3,238 research outputs found
Spontaneous rotational inversion in Phycomyces
The filamentary fungus Phycomyces blakesleeanus undergoes a series of remarkable transitions during aerial growth. During what is known as the Stage IV growth phase, the fungus extends while rotating in a counterclockwise manner when viewed from above (Stage IVa) and then, while continuing to grow, spontaneously reverses to a clockwise rotation (Stage IVb). This phase lasts for 24 - 48 hours and is sometimes followed by yet another reversal (Stage IVc) before the overall growth ends. Here, we propose a continuum mechanical model of this entire process using nonlinear, anisotropic, elasticity and show how helical anisotropy associated with the cell wall structure can induce spontaneous rotation and, under appropriate circumstances, the observed reversal of rotational handedness
On the mechanical stability of growing arteries
Arteries are modeled, within the framework of nonlinear elasticity, as incompressible two-layer cylindrical structures that are residually stressed through differential growth. These structures are loaded by an axial force, internal pressure and have nonlinear, anisotropic, hyperelastic response to stresses. Parameters for this model are directly related to experimental observations. The possible role of axial residual stress in regulating stress in arteries and preventing buckling instabilities is investigated. It is shown that axial residual stress lowers the critical internal pressure leading to buckling and that a reduction of axial loading may lead to a buckling instability which may eventually lead to arterial tortuosity
Large-scale QRPA calculation of E1-strength and its impact on the neutron capture cross section
Large-scale QRPA calculations of the E1-strength are performed as a first
attempt to microscopically derive the radiative neutron capture cross sections
for the whole nuclear chart. A folding procedure is applied to the QRPA
strength distribution to take the damping of the collective motion into
account. It is shown that the resulting E1-strength function based on the SLy4
Skyrme force is in close agreement with photoabsorption data as well as the
available experimental E1-strength at low energies. The increase of the
E1-strength at low energies for neutron-rich nuclei is qualitatively analyzed
and shown to affect the corresponding radiative neutron capture cross section
significantly. A complete set of E1-strength function is made available for
practical applications in a table format for all 7<Z<111 nuclei lying between
the proton and the neutron drip lines.Comment: 17 pages, 8 figures, accepted for publication in Nucl. Phys.
Self-diffusion in remodelling and growth
Self-diffusion, or the flux of mass of a single species within itself, is viewed as an independent phenomenon amenable to treatment by the introduction of an auxiliary field of diffusion velocities. The theory is shown to be heuristically derivable as a limiting case of that of an ordinary binary mixture
Riemann-Cartan Geometry of nonlinear dislocation mechanics
We present a geometric theory of nonlinear solids with distributed dislocations. In this theory the material manifold – where the body is stress free – is a Weitzenbock manifold, i.e. a manifold with a flat affine connection with torsion but vanishing non-metricity. Torsion of the material manifold is identified with the dislocation density tensor of nonlinear dislocation mechanics. Using Cartan’s moving frames we construct the material manifold for several examples of bodies with distributed dislocations. We also present non-trivial examples of zero-stress dislocation distributions. More importantly, in this geometric framework we are able to calculate the residual stress fields assuming that the nonlinear elastic body is incompressible. We derive the governing equations of nonlinear dislocation mechanics covariantly using balance of energy and its covariance
Nucleosynthesis of s-elements in zero-metal AGB stars
Contrary to previous expectations, recent evolutionary models of
zero-metallicity stars show that the development of mixing episodes at the
beginning of the AGB phase allows low- and intermediate-mass stars to
experience thermal pulses. If these stars, like their metal-rich counterparts,
also experience partial mixing of protons from the H-rich envelope into the
C-rich layers at the time of the third dredge-up, an extensive neutron capture
nucleosynthesis leads to the production of s-process nuclei up to Pb and Bi.
Nucleosynthesis calculations based on stellar AGB models are performed assuming
a parameterized H-abundance profile below the convective envelope at the time
of the third dredge-up. Despite the absence of Fe-group elements, the large
neutron flux resulting from the 13C(alpha,n)16O reaction leads to an efficient
production of s-process elements starting from the neutron captures on the C-Ne
isotopes. Provided partial mixing of protons takes place, it is shown that
population III AGB stars should be enriched in s-process elementsand overall in
Pb and Bi.Comment: 4 pages, 3 Postscript figures, uses aa.sty. Accepted for publication
in A&A Letter
Rotation, inversion, and perversion in anisotropic elastic cylindrical tubes and membranes
Cylindrical tubes and membranes are universal structural elements found in biology and engineering over a wide range of scales. Working in the framework of nonlinear elasticity we consider the possible deformations of elastic cylindrical shells reinforced by one or two families of anisotropic fibers. We consider both small and large deformations and the reduction from thick cylindrical shells (tubes) to thin shells (cylindrical membranes). In particular, a number of universal regimes can be identified including the possibility of inversion and perversion of rotation
Hope and Inquietudes in Nucleo-cosmochronology
Critical views are presented on some nucleo-cosmochronological questions.
Progress has been made recently in the development of the 187Re-187Os
cosmochronometry. From this, there is good hope for this clock to become of the
highest quality for the nuclear dating of the Universe. The simultaneous
observation of Th and U in ultra-metal-poor stars would also be a most
interesting prospect. In contrast, a serious inquietude is expressed about the
reliability of the chronometric attempts based on the classical 232Th-238U and
235U-238U pairs, as well as on the Th (without U) abundance determinations in
ultra-metal poor stars.Comment: 9 pages, no figures; ASP Conference Series: "Astrophysical Ages and
Time Scales
Large-scale Continuum Random Phase Approximation predictions of dipole strength for astrophysical applications
Large-scale calculations of the E1 strength are performed within the random
phase approximation (RPA) based on the relativistic point-coupling mean field
approach in order to derive the radiative neutron capture cross sections for
all nuclei of astrophysical interest. While the coupling to the single-particle
continuum is taken into account in an explicit and self-consistent way,
additional corrections like the coupling to complex configurations and the
temperature and deformation effects are included in a phenomenological way to
account for a complete description of the nuclear dynamical problem. It is
shown that the resulting E1-strength function based on the PCF1 force is in
close agreement with photoabsorption data as well as the available experimental
E1 strength data at low energies. For neutron-rich nuclei, as well as light
neutron-deficient nuclei, a low-lying so-called pygmy resonance is found
systematically in the 5-10 MeV region. The corresponding strength can reach 10%
of the giant dipole strength in the neutron-rich region and about 5% in the
neutron-deficient region, and is found to be reduced in the vicinity of the
shell closures. Finally, the neutron capture reaction rates of neutron-rich
nuclei is found to be about 2-5 times larger than those predicted on the basis
of the nonrelativistic RPA calculation and about a factor 50 larger than
obtained with traditional Lorentzian-type approaches.Comment: 11 pages, 12 figure
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