2,455 research outputs found
Paleomagnetism of the Avalonian Finn Hill sequence of Eastern Newfoundland, Canada
As part of a major study of the Avalon zone in eastern Newfoundland, the paleomagnetism of the Finn Hill ignimbrite sequence located in the Colliers-Harbour Main area is described. Detailed alternating field and thermal experiments performed on this Hadrynian ash-flow tuff sequence indicate that it is characterized by two significantly different mean directions of magnetization; I) A(S-l28, I=+25, α95 =7.1°, K-171, N=4 sites) in-situ and (D=225, D + 59, α95=13°, K=48) after tilt correction, 2) B (D=69, I=+52, N= 2 sites) in-sltu and (D=303, I=+40) after tilt correction.
The A component is believed to be secondary and the paleopole corresponding to the in-situ direction of magnetization is I4°S, l°W. The B tilt corrected component (D=303, I=+40) is interpreted as a pre- or syn-folding direction of magnetization corresponding with the time of formation of the ignimbrite sequence or slightly later; its corresponding paleopole position Is 39°S, 29°E. It is worth noting that the paleopole (14°S, 1°W) of the secondary component found in the present study is not significantly different from the paleopole (5°S, 8°W) obtained in the Cloud Mountain basalt overlying Grenville basement of northwestern Newfoundland, the age of which is close to latest Precambrian (≈620 Ma), this suggests that in tate Precambrian time, the eastern and western Newfoundland blocks were not very distant from each other or else they were far apart but at the same paleolatitude. The secondary component is Interpreted as representing the magnetic imprint of the Avalonian (Cadomlan) orogenic event. This last interpretation is reasonable since nearby Cambrian units (to the west) are located unconformably above the Precambrian units and are almost flat lying. Consequently, the chances of this secondary component being of Acadian age are relatively small. The new data set of this research fills a definite gap in the paleomagnetic record of the Avalon zone.
RÉSUMÉ
La description du paléomagnetlsme de la séquence d'ignimbrite de Finn Hill, située dans la région de Colliers - Harbour Main, s'insere dans le cadre d'une étude d'envergure de la zone Avalon de l’est de Terre-Neuve. Cette séquence hadrynienne de tuf volcanique a été soumise à des experiences poussées de champ alternatif et d'anlyse thermique. Les rèsultats indiquent que la séquence est caractérisée par deux sens moyens de L’aimantatlon qui différent de façon significative: 1) A(S-l28, I=+25, α95 =7.1°, K-171, N=4 sites) in-situ et (D=225, D + 59, α95=13°, K=48) après correction de l’incllnaison, 2) B(D=69, I=+52, N= 2 sites) In-situ et (D=303, I=+40) après correction de l'lnclinaison.
La composante A serait secondaire et le paléopôle correspondent au sens in-situ de L’aimantation est de I4°S, l°0. On interprète la composante d'inclinaison B corrigée (D=303, I=+40) com me le rèsultat d'une aimantation qui a précede ou qui était con tern poraine au plissement, et correspondent done au temps d'emplacement de la séquence d'ignimbrite, ou survenant peu apres; la position du palépôle correspondent est de 39°S, 29°E. Il est intéressant de noter que le palépôle (14°S, 1°O) de la composante secondaire, relevé dans la présente étude, ne différe pas de façon significative du palépôle {5°S, 8°O) obtenu à partir du basalte de Cloud Mountain qui recouvre le socle grenvillien dans le nord-ouest de Terre-Neuve et dont l'âge est proche du Précambrien tardif (≈620 Ma). Ceci suggère que tard durant le Précambrien, les blocs est et ouest de Terre-Neuve n'étaient pas très distants, ou bien qu'ils étaient très éloigners l'un de l’autre mais situes à la même paléolatitude. On interprète la composante secondaire comme une empreinte magnétique représentant l'orogenèse avalonienne (Cadomien). Cette derniere interprétation est raisonnable puisque les unités cambriennes voisines (a l’ouest) reposent subhorizontalement et en discordance sur les unités précambriennes. Conséquemment, il est peu probable que cette composante secondaire soit d'Sge Acadien. Ces nouvelles données comblent une lacune importante dans L’histoire palfio-magnétlque de la zone Avalon.
[Traduit par le journal
GaN/AlN Quantum Dots for Single Qubit Emitters
We study theoretically the electronic properties of -plane GaN/AlN quantum
dots (QDs) with focus on their potential as sources of single polarized photons
for future quantum communication systems. Within the framework of eight-band
k.p theory we calculate the optical interband transitions of the QDs and their
polarization properties. We show that an anisotropy of the QD confinement
potential in the basal plane (e.g. QD elongation or strain anisotropy) leads to
a pronounced linear polarization of the ground state and excited state
transitions. An externally applied uniaxial stress can be used to either induce
a linear polarization of the ground-state transition for emission of single
polarized photons or even to compensate the polarization induced by the
structural elongation.Comment: 6 pages, 9 figures. Accepted at Journal of Physics: Condensed Matte
Tidal Effects in Clusters of Galaxies
High-redshift clusters of galaxies show an over-abundance of spirals by a
factor of 2-3, and the corresponding under-abundance of S0 galaxies, relative
to the nearby clusters. This morphological evolution can be explained by tidal
interactions with neighboring galaxies and with the hierarchically growing
cluster halo. The efficiency of tidal interactions depends on the size and
structure of the cluster, as well as on the epoch of its formation. I simulate
the formation and evolution of Virgo-type clusters in three cosmologies: a
critical density model Omega_0=1, an open model Omega_0=0.4, and a flat model
Omega_0=0.4 with a cosmological constant. The orbits of identified halos are
traced with a high temporal resolution (~10^7 yr). Halos with low relative
velocities merge only shortly after entering the cluster; after virialization
mergers are suppressed. The dynamical evolution of galaxies is determined by
the tidal field along their trajectories. The maxima of the tidal force do not
always correspond to closest approach to the cluster center. They are produced
to a large extent by the local density structures, such as the massive galaxies
and the unvirialized remnants of infalling groups of galaxies. Collisions of
galaxies are intensified by the substructure, with about 10 encounters within
10 kpc per galaxy in the Hubble time. These very close encounters add an
important amount (10-50%) of the total heating rate. The integrated effect of
tidal interactions is insufficient to transform a spiral galaxy into an
elliptical, but can produce an S0 galaxy. Overall, tidal heating is stronger in
the low Omega_0 clusters
Generational differences in factors influencing physicians to choose a work location.
INTRODUCTION: Canadian medical schools have increased enrolment and recruited more rural students in an effort to address general and rural physician shortages. The success of this approach depends on the recruitment of these newly trained physicians to under-serviced areas. Studies from North America suggest that the career expectations and practice patterns of younger, more recently graduated physicians differ from those of their older counterparts. This study explored the factors that influenced the work location choices of physicians of differing generations, who trained at universities in Saskatchewan, and Newfoundland and Labrador, two Canadian provinces with large rural populations and no community larger than 235 000 population. METHODS: Semi-structured, qualitative interviews were conducted with physicians who graduated from either the Memorial University of Newfoundland or the University of Saskatchewan. Generation definitions were based on the graduation year. Early-career physicians graduated between 1995 and 1999; mid-career physician graduated between 1985 and 1989; late-career physicians graduated between 1975 and 1979; and end-career physicians graduated between 1965 and 1969. Each physician was asked questions about the number and nature of work location changes over the course of their careers and the factors related to their decision to choose each location. Interview transcripts and notes were analyzed using a thematic analysis approach. Although the study focus was on generational differences, similarities and differences between universities, sexes and specialties (family physicians/GPs vs specialists) were also examined. Recruitment to the provinces was focused on as a whole, because the largest communities in the provinces are small compared with most urban communities. RESULTS: Forty-eight physicians were interviewed, five to nine physicians who graduated in each decade and from each university. The desire to be near family and friends was cited as the primary consideration when choosing a work location, regardless of generation. Likewise, residency training location, the ability to use their skills and knowledge fully, and the quality of recruitment efforts were important considerations in choosing a work location for all physicians. For some, remuneration was very influential in their work location decision; however, many physicians who chose to remain in their smaller 'home' provinces noted the lower cost of living in these provinces. Physicians who graduated in the 1980s and 1990s placed greater emphasis on work-life balance and spouse's employment opportunities than their older generation counterparts. In contrast, physicians who graduated in the 1960s and 1070s highlighted the medical need of the community, and the desire for adventure and to see new places as important. CONCLUSIONS: While many factors for choosing a work location appear to be stable over generations, a number of generational differences were found. Younger physicians placed greater emphasis on work-life balance and spouse's employment than older generation physicians. These differences may have important implications for small population regions which may not be able to support physician-spouse pairs or certain subspecialties. Although economic factors have largely been the focus of recruitment and retention initiatives in these provinces, the findings highlight the importance of addressing the needs and expectations of younger generation physicians in order to attract these physicians
Accretion of a satellite onto a spherical galaxy. II. Binary evolution and orbital decay
We study the dynamical evolution of a satellite orbiting outside of a
companion spherical galaxy. The satellite is subject to a back-reaction force
resulting from the density fluctuations excited in the primary stellar system.
We evaluate this force using the linear response theory developed in Colpi and
Pallavicini (1997). The force is computed in the reference frame comoving with
the primary galaxy and is expanded in multipoles. To capture the relevant
features of the physical process determining the evolution of the detached
binary, we introduce in the Hamiltonian the harmonic potential as interaction
potential among stars. The dynamics of the satellite is computed
self-consistently. We determine the conditions for tidal capture of a satellite
from an asymptotic free state. If the binary comes to existence as a bound
pair, stability against orbital decay is lost near resonance. The time scale of
binary coalescence is computed as a function of the eccentricity and mass
ratio. In a comparison with Weinberg's perturbative technique we demonstrate
that pinning the center of mass of the galaxy would induce a much larger torque
on the satellite.Comment: 13 pages, Tex,+ 10 .ps figures Submitted to The Astrophysical Journa
Paleomagnetism of mafic dikes from the Avalon Peninsula, eastern Newfoundland
As part of a major study of the Avalon zone in eastern Newfoundland, we describe the palromagnetism of two series of dikes located in the Bauline and Colliers-Harbour Main areas, respectively. Detailed alternating field (AF) and thermal experiments performed on these dikes indicate that they are characterized by two significantly different mean directions of magnetization: SE++(D = 153°, I = +62*; K = 46, N = 12 sites) and SW (D = 217°, I = +58°, K = 58, N = 12 sites ) with corresponding paleopole positions at A (4°N, 34"U; dp,dm = 8", 10°) and C (2"N, 81°W; dp, dm = 6", 9'). Antipoles A and C are located in southerly latitudes, and a comparison with other poles of Avalonian rocks from New Brunswick, Nova Scotia and Newfoundland indicates a Silurian to Siluro-Devonian age for these dikes. Poles A and B are interpreted as representing rapid apparent polar wander with respect to eastern Newfoundland during Silurian time. Two components of magnetization, corresponding to two distinct geological events, are observed in the dikes investigated and this situation favours the following geological scenario for the evolution of the Avalon microcontinent: (a) intracratonic rift zone resulting in long-lived Late Hadrynian and Cambrian continental extension, (b) a very late phase of distension in separating miniplates during Silurian and Early Devonian time.
RÉSUMÉ
Dans le cadre d'une étude d'envergure de la zone d'Avalon menée dans l'est de Terre-Neuve, nous décrivons le paléomagnétisme de deux séries de dykes situées dans les régions de Bauline et de Colliers - Harbour Main respectivement. Des experiences détaillées de désaimantation par champ alternatif (CA) et par lavage thermique démontrent que deux directions moyennes de l'aimantation caractérisent les dykes: SE++(D = 153°, I = +62*; K = 46, N = 12 sites de prélèvement) et SO (D = 217°, I = +58°, K = 58, N = 12 sites de prélèvement) avec des paléopöles correspondants aux positions A (4°N, 34"U; dp,dm = 8", 10°) et C (2"N, 81°W; dp, dm = 6", 9'). Les antipôles A et C se situent à des latitudes méridionales et par comparaison avec d'autres pôles de roches avaloniennes du Nouveau-Brunswick, de la Nouvelle-Écosse et de Terre-Neuve, on attrlbue aux dykes un âge Silurien à Siluro-Dévonien. On interprète les pôles géomagnétiques A et B comme le résultat de displacements rapides du pôle par rapport avec l'est de Terre-Neuve durant le Silurien. On observe dans ces dykes deux composantes de magnétisation, correspondant à deux évènements géologiques distincts. Ceci suggère le scenario suivant pour expliquer l’évolution du microcontinent Avalon: (a) un rift intracratonique qui se traduit par une longue période d'extension continentale depuis l'Hadrynien tardif jusqu'au Cambrien; (b) du Silurien jusqu'au début du Dévonien, une phase tardive de distension lors de la séparation des miniplaques.
[Traduit par le journal
Darwin Tames an Andromeda Dwarf: Unraveling the Orbit of NGC 205 Using a Genetic Algorithm
NGC 205, a close satellite of the M31 galaxy, is our nearest example of a
dwarf elliptical galaxy. Photometric and kinematic observations suggest that
NGC 205 is undergoing tidal distortion from its interaction with M31. Despite
earlier attempts, the orbit and progenitor properties of NGC 205 are not well
known. We perform an optimized search for these unknowns by combining a genetic
algorithm with restricted N-body simulations of the interaction. This approach,
coupled with photometric and kinematic observations as constraints, allows for
an effective exploration of the parameter space. We represent NGC 205 as a
static Hernquist potential with embedded massless test particles that serve as
tracers of surface brightness. We explore 3 distinct, initially stable
configurations of test particles: cold rotating disk, warm rotating disk, and
hot, pressure-supported spheroid. Each model reproduces some, but not all, of
the observed features of NGC 205, leading us to speculate that a rotating
progenitor with substantial pressure support could match all of the
observables. Furthermore, plausible combinations of mass and scale length for
the pressure-supported spheroid progenitor model reproduce the observed
velocity dispersion profile. For all 3 models, orbits that best match the
observables place the satellite 11+/-9 kpc behind M31 moving at very large
velocities: 300-500 km/s on primarily radial orbits. Given that the observed
radial component is only 54 km/s, this implies a large tangential motion for
NGC 205, moving from the NW to the SE. These results suggest NGC 205 is not
associated with the stellar arc observed to the NE of NGC 205. Furthermore, NGC
205's velocity appears to be near or greater than its escape velocity,
signifying that the satellite is likely on its first M31 passage.Comment: 34 pages, 20 figures, accepted for publication in the Astrophysical
Journal, A pdf version with high-resolution figures may be obtained from
http://www.ucolick.org/~kirsten/ms.pd
Polarized Emission Lines from Single InGaN/GaN Quantum Dots: Role of the Valence-band Structure of Wurtzite Group-III Nitrides
We present a study of the polarization properties of emission lines from
single InGaN/GaN quantum dots (QDs). The QDs, formed by spinodal decomposition
within ultra-thin InGaN quantum wells, are investigated using single-QD
cathodoluminescence (CL). The emission lines exhibit a systematic linear
polarization in the orthogonal crystal directions [1 1 -2 0] and [-1 1 0 0]--a
symmetry that is non-native to hexagonal crystals.
Eight-band k.p calculations reveal a mechanism that can explain the observed
polarizations: The character of the hole(s) in an excitonic complex determines
the polarization direction of the respective emission if the QD is slightly
elongated. Transitions involving A-band holes are polarized parallel to the
elongation; transitions involving B-type holes are polarized in the orthogonal
direction. The energetic separation of both hole states is smaller than 10 meV.
The mechanism leading to the linear polarizations is not restricted to InGaN
QDs, but should occur in other wurtzite-nitride QDs and in materials with
similar valence band structure.Comment: Conf. Proc. of the MSS-13 in Genova 2007, accepted at Physica
Dynamical friction and the evolution of satellites in virialized halos: the theory of linear response
The evolution of a small satellite inside a more massive truncated isothermal
spherical halo is studied using both the Theory of Linear Response for
dynamical friction and N-Body simulations. The analytical approach includes the
effects of the gravitational wake, of the tidal deformation and the shift of
the barycenter of the primary, so unifying the local versus global
interpretation of dynamical friction. Sizes, masses, orbital energies and
eccentricities are chosen as expected in hierarchical clustering models. We
find that in general the drag force in self-gravitating backgrounds is weaker
than in uniform media and that the orbital decay is not accompanied by a
significant circularization. We also show that the dynamical friction time
scale is weakly dependent on the initial circularity. We provide a fitting
formula for the decay time that includes the effect of mass and angular
momentum loss. Live satellites with dense cores can survive disruption up to an
Hubble time within the primary, notwithstanding the initial choice of orbital
parameters. Dwarf spheroidal satellites of the Milky Way, like Sagittarius A
and Fornax, have already suffered mass stripping and, with their present
masses, the sinking times exceed 10 Gyr even if they are on very eccentric
orbits.Comment: 27 pages including 9 figures. Accepted for publication in the
Astrophysical Journal. Part 2, issue November 10 1999, Volume 52
The genetic diversity of the American oil palm, Elaeis oleifera (Kunth), Corés revealed by nuclear RFLP markers.
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