743 research outputs found
Clues about the scarcity of stripped-envelope stars from the evolutionary state of the sdO+Be binary system phi Persei
Stripped-envelope stars (SESs) form in binary systems after losing mass
through Roche-lobe overflow. They bear astrophysical significance as sources of
UV and ionizing radiation in older stellar populations and, if sufficiently
massive, as stripped supernova progenitors. Binary evolutionary models predict
them to be common, but only a handful of subdwarfs (i.e., SESs) with B-type
companions are known. This could be the result of observational biases
hindering detection, or an incorrect understanding of binary evolution. We
reanalyze the well-studied post-interaction binary phi Persei. Recently, new
data improved the orbital solution of the system, which contains a ~1.2 Msun
SES and a rapidly rotating ~9.6 Msun Be star. We compare with an extensive grid
of evolutionary models using a Bayesian approach and find initial masses of the
progenitor of 7.2+/-0.4 Msun for the SES and 3.8+/-0.4 Msun for the Be star.
The system must have evolved through near-conservative mass transfer. These
findings are consistent with earlier studies. The age we obtain, 57+/-9 Myr, is
in excellent agreement with the age of the alpha Persei cluster. We note that
neither star was initially massive enough to produce a core-collapse supernova,
but mass exchange pushed the Be star above the mass threshold. We find that the
subdwarf is overluminous for its mass by almost an order of magnitude, compared
to the expectations for a helium core burning star. We can only reconcile this
if the subdwarf is in a late phase of helium shell burning, which lasts only
2-3% of the total lifetime as a subdwarf. This could imply that up to ~50 less
evolved, dimmer subdwarfs exist for each system similar to phi Persei. Our
findings can be interpreted as a strong indication that a substantial
population of SESs indeed exists, but has so far evaded detection because of
observational biases and lack of large-scale systematic searches.Comment: 11 pages, 5 figures, accepted for publication in A&
Design of Hybrid Conductors for Electromagnetic Forming Coils
The use of hybrid coil turns made of steel (St) and copper (Cu) is originally motivated by the
increased mechanical strength compared to monolithic copper conductors. Due to the
differing electrical conductivities of the two materials, the hybrid design also changes the
current density distribution in the conductor cross section. This affects crucial process
parameters such as the magnetic pressure and the Joule heat losses.
The effect of the hybrid conductor design on the process efficiency is investigated. An
electromagnetic sheet metal forming operation using a one-turn coil with rectangular cross
section is used as reference case. The copper layer (CuCr1Zr) was deposited on a tool steel
substrate (X40CrMoV5-1) using a selective laser melting process. The copper layer
thickness is varied ranging from a monolithic steel conductor to a monolithic copper
conductor. The workpiece (EN AW-5083, t_w = 1 mm) is formed through a drawing ring so
that the final forming height is a qualitative measure for the process efficiency. The
experimental results prove that the efficiency in case of a properly designed hybrid
conductor can exceed the efficiency of a monolithic copper coil. The current density
distribution in the hybrid cross section is investigated by means of numerical simulations.
This way a deeper insight into the physical effects of a varying copper layer thickness is
gained. The results reveal that the optimum layer thickness is not just a function of the coil
cross section and the current frequency. It is also affected by the coil length and the
resistance of the pulse generator
INTEGRAL observations of SS433, a supercritically accreting microquasar with hard spectrum
Observations of SS433 by INTEGRAL carried out in March -- May 2003 are
presented. SS433 is evidently detected on the INTEGRAL images of the
corresponding sky region in the energy bands 25-50 and 50-100 keV. The
precessional variability of the hard X-ray flux is clearly seen. The X-ray
eclipse caused by the binary orbital motion is also detected. A possible origin
of the hard continuum is briefly discussed.Comment: 5 pages, 6 figures. Accepted to A&A INTEGRAL special volum
Strong laser fields as a probe for fundamental physics
Upcoming high-intensity laser systems will be able to probe the
quantum-induced nonlinear regime of electrodynamics. So far unobserved QED
phenomena such as the discovery of a nonlinear response of the quantum vacuum
to macroscopic electromagnetic fields can become accessible. In addition, such
laser systems provide for a flexible tool for investigating fundamental
physics. Primary goals consist in verifying so far unobserved QED phenomena.
Moreover, strong-field experiments can search for new light but weakly
interacting degrees of freedom and are thus complementary to accelerator-driven
experiments. I review recent developments in this field, focusing on photon
experiments in strong electromagnetic fields. The interaction of
particle-physics candidates with photons and external fields can be
parameterized by low-energy effective actions and typically predict
characteristic optical signatures. I perform first estimates of the accessible
new-physics parameter space of high-intensity laser facilities such as POLARIS
and ELI.Comment: 7 pages, Key Lecture at the ELI Workshop and School on "Fundamental
Physics with Ultra-High Fields", 9 September - 2 October 2008 at Frauenworth
Monastery, German
Casimir interaction between normal or superfluid grains in the Fermi sea
We report on a new force that acts on cavities (literally empty regions of
space) when they are immersed in a background of non-interacting fermionic
matter fields. The interaction follows from the obstructions to the (quantum
mechanical) motions of the fermions caused by the presence of bubbles or other
(heavy) particles in the Fermi sea, as, for example, nuclei in the neutron sea
in the inner crust of a neutron star or superfluid grains in a normal Fermi
liquid. The effect resembles the traditional Casimir interaction between
metallic mirrors in the vacuum. However, the fluctuating electromagnetic fields
are replaced by fermionic matter fields. We show that the fermionic Casimir
problem for a system of spherical cavities can be solved exactly, since the
calculation can be mapped onto a quantum mechanical billiard problem of a
point-particle scattered off a finite number of non-overlapping spheres or
disks. Finally we generalize the map method to other Casimir systems,
especially to the case of a fluctuating scalar field between two spheres or a
sphere and a plate under Dirichlet boundary conditions.Comment: 8 pages, 2 figures, submitted to the Proceedings of QFEXT'05,
Barcelona, Sept. 5-9, 200
Observable consequences of quantum gravity: Can light fermions exist?
Any theory of quantum gravity must ultimately be connected to observations.
This demand is difficult to be met due to the high energies at which we expect
the quantum nature of gravity to become manifest. Here we study, how viable
quantum gravity proposals can be restricted by investigating the interplay of
gravitational and matter degrees of freedom. Specifically we demand that a
valid quantum theory of gravity must allow for the existence of light (compared
to the Planck scale) fermions, since we observe these in our universe. Within
the effective theory framework, we can thus show that UV completions for
gravity are restricted, regardless of the details of the microscopic theory.
Specialising to asymptotically safe quantum gravity, we find indications that
universes with light fermions are favoured within this UV completion for
gravity.Comment: 4 pages, based on a talk given at Loops '11, Madrid, to appear in
Journal of Physics: Conference Series (JPCS
Asymptotically free scalar curvature-ghost coupling in Quantum Einstein Gravity
We consider the asymptotic-safety scenario for quantum gravity which
constructs a non-perturbatively renormalisable quantum gravity theory with the
help of the functional renormalisation group. We verify the existence of a
non-Gaussian fixed point and include a running curvature-ghost coupling as a
first step towards the flow of the ghost sector of the theory. We find that the
scalar curvature-ghost coupling is asymptotically free and RG relevant in the
ultraviolet. Most importantly, the property of asymptotic safety discovered so
far within the Einstein-Hilbert truncation and beyond remains stable under the
inclusion of the ghost flow.Comment: 8 pages, 3 figures, RevTe
Thermal Quantum Fields in Static Electromagnetic Backgrounds
We present and discuss, at a general level, new mathematical results on the
spatial nonuniformity of thermal quantum fields coupled minimally to static
background electromagnetic potentials. Two distinct examples are worked through
in some detail: uniform (parallel and perpendicular) background electric and
magnetic fields coupled to a thermal quantum scalar field.Comment: 22 page
The Effect of the Specimen–Platen Interface on Internal Cracking and Brittle Fracture of Ice Under Compression: High-Speed Photography
Uniaxial compression experiments at –10°C at 10−3s−1 on fresh-water, granular ice have established through the use of high-speed photography that internal cracks nucleate preferentially away from the ice/platen (i/p) interface under conditions of i/p contraint, but near the interface under conditions of i/p expansion. Under conditions of little i/p interaction, cracks nucleate more or less randomly throughout the specimen. Correspondingly, the brittle-fracture strength decreases as the i/p interaction changes from compressive to tensile. These effects are explained in terms of the spatial variation of the maximum shear stress and the crack density
Qualification of CuCr1Zr for the SLM Process
Working coils for electromagnetic forming processes need to comply with a wide list of
requirements such as durability, efficiency and a tailored pressure distribution. Due to its
unique combination of high strength and high electrical conductivity CuCr1Zr meets these
requirements and is a common material for coil turns. In combination with conventional coil
production processes like winding or waterjet cutting the use of this material is state of the
art. A promising approach for coil production is the use of additive manufacturing (AM)
processes. In comparison to conventional manufacturing processes, AM offers tremendous
advantages such as feature-integration e.g. undercuts or lattice structures. However, this
increased design freedom only leads to improved working coils if copper alloys with high
strength and high electrical conductivity such as CuCr1Zr can be processed. Due to the high
thermal conductivity and reflectivity the use of suchlike materials in additive manufacturing
processes is challenging. Considering the effects of the required pre- and post-processing
treatments for additive manufactured parts the need for research is further increased. The
objective of this paper is to develop a method for the qualification of CuCr1Zr for the
selective laser melting (SLM) process. This comprises the powder characterization, the
process parameter identification and the microstructure investigation of the generated test
geometries
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