26,467 research outputs found
Exoplanets: Gaia and the importance of ground based spectroscopy follow-up
The search for extrasolar planets has developed rapidly and, today, more than
1700 planets have been found orbiting stars. Thanks to Gaia, we will collect
high-accuracy astrometric orbits of thousands of new low-mass celestial
objects, such as extra-solar planets and brown dwarfs. These measurements in
combination with spectroscopy and with present day and future extrasolar planet
search programs (like HARPS, ESPRESSO) will have a crucial contribution to
several aspects of planetary astrophysics (formation theories, dynamical
evolution, etc.). Moreover, Gaia will have a strong contribution on the stellar
chemical and kinematic characterisation studies. In this paper we present a
short overview of the importance of Gaia in the context of exoplanet research.
As preparatory work for Gaia, we will then present a study where we derived
stellar parameters for a sample of field giant stars
Internamentos Compulsivos: revisão da casuÃstica dos HUC e HIP no triénio 2006-2008.
ão obstante terem passado mais de 10 anos sobre a publicação da Lei de Saúde Mental, o intenso debate
e reflexão ocorridos não foram acompanhados por publicação, igualmente profusa, de investigação clÃnica
sobre a epidemiologia dos internamentos compulsivos. Os autores procuraram caracterizar o arquétipo do
doente submetido a internamento involuntário em Portugal, apreciando variáveis demográficas, clÃnicas e legais.
Para esse fim, conduziu-se um estudo retrospectivo dos internamentos compulsivos ocorridos entre 2006
e 2008 em dois hospitais: os Hospitais da Universidade de Coimbra (HUC) e o Hospital Infante D. Pedro –
Aveiro (HIP). Uma amostra de 384 internamentos foi recolhida e caracterizada, apresentando-se resultados
desta investigação e discussão dos dados obtidos
Searching for solar siblings among the HARPS data
The search for the solar siblings has been particularly fruitful in the last
few years. Until now, there are four plausible candidates pointed out in the
literature: HIP21158, HIP87382, HIP47399, and HIP92831. In this study we
conduct a search for solar siblings among the HARPS high-resolution FGK dwarfs
sample, which includes precise chemical abundances and kinematics for 1111
stars. Using a new approach based on chemical abundance trends with the
condensation temperature, kinematics, and ages we found one (additional)
potential solar sibling candidate: HIP97507.Comment: 4 pages, 2 figures, 1 table. Accepted in A&
Nonlinear viscosity and velocity distribution function in a simple longitudinal flow
A compressible flow characterized by a velocity field is
analyzed by means of the Boltzmann equation and the Bhatnagar-Gross-Krook
kinetic model. The sign of the control parameter (the longitudinal deformation
rate ) distinguishes between an expansion () and a condensation ()
phenomenon. The temperature is a decreasing function of time in the former
case, while it is an increasing function in the latter. The non-Newtonian
behavior of the gas is described by a dimensionless nonlinear viscosity
, that depends on the dimensionless longitudinal rate . The
Chapman-Enskog expansion of in powers of is seen to be only
asymptotic (except in the case of Maxwell molecules). The velocity distribution
function is also studied. At any value of , it exhibits an algebraic
high-velocity tail that is responsible for the divergence of velocity moments.
For sufficiently negative , moments of degree four and higher may diverge,
while for positive the divergence occurs in moments of degree equal to or
larger than eight.Comment: 18 pages (Revtex), including 5 figures (eps). Analysis of the heat
flux plus other minor changes added. Revised version accepted for publication
in PR
Noncommutative fluid dynamics in the K\"{a}hler parametrization
In this paper, we propose a first order action functional for a large class
of systems that generalize the relativistic perfect fluids in the K\"{a}hler
parametrization to noncommutative spacetimes. We calculate the equations of
motion for the fluid potentials and the energy-momentum tensor in the first
order in the noncommutative parameter. The density current does not receive any
noncommutative corrections and it is conserved under the action of the
commutative generators but the energy-momentum tensor is not.
Therefore, we determine the set of constraints under which the energy-momentum
tensor is divergenceless. Another set of constraints on the fluid potentials is
obtained from the requirement of the invariance of the action under the
generalization of the volume preserving transformations of the noncommutative
spacetime. We show that the proposed action describes noncommutative fluid
models by casting the energy-momentum tensor in the familiar fluid form and
identifying the corresponding energy and momentum densities. In the commutative
limit, they are identical to the corresponding quantities of the relativistic
perfect fluids. The energy-momentum tensor contains a dissipative term that is
due to the noncommutative spacetime and vanishes in the commutative limit.
Finally, we particularize the theory to the case when the complex fluid
potentials are characterized by a function that is a deformation
of the complex plane and show that this model has important common features
with the commutative fluid such as infinitely many conserved currents and a
conserved axial current that in the commutative case is associated to the
topologically conserved linking number.Comment: References fixed. Typos corrected. 12 page
Aging to non-Newtonian hydrodynamics in a granular gas
The evolution to the steady state of a granular gas subject to simple shear
flow is analyzed by means of computer simulations. It is found that, regardless
of its initial preparation, the system reaches (after a transient period
lasting a few collisions per particle) a non-Newtonian (unsteady) hydrodynamic
regime, even at strong dissipation and for states where the time scale
associated with inelastic cooling is shorter than the one associated with the
irreversible fluxes. Comparison with a simplified rheological model shows a
good agreement.Comment: 6 pages, 4 figures; v2: improved version to be published in EP
Expansion-Free Cavity Evolution: Some exact Analytical Models
We consider spherically symmetric distributions of anisotropic fluids with a
central vacuum cavity, evolving under the condition of vanishing expansion
scalar. Some analytical solutions are found satisfying Darmois junction
conditions on both delimiting boundary surfaces, while some others require the
presence of thin shells on either (or both) boundary surfaces. The solutions
here obtained model the evolution of the vacuum cavity and the surrounding
fluid distribution, emerging after a central explosion. This study complements
a previously published work where modeling of the evolution of such kind of
systems was achieved through a different kinematical condition.Comment: 9 pages, Revtex. Typos corrected. Published in Int. J. Mod. Phys.
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