a test of series analysis techniques
We have calculated long series expansions for self-avoiding walks and
polygons on the honeycomb lattice, including series for metric properties such
as mean-squared radius of gyration as well as series for moments of the
area-distribution for polygons. Analysis of the series yields accurate
estimates for the connective constant, critical exponents and amplitudes of
honeycomb self-avoiding walks and polygons. The results from the numerical
analysis agree to a high degree of accuracy with theoretical predictions for
these quantities.Comment: 16 pages, 9 figures, jpconf style files. Presented at the conference
"Counting Complexity: An international workshop on statistical mechanics and
combinatorics." In celebration of Prof. Tony Guttmann's 60th birthda
Layer Features of the Lattice Gas Model for Self-Organized Criticality
A layer-by-layer description of the asymmetric lattice gas model for
1/f-noise suggested by Jensen [Phys. Rev. Lett. 64, 3103 (1990)] is presented.
The power spectra of the lattice layers in the direction perpendicular to the
particle flux is studied in order to understand how the white noise at the
input boundary evolves, on the average, into 1/f-noise for the system. The
effects of high boundary drive and uniform driving force on the power spectrum
of the total number of diffusing particles are considered. In the case of
nearest-neighbor particle interactions, high statistics simulation results show
that the power spectra of single lattice layers are characterized by different
exponents such that as one approaches the outer
boundary.Comment: LaTeX, figures upon reques
Perimeter Generating Functions For The Mean-Squared Radius Of Gyration Of Convex Polygons
We have derived long series expansions for the perimeter generating functions
of the radius of gyration of various polygons with a convexity constraint.
Using the series we numerically find simple (algebraic) exact solutions for the
generating functions. In all cases the size exponent .Comment: 8 pages, 1 figur
Assignment of transition multipole orders from L subshell internal conversion coefficient ratios
Internal conversion coefficients, hereafter symbolized by ICCs, play an important role in the field .of nuclear physics. To date physicists are unable to describe adequately the structure of the nucleus. One of the tools being used to determine nuclear structure is that of internal conversion coefficients
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