949 research outputs found
Dynamo models and differential rotation in late-type rapidly rotating stars
Increasing evidence is becoming available about not only the surface
differential rotation of rapidly rotating cool stars but, in a small number of
cases, also about temporal variations, which possibly are analogous to the
solar torsional oscillations. Given the present difficulties in resolving the
precise nature of such variations, due to both the short length and poor
resolution of the available data, theoretical input is vital to help assess the
modes of behaviour that might be expected, and will facilitate interpretation
of the observations. Here we take a first step in this direction by studying
the variations in the convection zones of such stars, using a two dimensional
axisymmetric mean field dynamo model operating in a spherical shell in which
the only nonlinearity is the action of the azimuthal component of the Lorentz
force of the dynamo generated magnetic field on the stellar angular velocity.
We consider three families of models with different depths of dynamo-active
regions. For moderately supercritical dynamo numbers we find torsional
oscillations that penetrate all the way down to the bottom of the convection
zones, similar to the case of the Sun. For larger dynamo numbers we find
fragmentation in some cases and sometimes there are other dynamical modes of
behaviour, including quasi-periodicity and chaos. We find that the largest
deviations in the angular velocity distribution caused by the Lorentz force are
of the order of few percent, implying that the original assumed `background'
rotation field is not strongly distorted.Comment: Astronomy and Astrophysics, in pres
Mean Field Dynamos with Algebraic and Dynamic alpha-Quenchings
Calculations for mean field dynamo models (in both full spheres and spherical
shells), with both algebraic and dynamic --quenchings, show qualitative
as well as quantitative differences and similarities in the dynamical behaviour
of these models. We summarise and enhance recent results with extra examples.
Overall, the effect of using a dynamic appears to be complicated and
is affected by the region of parameter space examined.Comment: 6 pages, 2 postscript figures, also available at
http://www.maths.qmw.ac.uk/~eo
Dynamical variations of the differential rotation in the solar convection zone
Recent analyses of helioseismological observations seem to suggest the
presence of two new phenomena connected with the dynamics of the solar
convective zone. Firstly, there are present torsional oscillations with periods
of about 11 years, which penetrate significantly into the solar convection zone
and secondly, oscillatory regimes exist near the base of the convection which
are markedly different from those observed near the top, having either
significantly reduced periods or being non-periodic.
Recently spatiotemporal fragmentation/bifurcation has been proposed as a
possible dynamical mechanism to account for such observed multi-mode behaviours
in different parts of the solar convection zone. Evidence for this scenario was
produced in the context of an axisymmetric mean field dynamo model operating in
a spherical shell, with a semi-open outer boundary condition and a zero order
angular velocity obtained by the inversion of the MDI data, in which the only
nonlinearity was the action of the Lorentz force of the dynamo generated
magnetic field on the solar angular velocity.
Here we make a detailed study of the robustness of this model with respect to
plausible changes to its main ingredients, including changes to the alpha and
eta profiles as well as the inclusion of a nonlinear alpha quenching. We find
that spatiotemporal fragmentation is present in this model for different
choices of the rotation data and as the details of the model are varied. Taken
together, these results give strong support to the idea that spatiotemporal
fragmentation is likely to occur in general dynamo settings.Comment: 14 pages, 30 figures, submitted to Astronomy and Astrophysics, also
available at http://www.eurico.web.co
Improved all-sky search method for continuous gravitational waves from unknown neutron stars in binary systems
Continuous gravitational waves from spinning deformed neutron stars have not been detected yet, and are one of the most promising signals for future detection. All-sky searches for continuous gravitational waves from unknown neutron stars in binary systems are the most computationally challenging search type. Consequently, very few search algorithms and implementations exist for these sources, and only a handful of such searches have been performed so far. In this paper, we present a new all-sky binary search method, BinarySkyHou, which extends and improves upon the earlier BinarySkyHough method, and which was the basis for a recent search (Covas et al. [1]). We compare the sensitivity and computational cost to previous methods, showing that it is both more sensitive and computationally efficient, which allows for broader and more sensitive searches. <br
Improved short-segment detection statistic for continuous gravitational waves
Continuous gravitational waves represent one of the long-sought types of signals that have yet to be detected. Due to their small amplitude, long observational datasets (months-years) have to be analyzed together, thereby vastly increasing the computational cost of these searches. All-sky searches face the most severe computational obstacles, especially searches for sources in unknown binary systems, which need to break the data into very short segments in order to be computationally feasible. In this paper, we present a new detection statistic that improves sensitivity by up to 19% compared to the standard -statistic for segments shorter than a few hours
Constraints on r-modes and Mountains on Millisecond Neutron Stars in Binary Systems
Continuous gravitational waves are nearly monochromatic signals emitted by asymmetries in rotating neutron stars. These signals have not yet been detected. Deep all-sky searches for continuous gravitational waves from isolated neutron stars require significant computational expense. Deep searches for neutron stars in binary systems are even more expensive, but these targets are potentially more promising emitters, especially in the hundreds of Hertz region, where ground-based gravitational-wave detectors are most sensitive. We present here an all-sky search for continuous signals with frequency between 300 and 500 Hz, from neutron stars in binary systems with orbital periods between 15 and 60 days and projected semimajor axes between 10 and 40 lt-s. This is the only binary search on Advanced LIGO data that probes this frequency range. Compared to previous results, our search is over an order of magnitude more sensitive. We do not detect any signals, but our results exclude plausible and unexplored neutron star configurations, for example, neutron stars with relative deformations greater than 3 × 10-6 within 1 kpc from Earth and r-mode emission at the level of α ∼a few 10-4 within the same distance. © 2022. The Author(s). Published by the American Astronomical Society
A comparative study of the dispersion of carbon nanofibres in polymer melts
The dispersion of carbon nanofibres (CNF) in a polymer matrix using two melt mixing methods is studied. Distributive and dispersive mixing were evaluated by optical and electron microscopy. The CNF were chemically modified to improve the interface with the matrix. The results showed that the two methods produced good distribution of the filler, but extensional stresses induced higher dispersion. The latter correlated well with a decrease in electrical resistivity. Also, the chemical modification largely improved the CNF/polymer interfaceFundação para a Ciência e a Tecnologia (FCT
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