1,463 research outputs found
Fidelity for imperfect postselection
We describe a simple measure of fidelity for mixed state postselecting
devices. The measure is most appropriate for postselection where the task
performed by the output is only effected by a specific state.Comment: 8 Pages, 8 Figure
Magnetic activity on AB Doradus: Temporal evolution of starspots and differential rotation from 1988 to 1994
Surface brightness maps for the young K0 dwarf AB Doradus are reconstructed
from archival data sets for epochs spanning 1988 to 1994. By using the
signal-to-noise enhancement technique of Least-Squares Deconvolution, our
results show a greatly increased resolution of spot features than obtained in
previously published surface brightness reconstructions. These images show that
for the exception of epoch 1988.96, the starspot distributions are dominated by
a long-lived polar cap, and short-lived low to high latitude features. The
fragmented polar cap at epoch 1988.96 could indicate a change in the nature of
the dynamo in the star. For the first time we measure differential rotation for
epochs with sufficient phase coverage (1992.05, 1993.89, 1994.87). These
measurements show variations on a timescale of at least one year, with the
strongest surface differential rotation ever measured for AB Dor occurring in
1994.86. In conjunction with previous investigations, our results represent the
first long-term analysis of the temporal evolution of differential rotation on
active stars.Comment: accepted by MNRAS 18 pages 18 figure
Retrodiction as a tool for micromaser field measurements
We use retrodictive quantum theory to describe cavity field measurements by
successive atomic detections in the micromaser. We calculate the state of the
micromaser cavity field prior to detection of sequences of atoms in either the
excited or ground state, for atoms that are initially prepared in the excited
state. This provides the POM elements, which describe such sequences of
measurements.Comment: 20 pages, 4(8) figure
A validated finite element analysis procedure for porous structures
Cellular materials are gaining interest thanks to developments in additive manufacturing. Whilst Finite Element Analysis (FEA) is commonly used to obtain the mechanical behaviour of these structures, different modelling and simulation methodologies are followed in literature. Consequently, there is not a clear procedure to accurately evaluate the mechanical properties of porous structures. This study presents a method to perform FEA of lattice structures with accurate results. All inputs required to simulate compression testing of lattices in FEA were investigated, these included the modelling type, element size, number of unit cells required, boundary conditions and the material model. The effect of these variables on the modulus and yield strength of a lattice structure was studied. Lattices with two unit cell structures, varying unit cell sizes and relative densities were additively manufactured in stainless steel, compression tested and compared to FE simulations. The material model for the FE simulations was obtained from tensile testing individual micro-struts of varying diameters. FE simulation results were in good agreement with the experimental results with an average error for the modulus and yield strength of ~10% and 17% respectively. The methodology presented should provide a foundation to accelerate development and adoption of these structures
Surprisingly different star-spot distributions on the near equal-mass equal-rotation-rate stars in the M dwarf binary GJ 65 AB
We aim to understand how stellar parameters such as mass and rotation impact the distribution of star-spots on the stellar surface. To this purpose, we have used Doppler imaging to reconstruct the surface brightness distributions of three fully convective M dwarfs with similar rotation rates. We secured high cadence spectral time series observations of the 5.5 au separation binary GJ 65, comprising GJ 65A (M5.5V, Prot = 0.24 d) and GJ 65B (M6V, Prot = 0.23 d). We also present new observations of GJ 791.2A (M4.5V, Prot = 0.31 d). Observations of each star were made on two nights with UVES, covering a wavelength range from 0.64 - 1.03μm. The time series spectra reveal multiple line distortions that we interpret as cool star-spots and which are persistent on both nights suggesting stability on the time-scale of 3 d. Spots are recovered with resolutions down to 8.3° at the equator. The global spot distributions for GJ 791.2A are similar to observations made a year earlier. Similar high latitude and circumpolar spot structure is seen on GJ 791.2A and GJ 65A. However, they are surprisingly absent on GJ 65B, which instead reveals more extensive, larger, spots concentrated at intermediate latitudes. All three stars show small amplitude latitude-dependent rotation that is consistent with solid body rotation. We compare our measurements of differential rotation with previous Doppler imaging studies and discuss the results in the wider context of other observational estimates and recent theoretical predictions
Estatus de las tortugas marinas en Montserrat (Caribe oriental)
The status of marine turtles in Montserrat (Eastern Caribbean) is reviewed following five years of monitoring (1999–2003). The mean number of nests recorded during the annual nesting season (June–October) was 53 (± 24.9 SD; range: 13–43). In accordance with earlier reports, the nesting of hawksbill (Eretmochelys imbricata) and green (Chelonia mydas) turtles was confirmed on several beaches around the island. Only non–nesting emergences were documented for loggerhead turtles (Caretta caretta) and there was no evidence of nesting by leatherback turtles (Dermochelys coriacea); however, it is possible that additional survey effort would reveal low density nesting by these species. Officially reported turtle capture data for 1993–2003 suggest that a mean of 0.9 turtle per year (± 1.2 SD; range: 0–4) were landed island–wide, with all harvest having occurred during the annual open season (1 October to 31 May). Informed observers believe that the harvest is significantly under–reported and that fishermen avoid declaring their catch by butchering turtles at sea (both during and outside the open season). Of concern is the fact that breeding adults are potentially included in the harvest, and that the open season partially coincides with the breeding season. The present study has shown that although Montserrat is not a major nesting site for sea turtles, it remains important on a regional basis for the Eastern Caribbean.Se ha estudiado la situaciĂłn de las tortugas marinas en Montserrat (Caribe oriental) mediante un seguimiento de cinco años (1999–2003). El nĂşmero medio de nidos registrados durante la estaciĂłn anual de nidificaciĂłn (junio–octubre) fue de 53 (± 24.9 SD; rango: 13–143). En concordancia con informes anteriores, se confirmĂł la nidificacĂłn de las tortugas carey (Eretmochelys imbricata) y verde (Chelonia mydas) en varias playas alrededor de la isla. En la tortuga boba (Caretta caretta) sĂłlo se registraron salidas sin nidificaciĂłn, y no se encontraron pruebas de que la tortuga laĂşd (Dermochelys coriacea) nidificase; sin embargo, es posible que ulteriores estudios pongan de manifiesto una baja densidad de nidificaciĂłn de esta especie. Los datos oficiales de capturas de tortugas (1993–2003) sugieren que en toda la isla llegaban a tierra una media de 0.9 tortugas anuales (± 1.2 SD; rango: 0–4), produciĂ©ndose todas las capturas cuando se habĂa levantado la veda. Observadores bien documentados creen que las cifras de recolecciĂłn están significativamente falseadas a la baja, y que los pescadores evitan declarar sus capturas sacrificando las tortugas en el mar (con la veda abierta o cerrada). Es preocupante que en esta caza puedan incluirse tortugas que crĂan, y que el perĂodo de captura permitida coincide en parte con la estaciĂłn reproductora. Este estudio demuestra que aunque Montserrat no es un lugar principal de nidificaciĂłn de las tortugas marinas, sigue siendo importante a escala regional en el Caribe oriental
The relation between stellar magnetic field geometry and chromospheric activity cycles - I. The highly variable field of É› Eridani at activity minimum
The young and magnetically active K dwarf Epsilon Eridani exhibits a chromospheric activity cycle of about 3 years. Previous reconstructions of its large-scale magnetic field show strong variations at yearly epochs. To understand how Epsilon Eridani's large-scale magnetic field geometry evolves over its activity cycle we focus on high cadence observations spanning 5 months at its activity minimum. Over this timespan we reconstruct 3 maps of Epsilon Eridani's large-scale magnetic field using the tomographic technique of Zeeman Doppler Imaging. The results show that at the minimum of its cycle, Epsilon Eridani's large-scale field is more complex than the simple dipolar structure of the Sun and 61 Cyg A at minimum. Additionally we observe a surprisingly rapid regeneration of a strong axisymmetric toroidal field as Epsilon Eridani emerges from its S-index activity minimum. Our results show that all stars do not exhibit the same field geometry as the Sun and this will be an important constraint for the dynamo models of active solar-type stars
Additive manufactured push-fit implant fixation with screw-strength pull out
Additive manufacturing offers exciting new possibilities for improving long-term metallic implant fixation in bone through enabling open porous structures for bony ingrowth. The aim of this research was to investigate how the technology could also improve initial fixation, a precursor to successful long-term fixation. A new barbed fixation mechanism, relying on flexible struts was proposed and manufactured as a push-fit peg. The technology was optimized using a synthetic bone model and compared with conventional press-fit peg controls tested over a range of interference fits. Optimum designs, achieving maximum pull-out force, were subsequently tested in a cadaveric femoral condyle model. The barbed fixation surface provided more than double the pull-out force for less than a third of the insertion force compared to the best performing conventional press-fit peg (p < 0.001). Indeed, it provided screw-strength pull out from a push-fit device (1,124 ± 146 N). This step change in implant fixation potential offers new capabilities for low profile, minimally invasive implant design, while providing new options to simplify surgery, allowing for one-piece push-fit components with high levels of initial stability
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