757 research outputs found
Spectroscopy of SMC Wolf-Rayet Stars Suggests that Wind-Clumping does not Depend on Ambient Metallicity
The mass-loss rates of hot, massive, luminous stars are considered a decisive
parameter in shaping the evolutionary tracks of such stars and influencing the
interstellar medium on galactic scales. The small-scale structures (clumps)
omnipresent in such winds may reduce empirical estimates of mass-loss rates by
an evolutionarily significant factor of >=3. So far, there has been no direct
observational evidence that wind-clumping may persist at the same level in
environments with a low ambient metallicity, where the wind-driving opacity is
reduced. Here we report the results of time-resolved spectroscopy of three
presumably single Population I Wolf-Rayet stars in the Small Magellanic Cloud,
where the ambient metallicity is ~1/5 Z_Sun.We detect numerous small-scale
emission peaks moving outwards in the accelerating parts of the stellar
winds.The general properties of the moving features, such as their velocity
dispersions,emissivities and average accelerations, closely match the
corresponding characteristics of small-scale inhomogeneities in the winds of
Galactic Wolf-Rayet stars.Comment: 9 pages, 3 figures; accepted by ApJ Letter
Unified continuum approach to crystal surface morphological relaxation
A continuum theory is used to predict scaling laws for the morphological
relaxation of crystal surfaces in two independent space dimensions. The goal is
to unify previously disconnected experimental observations of decaying surface
profiles. The continuum description is derived from the motion of interacting
atomic steps. For isotropic diffusion of adatoms across each terrace, induced
adatom fluxes transverse and parallel to step edges obey different laws,
yielding a tensor mobility for the continuum surface flux. The partial
differential equation (PDE) for the height profile expresses an interplay of
step energetics and kinetics, and aspect ratio of surface topography that
plausibly unifies observations of decaying bidirectional surface corrugations.
The PDE reduces to known evolution equations for axisymmetric mounds and
one-dimensional periodic corrugations.Comment: 5 pages, 1 figur
Structural compliance, misfit strain and stripe nanostructures in cuprate superconductors
Structural compliance is the ability of a crystal structure to accommodate
variations in local atomic bond-lengths without incurring large strain
energies. We show that the structural compliance of cuprates is relatively
small, so that short, highly doped, Cu-O-Cu bonds in stripes are subject to a
tensile misfit strain. We develop a model to describe the effect of misfit
strain on charge ordering in the copper oxygen planes of oxide materials and
illustrate some of the low energy stripe nanostructures that can result.Comment: 4 pages 5 figure
The alpha-dependence of transition frequencies for some ions of Ti, Mn, Na, C, and O, and the search for variation of the fine structure constant
We use the relativistic Hartree-Fock method, many-body perturbation theory
and configuration-interaction method to calculate the dependence of atomic
transition frequencies on the fine structure constant, alpha. The results of
these calculations will be used in the search for variation of the fine
structure constant in quasar absorption spectra.Comment: 4 pages, 5 table
Absence of the Transition into Abrikosov Vortex State of Two-Dimensional Type-II Superconductor with Weak Pinning
The resistive properties of thin amorphous NbO_{x} films with weak pinning
were investigated experimentally above and below the second critical field
H_{c2}. As opposed to bulk type II superconductors with weak pinning where a
sharp change of resistive properties at the transition into the Abrikosov state
is observed at H_{c4}, some percent below H_{c2} (V.A.Marchenko and
A.V.Nikulov, 1981), no qualitative change of resistive properties is observed
down to a very low magnetic field, H_{c4} < 0.006 H_{c2}, in thin films with
weak pinning. The smooth dependencies of the resistivity observed in these
films can be described by paraconductivity theory both above and below H_{c2}.
This means that the fluctuation superconducting state without phase coherence
remains appreciably below H_{c2} in the two-dimensional superconductor with
weak pinning. The difference the H_{c4}/H_{c2} values, i.e. position of the
transition into the Abrikosov state, in three- and two-dimensional
superconductors conforms to the Maki-Takayama result 1971 year according to
which the Abrikosov solution 1957 year is valid only for a superconductor with
finite dimensions. Because of the fluctuation this solution obtained in the
mean field approximation is not valid in a relatively narrow region below
H_{c2} for bulk superconductors with real dimensions and much below H_{c2} for
thin films with real dimensions. The superconducting state without phase
coherence should not be identified with the mythical vortex liquid because the
vortex, as a singularity in superconducting state with phase coherence, can not
exist without phase coherence.Comment: 4 pages, 4 figure
Qualitative features of periodic solutions of KdV
In this paper we prove new qualitative features of solutions of KdV on the
circle. The first result says that the Fourier coefficients of a solution of
KdV in Sobolev space , admit a WKB type expansion up to first
order with strongly oscillating phase factors defined in terms of the KdV
frequencies. The second result provides estimates for the approximation of such
a solution by trigonometric polynomials of sufficiently large degree
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