1,301 research outputs found
Research on "internal abnormalities and quality of Rocha pears" : September 1997 through January 1998
Chandra observation of two shock fronts in the merging galaxy cluster Abell 2146
We present a new Chandra observation of the galaxy cluster Abell 2146 which has revealed a complex merging system with a gas structure that is remarkably similar to the Bullet cluster. The X-ray image and temperature map show a cool 2 ā3 keV subcluster with a ram pressure stripped tail of gas just exiting the disrupted 6 ā 7 keV primary cluster. From the sharp jump in the temperature and density of the gas, we determine that the subcluster is preceded by a bow shock with a Mach number M= 2.2 Ā± 0.8, corresponding to a velocity v= 2200+1000ā900 km sā1 relative to the main cluster. We estimate that the subcluster passed through the primary core only 0.1 ā0.3 Gyr ago. In addition, we observe a slower upstream shock propagating through the outer region of the primary cluster and calculate a Mach number M= 1.7 Ā± 0.3. Based on the measured shock Mach numbers Mā¼ 2 and the strength of the upstream shock, we argue that the mass ratio between the two merging clusters is between 3 and 4 to one. By comparing the Chandra observation with an archival Hubble Space Telescope observation, we find that a group of galaxies is located in front of the X-ray subcluster core but the brightest cluster galaxy is located immediately behind the X-ray peak
Generation of arbitrary two dimensional motional state of a trapped ion
We present a scheme to generate an arbitrary two-dimensional quantum state of
motion of a trapped ion. This proposal is based on a sequence of laser pulses,
which are tuned appropriately to control transitions on the sidebands of two
modes of vibration. Not more than laser pulses are needed to
generate a pure state with upper phonon number and in the and
direction respectively.Comment: to appear in PR
Young women and limits to the normalisation of condom use: a qualitative study
Encouraging condom use among young women is a major focus of HIV/STI prevention efforts but the degree to which they see themselves as being at risk limits their use of the method. In this paper, we examine the extent to which condom use has become normalised among young women. In-depth interviews were conducted with 20 year old women from eastern Scotland (N = 20). Purposive sampling was used to select a heterogeneous group with different levels of sexual experience and from different social backgrounds. All of the interviewees had used (male) condoms but only three reported consistent use. The rest had changed to other methods, most often the pill, though they typically went back to using condoms occasionally. Condoms were talked about as the most readily available contraceptive method, and were most often the first contraceptive method used. The young women had ingrained expectations of use, but for most, these norms centred only on their new or casual partners, with whom not using condoms was thought to be irresponsible. Many reported negative experiences with condoms, and condom dislike and failure were common, lessening trust in the method. Although the sexually transmitted infection (STI) prevention provided by condoms was important, this was seen as additional, and secondary, to pregnancy prevention. As the perceived risks of STIs lessened in relationships with boyfriends, so did condom use. The promotion of condoms for STI prevention alone fails to consider the wider influences of partners and young women's negative experiences of the method. Focusing on the development of condom negotiation skills alone will not address these issues. Interventions to counter dislike, method failure, and the limits of the normalisation of condom use should be included in STI prevention efforts
On the spin-statistics connection in curved spacetimes
The connection between spin and statistics is examined in the context of
locally covariant quantum field theory. A generalization is proposed in which
locally covariant theories are defined as functors from a category of framed
spacetimes to a category of -algebras. This allows for a more operational
description of theories with spin, and for the derivation of a more general
version of the spin-statistics connection in curved spacetimes than previously
available. The proof involves a "rigidity argument" that is also applied in the
standard setting of locally covariant quantum field theory to show how
properties such as Einstein causality can be transferred from Minkowski
spacetime to general curved spacetimes.Comment: 17pp. Contribution to the proceedings of the conference "Quantum
Mathematical Physics" (Regensburg, October 2014
Multicomponent theory of buoyancy instabilities in magnetized plasmas: The case of magnetic field parallel to gravity
We investigate electromagnetic buoyancy instabilities of the electron-ion
plasma with the heat flux based on not the magnetohydrodynamic (MHD) equations,
but using the multicomponent plasma approach when the momentum equations are
solved for each species. We consider a geometry in which the background
magnetic field, gravity, and stratification are directed along one axis. The
nonzero background electron thermal flux is taken into account. Collisions
between electrons and ions are included in the momentum equations. No
simplifications usual for the one-fluid MHD-approach in studying these
instabilities are used. We derive a simple dispersion relation, which shows
that the thermal flux perturbation generally stabilizes an instability for the
geometry under consideration. This result contradicts to conclusion obtained in
the MHD-approach. We show that the reason of this contradiction is the
simplified assumptions used in the MHD analysis of buoyancy instabilities and
the role of the longitudinal electric field perturbation which is not captured
by the ideal MHD equations. Our dispersion relation also shows that the medium
with the electron thermal flux can be unstable, if the temperature gradients of
ions and electrons have the opposite signs. The results obtained can be applied
to the weakly collisional magnetized plasma objects in laboratory and
astrophysics.Comment: Accepted for publication in Astrophysics & Space Scienc
Hadamard states from null infinity
Free field theories on a four dimensional, globally hyperbolic spacetime,
whose dynamics is ruled by a Green hyperbolic partial differential operator,
can be quantized following the algebraic approach. It consists of a two-step
procedure: In the first part one identifies the observables of the underlying
physical system collecting them in a *-algebra which encodes their relational
and structural properties. In the second step one must identify a quantum
state, that is a positive, normalized linear functional on the *-algebra out of
which one recovers the interpretation proper of quantum mechanical theories via
the so-called Gelfand-Naimark-Segal theorem. In between the plethora of
possible states, only few of them are considered physically acceptable and they
are all characterized by the so-called Hadamard condition, a constraint on the
singular structure of the associated two-point function. Goal of this paper is
to outline a construction scheme for these states which can be applied whenever
the underlying background possesses a null (conformal) boundary. We discuss in
particular the examples of a real, massless conformally coupled scalar field
and of linearized gravity on a globally hyperbolic and asymptotically flat
spacetime.Comment: 23 pages, submitted to the Proceedings of the conference "Quantum
Mathematical Physics", held in Regensburg from the 29th of September to the
02nd of October 201
Towards a New Standard Model for Black Hole Accretion
We briefly review recent developments in black hole accretion disk theory,
emphasizing the vital role played by magnetohydrodynamic (MHD) stresses in
transporting angular momentum. The apparent universality of accretion-related
outflow phenomena is a strong indicator that large-scale MHD torques facilitate
vertical transport of angular momentum. This leads to an enhanced overall rate
of angular momentum transport and allows accretion of matter to proceed at an
interesting rate. Furthermore, we argue that when vertical transport is
important, the radial structure of the accretion disk is modified at small
radii and this affects the disk emission spectrum. We present a simple model
demonstrating how energetic, magnetically-driven outflows modify the emergent
disk emission spectrum with respect to that predicted by standard accretion
disk theory. A comparison of the predicted spectra against observations of
quasar spectral energy distributions suggests that mass accretion rates
inferred using the standard disk model may severely underestimate their true
values.Comment: To appear in the Fifth Stromlo Symposium Proceedings special issue of
ApS
Deformations of quantum field theories on spacetimes with Killing vector fields
The recent construction and analysis of deformations of quantum field
theories by warped convolutions is extended to a class of curved spacetimes.
These spacetimes carry a family of wedge-like regions which share the essential
causal properties of the Poincare transforms of the Rindler wedge in Minkowski
space. In the setting of deformed quantum field theories, they play the role of
typical localization regions of quantum fields and observables. As a concrete
example of such a procedure, the deformation of the free Dirac field is
studied.Comment: 35 pages, 3 figure
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