755 research outputs found
Quasi integral of motion for axisymmetric potentials
We present an estimate of the third integral of motion for axisymmetric
three-dimensional potentials. This estimate is based on a Staeckel
approximation and is explicitly written as a function of the potential. We
tested this scheme for the Besancon Galactic model and two other disc-halo
models and find that orbits of disc stars have an accurately conserved third
quasi integral.
The accuracy ranges from of 0.1% to 1% for heights varying from z = 0~kpc to
z= 6 kpc and Galactocentric radii R from 5 to 15kpc.
We also tested the usefulness of this quasi integral in analytic distribution
functions of disc stellar populations: we show that the distribution function
remains approximately stationary and that it allows to recover the potential
and forces by applying Jeans equations to its moments.Comment: 9 pages, 9 figures, accepted for publication in Astron. and Astrophy
The distribution of nearby stars in phase space mapped by Hipparcos III. Clustering and streaming among A-F type stars
This paper presents the detailed results obtained in the search of density-
velocity inhomogeneities in a volume limited and absolute magnitude limited
sample of A-F type dwarfs within 125 parsecs of the Sun. A 3-D wavelet analysis
is used to extract inhomogeneities, both in the density and velocity
distributions. Having established a real picture of the phase space without
assumption we come back to previously known observational facts regarding
clusters and associations, superclusters. In the 3-D position space, well known
open clusters (Hyades, Coma Berenices and Ursa Major), associations (parts of
the Scorpio-Centaurus association) as well as the Hyades evaporation track are
retrieved. Three new probably loose clusters are identified (Bootes, Pegasus 1
and 2). The sample is relatively well mixed in the position space since less
than 7 per cent of the stars belong to structures with coherent kinematics,
most likely gravitationally bound. In the velocity space, the majority of large
scale velocity structures ( ~ 6.3 ) are Eggen's superclusters
(Pleiades SCl, Hyades SCl and Sirius SCl) with the whole Centaurus association.
A new supercluster-like structure is found with a mean velocity between the Sun
and Sirius SCl velocities. These structures are all characterized by a large
age range which reflects the overall sample age distribution. Moreover, a few
old streams of ~ 2 Gyr are also extracted at this scale with high U components.
We show that all these large velocity dispersion structures represent 46% of
the sample. Smaller scales (\sigma ~ 3.8 and 2.4 ) reveal that
superclusters are always substructured by 2 or more streams which generally
exhibit a coherent age distribution. Percentages of stars in these streams are
38% and 18% respectively.Comment: 25 pages, Latex, 29 figures, 4 tables to be published in A&A
Supplements Serie
Economie politique de la gouvernance de lâUnion europĂ©enne.
Le rapport sâinscrit dans le dĂ©bat approfondissement / Ă©largissement de lâUnion. Il souligne la dimension politique de la construction Ă©conomique europĂ©enne en abordant les thĂšmes du nombre et de la taille des nations, de lâintĂ©gration Ă©conomique, de lâextension des âbonnes pratiquesâ de âgouvernanceâ Ă©conomique. Il passe en revue les principaux instruments de politique Ă©conomique dont les Ătats se sont en tout ou partie dessaisis â le policy mix monnaie -budget, les instruments dâaction destinĂ©s Ă orienter lâoffre globale et le tissu industriel-. Il Ă©voque pour terminer la question du dĂ©coupage des compĂ©tences entre Ătats, RĂ©gions et AutoritĂ©s supranationales. Les responsabilitĂ©s politiques encourues devant les coĂ»ts dâajustement entraĂźnĂ©s par la dynamique Ă©conomique posent le problĂšme de la lĂ©gitimitĂ© comme de lâefficacitĂ© des autoritĂ©s europĂ©ennes. Ces questions passent par une rĂ©flexion sur lâapprofondissement du modĂšle europĂ©en. NĂ©cessitent â elles une pause dans lâagrandissement de lâUE ou lâinvention dâune autre solution ?Gouvernance; construction Ă©conomique europĂ©enne; bilan Ă©conomique; Ă©conomie pure; mobilitĂ© du travail; mobilitĂ© des capitaux; intĂ©gration; efficacitĂ©; lĂ©gitimitĂ© politique;
On high proper motion white dwarfs from photographic surveys
The interpretation of high proper motion white dwarfs detected by Oppenheimer
et al (2001) was the start of a lively controversy. While the discoverers
identify a large fraction of their findings as dark halo members, others
interpret the same sample as essentially made of disc and/or thick disc stars.
We use the comprehensive description of Galactic stellar populations provided
by the "Besancon" model to produce a realistic simulation of Oppenheimer et al.
data, including all observational selections and calibration biases. The
conclusion is unambiguous: Thick disc white dwarfs resulting from ordinary
hypotheses on the local density and kinematics are sufficient to explain the
observed objects, there is no need for halo white dwarfs. This conclusion is
robust to reasonable changes in model ingredients. The main cause of the
misinterpretation seems to be that the velocity distribution of a proper motion
selected star sample is severely biased in favour of high velocities. This has
been neglected in previous analyses. Obviously this does not prove that no such
objects like halo white dwarfs can exist, but Oppenheimer et al. observations
drive their possible contribution in the dark matter halo down to an extremely
low fraction.Comment: 4 pages, 1 figure, A&A Letters, accepte
Approximate integrals of motion
We determine approximate numerical integrals of motion of 2D symmetric
Hamiltonian systems. We detail for a few gravitational potentials the
conditions under which quasi-integrals are obtained as polynomial series. We
show that each of these potentials has a wide range of regular orbits that are
accurately modelled with a unique approximate integral of motion.Comment: 11 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
On the kinematic deconvolution of the local neighbourhood luminosity function
A method for inverting the statistical star counts equation, including proper
motions, is presented; in order to break the degeneracy in that equation it
uses the supplementary constraints required by dynamical consistency. The
inversion gives access to both the kinematics and the luminosity function of
each population in three r\'egimes: the singular ellipsoid, the constant ratio
Schwarzschild ellipsoid plane parallel models and the epicyclic model. This
more realistic model is taylored to account for local neighbourhood density and
velocity distribution.
The first model is fully investigated both analytically and via means of a
non-parametric inversion technique, while the second model is shown to be
formally its equivalent. The effect of noise and incompleteness in apparent
magnitude is investigated. The third model is investigated via a 5D+2D
non-parametric inversion technique where positivity of the underlying
luminosity function is explicitely accounted for.
It is argued that its future application to data such as the Tycho catalogue
(and in the upcoming satellite GAIA) could lead -- provided the vertical
potential, and/or the asymmetric drift or w_0 are known -- to a non-parametric
determination of the local neighbourhood luminosity function without any
reference to stellar evolution tracks. It should also yield the proportion of
stars for each kinematic component and a kinematic diagnostic to split the thin
disk from the thick disk or the halo.Comment: 18 pages, LateX (or Latex, etc), mnras, accepted for publicatio
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