7,023 research outputs found
FILOU oscillation code
The present paper provides a description of the oscillation code FILOU, its
main features, type of applications it can be used for, and some representative
solutions. The code is actively involved in CoRoT/ESTA exercises (this volume)
for the preparation for the proper interpretation of space data from the CoRoT
mission. Although CoRoT/ESTA exercises have been limited to the oscillations
computations for non-rotating models, the main characteristic of FILOU is,
however, the computation of radial and non-radial oscillation frequencies in
presence of rotation. In particular, FILOU calculates (in a perturbative
approach) adiabatic oscillation frequencies corrected for the effects of
rotation (up to the second order in the rotation rate) including near
degeneracy effects. Furthermore, FILOU works with either a uniform rotation or
a radial differential rotation profile (shellular rotation), feature which
makes the code singular in the field.Comment: 6 pages, 5 figures. Astrophysics and Space Science (in press
An updated catalog of OH-maser-emitting planetary nebulae
Aims. We studied the characteristics of planetary nebulae (PNe) that show
both OH maser and radio continuum emission (hereafter OHPNe). These have been
proposed to be very young PNe, and therefore, they could be key objects for
understanding the formation and evolution of PNe. Methods. We consulted the
literature searching for interferometric observations of radio continuum and OH
masers toward evolved stars, including the information from several surveys. We
also processed radio continuum and OH maser observations toward PNe in the Very
Large Array data archive. The high positional accuracy provided by
interferometric observations allow us to confirm or reject the association
between OH maser and radio continuum emission. Results. We found a total of six
PNe that present both OH maser and radio continuum emissions, as confirmed with
radio interferometric observations. These are bona fide OHPNe. The confirmed
OHPNe present a bipolar morphology in resolved images of their ionized emission
at different wavelengths, suggesting that the OH maser emission in PNe is
related to nonspherical mass-loss phenomena. The OH maser spectra in PNe
present a clear asymmetry, tending to show blueshifted emission with respect to
the systemic velocity. Their infrared colors suggest that most of these objects
are very young PNe. OHPNe do not form a homogeneous group, and seem to
represent a variety of different evolutionary stages. We suggest that OH masers
pumped in the AGB phase may disappear during the post-AGB phase, but reappear
once the source becomes a PN and its radio continuum emission is amplified by
the OH molecules. Therefore, OH maser emission could last significantly longer
than the previously assumed 1000 yr after the end of the AGB phase. This maser
lifetime may be longer in PNe with more massive central stars, which ionize a
larger amount of gas in the envelope.Comment: 16 pages, 5 figures, 4 tables. Accepted for publication by Astronomy
& Astrophysic
New detections of H2O masers in planetary nebulae and post-AGB stars using the Robledo-70m antenna
Aims: We investigated the possible relationship between the evolutionary
stage of post-AGB stars and planetary nebulae (PNe) and the presence of water
masers in their envelopes.
Methods: We have used NASA's 70-m antenna in Robledo de Chavela (Spain) to
search for the water maser transition at 22235.08 MHz, towards a sample of 105
sources with IRAS colour characteristic of post-AGB stars and PNe at
declination >-32 deg. 83% of the sources in the sample are post-AGB stars, 15%
PNe or PN candidates, while only 2% seem to be HII regions.
Results: We have detected five water masers, of which four are reported for
the first time: two in PNe (IRAS 17443-2949 and IRAS 18061-2505), a ``water
fountain'' in a post-AGB star (IRAS 16552-3050), and one in a source previously
catalogued as a PN, but whose classification is uncertain (IRAS 17580-3111).
Conclusions: The unexpected detections of water masers in two objects among
the small subset of PNe led us to suggest that the PNe harbouring water masers
are a special type of massive, rapidly evolving PNe.Comment: 15 pages, 7 figures. Accepted for publication in Astronomy and
Astrophysic
Radiation effects on the electronic structure of bilayer graphene
We report on the effects of laser illumination on the electronic properties
of bilayer graphene. By using Floquet theory combined with Green's functions we
unveil the appeareance of laser-induced gaps not only at integer multiples of
but also at the Dirac point with features which are shown to
depend strongly on the laser polarization. Trigonal warping corrections are
shown to lead to important corrections for radiation in the THz range, reducing
the size of the dynamical gaps. Furthermore, our analysis of the topological
properties at low energies reveals that when irradiated with linearly polarized
light, ideal bilayer graphene behaves as a trivial insulator, whereas circular
polarization leads to a non-trivial insulator per valley.Comment: 5 pages 3 figure
Computational analysis of projectile impact resistance on aluminium (a356) curvilinear surface reinforced with carbon nanotubes (cnts) for applications in systems of protection
Computational tests for ballistic impact energy absorption were developed on A356/CNTs composite material with the goal of estimating the improvement of the material’s mechanical properties by the contribution of the CNTs [1]. For the implementation of computational tests on the material exposed to projectile impact, A356/CNTs was configured by means of generalized Hooke’s model for anisotropic materials [1] and Johnson-Cook’s model was used to determine material failure and propagation of energy [2]. A curvilinear surface (semi-spheres on a plaque) with an area of 23x23 cm and thickness of 12 mm was elaborated to represent the composite material. The impact on surface was done with a 9 mm projectile and the surface was developed with 4.5 mm radium semi-spheres. It was used a 0.3% of nanotube insertions on the composite total volume. The results indicated the plaque stopped the impact without drilling. Incidence of damage to wearer, as well as possibility of composite material improvement and the diffusion/dispersion analysis on the curvilinear surface was also done
Crafting zero-bias one-way transport of charge and spin
We explore the electronic structure and transport properties of a metal on
top of a (weakly coupled) two-dimensional topological insulator. Unlike the
widely studied junctions between topological non-trivial materials, the systems
studied here allow for a unique bandstructure and transport steering. First,
states on the topological insulator layer may coexist with the gapless bulk
and, second, the edge states on one edge can be selectively switched-off,
thereby leading to nearly perfect directional transport of charge and spin even
in the zero bias limit. We illustrate these phenomena for Bernal stacked
bilayer graphene with Haldane or intrinsic spin-orbit terms and a perpendicular
bias voltage. This opens a path for realizing directed transport in materials
such as van der Waals heterostructures, monolayer and ultrathin topological
insulators.Comment: 7 pages, 7 figure
On Fabry P\'erot Etalon based Instruments. I. The Isotropic Case
Here we assess the spectral and imaging properties of Fabry P\'erot etalons
when located in solar magnetographs. We discuss the chosen configuration
(collimated or telecentric) for both ideal and real cases. For the real cases,
we focus on the effects caused by the polychromatic illumination of the filter
by the irregularities in the optical thickness of the etalon and by deviations
from the ideal illumination in both setups. We first review the general
properties of Fabry P\'erots and we then address the different sources of
degradation of the spectral transmission profile. We review and extend the
general treatment of defects followed by different authors. We discuss the
differences between the point spread functions (PSFs) of the collimated and
telecentric configurations for both monochromatic and (real)
quasi-monochromatic illumination of the etalon. The PSF corresponding to
collimated mounts is shown to have a better performance, although it varies
from point to point due to an apodization of the image inherent to this
configuration. This is in contrast to the (perfect) telecentric case, where the
PSF remains constant but produces artificial velocities and magnetic field
signals because of its strong spectral dependence. We find that the unavoidable
presence of imperfections in the telecentrism produces a decrease of flux of
photons and a shift, a broadening and a loss of symmetrization of both the
spectral and PSF profiles over the field of view, thus compromising their
advantages over the collimated configuration. We evaluate these effects for
different apertures of the incident beam.Comment: 20 pages 22 figures 2 Appendice
- …