3,638 research outputs found
Cosmological perturbations in coherent oscillating scalar field models
The fact that fast oscillating homogeneous scalar fields behave as perfect
fluids in average and their intrinsic isotropy have made these models very
fruitful in cosmology. In this work we will analyse the perturbations dynamics
in these theories assuming general power law potentials . At leading order in the wavenumber expansion, a simple
expression for the effective sound speed of perturbations is obtained
with the effective equation of
state. We also obtain the first order correction in
, when the wavenumber of the perturbations is
much smaller than the background oscillation frequency, .
For the standard massive case we have also analysed general anharmonic
contributions to the effective sound speed. These results are reached through a
perturbed version of the generalized virial theorem and also studying the exact
system both in the super-Hubble limit, deriving the natural ansatz for
; and for sub-Hubble modes, exploiting Floquet's theorem.Comment: 13 pages, 6 figures. Published on JHE
Isotropy theorem for cosmological Yang-Mills theories
We consider homogeneous non-abelian vector fields with general potential
terms in an expanding universe. We find a mechanical analogy with a system of N
interacting particles (with N the dimension of the gauge group) moving in three
dimensions under the action of a central potential. In the case of bounded and
rapid evolution compared to the rate of expansion, we show by making use of a
generalization of the virial theorem that for arbitrary potential and
polarization pattern, the average energy-momentum tensor is always diagonal and
isotropic despite the intrinsic anisotropic evolution of the vector field. We
consider also the case in which a gauge-fixing term is introduced in the action
and show that the average equation of state does not depend on such a term.
Finally, we extend the results to arbitrary background geometries and show that
the average energy-momentum tensor of a rapidly evolving Yang-Mills fields is
always isotropic and has the perfect fluid form for any locally inertial
observer.Comment: 8 pages, 3 figure
Isotropy theorem for arbitrary-spin cosmological fields
We show that the energy-momentum tensor of homogeneous fields of arbitrary
spin in an expanding universe is always isotropic in average provided the
fields remain bounded and evolve rapidly compared to the rate of expansion. An
analytic expression for the average equation of state is obtained for
Lagrangians with generic power-law kinetic and potential terms. As an example
we consider the behavior of a spin-two field in the standard Fierz-Pauli theory
of massive gravity. The results can be extended to general space-time
geometries for locally inertial observers.Comment: 4 pages, 0 figures. Published on JCA
New membership determination and proper motions of NGC 1817. Parametric and non-parametric approach
We have calculated proper motions and re-evaluated the membership
probabilities of 810 stars in the area of two NGC objects, NGC 1817 and NGC
1807. We have obtained absolute proper motions from 25 plates in the reference
system of the Tycho-2 Catalogue. The plates have a maximum epoch difference of
81 years; and they were taken with the double astrograph at Zo-Se station of
Shanghai Observatory, which has an aperture of 40 cm and a plate scale of 30
arcsec/mm. The average proper motion precision is 1.55 mas/yr. These proper
motions are used to determine the membership probabilities of stars in the
region, based on there being only one very extended physical cluster: NGC 1817.
With that aim, we have applied and compared parametric and non-parametric
approaches to cluster/field segregation. We have obtained a list of 169
probable member stars.Comment: 11 pages, 8 figures, A&A in pres
An XMM-Newton view of FeK{\alpha} in HMXBs
We present a comprehensive analysis of the whole sample of available
XMM-Newton observations of High Mass X-ray Binaries (HMXBs) until August, 2013,
focusing on the FeK{\alpha} emission line. This line is a key tool to better
understand the physical properties of the material surrounding the X-ray source
within a few stellar radii (the circumstellar medium). We have collected
observations from 46 HMXBs, detecting FeK{\alpha} in 21 of them. We have used
the standard classification of HMXBs to divide the sample in different groups.
We find that: (1) FeK{\alpha} is centred at a mean value of 6.42 keV.
Considering the instrumental and fits uncertainties, this value is compatible
with ionization states lower than FeXVIII. (2) The flux of the continuum is
well correlated with the flux of the line, as expected. Eclipse observations
show that the Fe fluorescence emission comes from an extended region
surrounding the X-ray source. (3) FeK{\alpha} is narrow (width lower than
0.15keV), reflecting that the reprocessing material does not move at high
speeds. We attempt to explain the broadness of the line in terms of three
possible broadening phenomena: line blending, Compton scattering and Doppler
shifts (with velocities of the reprocessing material V=1000-2000 km/s). (4) The
equivalent hydrogen column (NH) directly correlates with the EW of FeK{\alpha},
displaying clear similarities to numerical simulations. It highlights the
strong link between the absorbing and the fluorescent matter. The obtained
results clearly point to a very important contribution of the donors wind in
the FeK{\alpha} emission and the absorption when the donor is a supergiant
massive star.Comment: Accepted for publication in A&A. 13 pages, 16 figures + Appendice
Effective t-J Hamiltonian for the Copper Oxides
Starting from the Emery model, which is assumed to describe the copper oxygen
planes, and including direct oxygen hopping matrix elements, we have been able
to derive the effective t-J Hamiltonian for the copper orbitals using the
Linked Cluster Expansion Method up to fourth order in the hybridization matrix
element.Comment: (ps version of the dvi file, resubmitted because previous
uucompressed version was corrupted), 9 page
Primeres observacions de la papallona tigre (Danaus chrysippus L.) a les Illes Balears
Abstract not availabl
Pressure effects in the triangular layered cobaltites NaxCoO2
We have measured transport properties as a function of temperature and
pressure up to 30GPa in the NaxCoO2 system. For the x=0.5 sample the transition
temperature at 53K increases with pressure, while paradoxically the sample
passes from an insulating to a metallic ground state. A similar transition is
observed in the x=0.31 sample under pressure. Compression on the x=0.75 sample
transforms the sample from a metallic to an insulating state. We discuss our
results in terms of interactions between band structure effects and Na+ order.Comment: 18 pages, 5 figure
- âŠ