257 research outputs found
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
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: I. The potential well and local dynamical mass
Hipparcos data provide the first, volume limited and absolute magnitude
limited homogeneous tracer of stellar density and velocity distributions in the
solar neighbourhood. The density of A-type stars more luminous than
can be accurately mapped within a sphere of 125 pc radius, while proper motions
in galactic latitude provide the vertical velocity distribution near the
galactic plane. The potential well across the galactic plane is traced
practically hypothesis-free and model-free. The local dynamical density comes
out as \rho_{0}=0.076 \pm0.015~M_{\sun}~{pc}^{-3} a value well below all
previous determinations leaving no room for any disk shaped component of dark
matter.Comment: 24 pages, 13 figures, latex. To appear in A&A (main journal
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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