12,331 research outputs found
Binaries at Low Metallicity: ranges for case A, B and C mass transfer
The evolution of single stars at low metallicity has attracted a large
interest, while the effect of metallicity on binary evolution remains still
relatively unexplored. We study the effect of metallicity on the number of
binary systems that undergo different cases of mass transfer. We find that
binaries at low metallicity are more likely to start transferring mass after
the onset of central helium burning, often referred to as case C mass transfer.
In other words, the donor star in a metal poor binary is more likely to have
formed a massive CO core before the onset of mass transfer.
At solar metallicity the range of initial binary separations that result in
case C evolution is very small for massive stars, because they do not expand
much after the ignition of helium and because mass loss from the system by
stellar winds causes the orbit to widen, preventing the primary star to fill
its Roche lobe. This effect is likely to have important consequences for the
metallicity dependence of the formation rate of various objects through binary
evolution channels, such as long GRBs, double neutron stars and double white
dwarfs.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New
Mexico, July 16-20, 2007, 3 pages, 3 figure
Off-diagonal magnetoimpedance in field-annealed Co-based amorphous ribbons
The off-diagonal magnetoimpedance in field-annealed CoFeSiB amorphous ribbons
was measured in the low-frequency range using a pick-up coil wound around the
sample. The asymmetric two-peak behavior of the field dependence of the
off-diagonal impedance was observed. The asymmetry is attributed to the
formation of a hard magnetic crystalline phase at the ribbon surface. The
experimental results are interpreted in terms of the surface impedance tensor.
It is assumed that the ribbon consists of an inner amorphous region and surface
crystalline layers. The coupling between the crystalline and amorphous phases
is described through an effective bias field. A qualitative agreement between
the calculated dependences and experimental data is demonstrated. The results
obtained may be useful for development of weak magnetic-field sensors.Comment: 19 pages, 6 figure
Reconstruction of plasma density profiles by measuring spectra of radiation emitted from oscillating plasma dipoles
We suggest a new method for characterising non-uniform density distributions of plasma by measuring the spectra of radiation emitted from a localised plasma dipole oscillator excited by colliding electromagnetic pulses. The density distribution can be determined by scanning the collision point in space. Two-dimensional particle-in-cell simulations demonstrate the reconstruction of linear and nonlinear density profiles corresponding to laser-produced plasma. The method can be applied to a wide range of plasma, including fusion and low temperature plasmas. It overcomes many of the disadvantages of existing methods that only yield average densities along the path of probe pulses, such as interferometry and spectroscopy
Adjoint Trapping: A New Phenomenon at Strong 't Hooft Coupling
Adding matter of mass m, in the fundamental representation of SU(N), to N=4
supersymmetric Yang-Mills theory, we study ``generalized quarkonium''
containing a (s)quark, an anti(s)quark, and J massless (or very light) adjoint
particles. At large 't Hooft coupling >> 1, the states of spin <= 1
are surprisingly light (Kruczenski et al., hep-th/0304032) and small
(hep-th/0312071) with a J-independent size of order . This
``trapping'' of adjoint matter in a region small compared with its Compton
wavelength and compared to any confinement scale in the theory is an unfamiliar
phenomenon, as it does not occur at small . We explore adjoint
trapping further by considering the limit of large J. In particular, for J >>
>> 1, we expect the trapping phenomenon to become unstable.
Using Wilson loop methods, we show that a sharp transition, in which the
generalized quarkonium states become unbound (for massless adjoints) occurs at
. If the adjoint scalars of N=4 are massive and
the theory is confining (as, for instance, in N=1* theories) then the
transition becomes a cross-over, across which the size of the states changes
rapidly from ~ to something of order the confinement scale ~
.Comment: Clarified transition with a better figure and improved presentation;
added careful discussion of the small regime of validity of the
Born-Oppenheimer computation and adjusted some remarks appropriately; also
added two reference
Cosmic-Ray Proton and Helium Spectra from the First CREAM Flight
Cosmic-ray proton and helium spectra have been measured with the
balloon-borne Cosmic Ray Energetics And Mass experiment flown for 42 days in
Antarctica in the 2004-2005 austral summer season. High-energy cosmic-ray data
were collected at an average altitude of ~38.5 km with an average atmospheric
overburden of ~3.9 g cm. Individual elements are clearly separated with
a charge resolution of ~0.15 e (in charge units) and ~0.2 e for protons and
helium nuclei, respectively. The measured spectra at the top of the atmosphere
are represented by power laws with a spectral index of -2.66 0.02 for
protons from 2.5 TeV to 250 TeV and -2.58 0.02 for helium nuclei from 630
GeV/nucleon to 63 TeV/nucleon. They are harder than previous measurements at a
few tens of GeV/nucleon. The helium flux is higher than that expected from the
extrapolation of the power law fitted to the lower-energy data. The relative
abundance of protons to helium nuclei is 9.1 0.5 for the range from 2.5
TeV/nucleon to 63 TeV/nucleon. This ratio is considerably smaller than the
previous measurements at a few tens of GeV/nucleon.Comment: 20 pages, 4 figure
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