40,137 research outputs found
Algebraic approach to the Hulthen potential
In this paper the energy eigenvalues and the corresponding eigenfunctions are
calculated for Hulthen potential. Then we obtain the ladder operators and show
that these operators satisfy SU(2) commutation relation.Comment: 8 Pages, 1 Tabl
An updated stellar census of the Quintuplet cluster
Context. Found within the central molecular zone, the Quintuplet is one of the most massive young clusters in the Galaxy. As a consequence it offers the prospect of constraining stellar formation and evolution in extreme environments. However, current observations suggest that it comprises a remarkably diverse stellar population that is difficult to reconcile with an instantaneous formation event.
Aims. To better understand the nature of the cluster our aim is to improve observational constraints on the constituent stars.
Methods. In order to accomplish this goal we present Hubble Space Telescope/NICMOS+WFC3 photometry and Very Large Telescope/SINFONI+KMOS spectroscopy for ∼100 and 71 cluster members, respectively.
Results. Spectroscopy of the cluster members reveals the Quintuplet to be far more homogeneous than previously expected. All supergiants are classified as either O7–8 Ia or O9–B0 Ia, with only one object of earlier (O5 I–III) spectral type. These stars form a smooth morphological sequence with a cohort of seven early-B hypergiants and six luminous blue variables and WN9-11h stars, which comprise the richest population of such stars of any stellar aggregate known. In parallel, we identify a smaller population of late-O hypergiants and spectroscopically similar WN8–9ha stars. No further H-free Wolf–Rayet (WR) stars are identified, leaving an unexpectedly extreme ratio of 13:1 for WC/WN stars. A subset of the O9–B0 supergiants are unexpectedly faint, suggesting they are both less massive and older than the greater cluster population. Finally, no main sequence objects were identifiable.
Conclusions. Due to uncertainties over which extinction law to apply, it was not possible to quantitatively determine a cluster age via isochrone fitting. Nevertheless, we find an impressive coincidence between the properties of cluster members preceding the H-free WR phase and the evolutionary predictions for a single, non-rotating 60 M⊙ star; in turn this implies an age of ∼3.0–3.6 Myr for the Quintuplet. Neither the late O-hypergiants nor the low luminosity supergiants are predicted by such a path; we suggest that the former either result from rapid rotators or are the products of binary driven mass-stripping, while the latter may be interlopers. The H-free WRs must evolve from stars with an initial mass in excess of 60 M⊙ but it appears difficult to reconcile their observational properties with theoretical expectations. This is important since one would expect the most massive stars within the Quintuplet to be undergoing core-collapse/SNe at this time; since the WRs represent an evolutionary phase directly preceding this event,their physical properties are crucial to understanding both this process and the nature of the resultant relativistic remnant. As such, the Quintuplet provides unique observational constraints on the evolution and death of the most massive stars forming in the local, high metallicity Universe
Coexistence and competition of multiple charge-density-wave orders in rare-earth tri-telluride RTe3
The occurrences of collective quantum states, such as superconductivity (SC)
and charge- or spin-densitywaves (CDWs or SDWs), are among the most fascinating
phenomena in solids. To date much effort has been made to explore the interplay
between different orders, yet little is known about the relationship of
multiple orders of the same type. Here we report optical spectroscopy study on
CDWs in the rare-earth tri-telluride compounds RTe3 (R = rare earth elements).
Besides the prior reported two CDW orders, the study reveals unexpectedly the
presence of a third CDW order in the series which evolves systematically with
the size of R element. With increased chemical pressure, the first and third
CDW orders are both substantially suppressed and compete with the second one by
depleting the low energy spectral weight. A complete phase diagram for the
multiple CDW orders in this series is established.Comment: 7 pages, 4 figures, 1 tabl
Unified modeling and measurement of current-programmed converters
A unified model is established for a
current-programmed converter, which is both a
modification and an extension of familiar models.
Inclusion of the sampling effect allows the
presence of an additional pole in the current-loop
gain to be inferred. The resulting final
double-slope asymptote is fixed in position, and
the crossover frequency cannot exceed half the
switching frequency. A new “stability parameter”
Q_s determines the additional pole and describes
the degree of peaking in the closed-loop
transfer functions. Experimental verification
employs an analog signal injection technique
The Volume Source Technique for flavor singlets: a second look
We reconsider the Volume Source Technique (VST) for the determination of
flavor singlet quantities on the lattice. We point out a difficulty arising in
the case of fermions in real representations of the gauge group and propose an
improved version of the method (IVST) based on random gauge transformations of
the background configuration. We compare the performance of IVST with the
method based on stochastic estimators (SET). We consider the case of the N=1
Supersymmetric Yang-Mills Theory (SYM), where just one fermionic flavor is
present, the gluino in the adjoint representation, and only flavor singlet
states are possible. The work is part of an inclusive analysis of the spectrum
of the lightest particles of the theory, based on the simulation of the model
on a lattice with dynamical gluinos in the Wilson scheme.Comment: 11 pages, 6 figures, some formulations change
Mock Modular Mathieu Moonshine Modules
We construct super vertex operator algebras which lead to modules for
moonshine relations connecting the four smaller sporadic simple Mathieu groups
with distinguished mock modular forms. Starting with an orbifold of a free
fermion theory, any subgroup of Co_0 that fixes a 3-dimensional subspace of its
unique non-trivial 24-dimensional representation commutes with a certain N=4
superconformal algebra. Similarly, any subgroup of Co_0 that fixes a
2-dimensional subspace of the 24-dimensional representation commutes with a
certain N=2 superconformal algebra. Through the decomposition of the
corresponding twined partition functions into characters of the N=4 (resp. N=2)
superconformal algebra, we arrive at mock modular forms which coincide with the
graded characters of an infinite-dimensional Z-graded module for the
corresponding group. The Mathieu groups are singled out amongst various other
possibilities by the moonshine property: requiring the corresponding weak
Jacobi forms to have certain asymptotic behaviour near cusps. Our constructions
constitute the first examples of explicitly realized modules underlying
moonshine phenomena relating mock modular forms to sporadic simple groups.
Modules for other groups, including the sporadic groups of McLaughlin and
Higman--Sims, are also discussed.Comment: 94 pages, including 56 pages of tables; v2: updated references and
minor revisions to abstract, introduction and sections 8 and
A Unified Model for Current-Programmed Converters
A unified model is established for a current-programmed power converter, which is both a modification and an extension of familiar models. Inclusion of the sampling effect allows the presence of an additional pole ω_p in the current-loop gain to be derived. The resulting final double-slope asymptote is fixed in position, and the crossover frequency cannot exceed half the switching frequency. A stability parameter, Q_s, determines the additional pole and describes the degree of peaking in the closed-loop transfer function. Experimental verification employs an analog signal injection techniqu
- …