52 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
Galactic Spiral Structure
We describe the structure and composition of six major stellar streams in a
population of 20 574 local stars in the New Hipparcos Reduction with known
radial velocities. We find that, once fast moving stars are excluded, almost
all stars belong to one of these streams. The results of our investigation have
lead us to re-examine the hydrogen maps of the Milky Way, from which we
identify the possibility of a symmetric two-armed spiral with half the
conventionally accepted pitch angle. We describe a model of spiral arm motions
which matches the observed velocities and composition of the six major streams,
as well as the observed velocities of the Hyades and Praesepe clusters at the
extreme of the Hyades stream. We model stellar orbits as perturbed ellipses
aligned at a focus in coordinates rotating at the rate of precession of
apocentre. Stars join a spiral arm just before apocentre, follow the arm for
more than half an orbit, and leave the arm soon after pericentre. Spiral
pattern speed equals the mean rate of precession of apocentre. Spiral arms are
shown to be stable configurations of stellar orbits, up to the formation of a
bar and/or ring. Pitch angle is directly related to the distribution of orbital
eccentricities in a given spiral galaxy. We show how spiral galaxies can evolve
to form bars and rings. We show that orbits of gas clouds are stable only in
bisymmetric spirals. We conclude that spiral galaxies evolve toward grand
design two-armed spirals. We infer from the velocity distributions that the
Milky Way evolved into this form about 9 Gyrs ago.Comment: Published in Proc Roy Soc A. A high resolution version of this file
can be downloaded from http://papers.rqgravity.net/SpiralStructure.pdf. A
simplified account with animations begins at
http://rqgravity.net/SpiralStructur
Reconstructing the Star Formation History of the Galaxy
The evolution of the star formation rate in the Galaxy is one of the key
ingredients quantifying the formation and determining the chemical and
luminosity evolution of galaxies. Many complementary methods exist to infer the
star formation history of the components of the Galaxy, from indirect methods
for analysis of low-precision data, to new exact analytic methods for analysis
of sufficiently high quality data. We summarise available general constraints
on star formation histories, showing that derived star formation rates are in
general comparable to those seen today. We then show how colour-magnitude
diagrams of volume- and absolute magnitude-limited samples of the solar
neighbourhood observed by Hipparcos may be analysed, using variational calculus
techniques, to reconstruct the local star formation history. The remarkable
accuracy of the data coupled to our maximum-likelihood variational method
allows objective quantification of the local star formation history with a time
resolution of ~ 50 Myr. Over the past 3Gyr, the solar neighbourhood star
formation rate has varied by a factor of ~ 4, with characteristic timescale
about 0.5Gyr, possibly triggered by interactions with spiral arms.Comment: 12 pages, Proc. of the Sept. 20-24, 1999 Vulcano Workshop ``The
chemical evolution of the Milky Way: stars vs. clusters'', eds. F. Matteucci
& F. Giovanell
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
The Pattern Speed of the Galactic Bar
Most late-type stars in the solar neighborhood have velocities similar to the
local standard of rest (LSR), but there is a clearly separated secondary
component corresponding to a slower rotation and a mean outward motion.
Detailed simulations of the response of a stellar disk to a central bar show
that such a bi-modality is expected from outer-Lindblad resonant scattering.
When constraining the run of the rotation curve by the proper motion of Sgr A*
and the terminal gas velocities, the value observed for the rotation velocity
separating the two components results in a value of (53+/-3)km/s/kpc for the
pattern speed of the bar, only weakly dependent on the precise values for Ro
and bar angle phi.Comment: 5 pages LaTeX, 2 Figs, accepted for publication in ApJ Letter
The Effect of the Outer Lindblad Resonance of the Galactic Bar on the Local Stellar Velocity Distribution
Hydro-dynamical modeling of the inner Galaxy suggest that the radius of the
outer Lindblad resonance (OLR) of the Galactic bar lies in the vicinity of the
Sun. How does this resonance affect the distribution function in the outer
parts of a barred disk, and can we identify any effect of the resonance in the
velocity distribution f(v) actually observed in the solar neighborhood? To
answer these questions, detailed simulations of f(v) in the outer parts of an
exponential stellar disks with nearly flat rotation curves and a rotating
central bar have been performed. For a model resembling the old stellar disk,
the OLR causes a distinct feature in f(v) over a significant fraction of the
outer disk. For positions <2kpc outside the OLR radius and at bar angles of
\~10-70 degrees, f(v) inhibits a bi-modality between the low-velocity stars
moving like the local standard of rest (LSR) and a secondary mode of stars
predominantly moving outward and rotating more slowly than the LSR.
Such a bi-modality is indeed present in f(v) inferred from the Hipparcos data
for late-type stars in the solar neighborhood. If one interpretes this observed
bi-modality as induced by the OLR -- and there are hardly any viable
alternatives -- then one is forced to deduce that the OLR radius is slightly
smaller than Ro. Moreover, by a quantitative comparison of the observed with
the simulated distributions one finds that the pattern speed of the bar is
1.85+/-0.15 times the local circular frequency, where the error is dominated by
the uncertainty in bar angle and local circular speed.
Also other, less prominent but still significant, features in the observed
f(v) resemble properties of the simulated velocity distributions, in particular
a ripple caused by orbits trapped in the outer 1:1 resonance.Comment: 14 pages, 10 figures (Fig.2 in full resolution available upon
request), accepted for publication in A
Segmentation d'image par déformation multirésolution sur bases d'ondelettes combinant données et modÚle
- Nous proposons une nouvelle méthode de segmentation par ajustement d'un modÚle actif multirésolution à chaque niveau de résolution de données décomposées sur une base d'ondelettes orthogonales. La combinaison de l'approche multirésolution sur bases d'ondelettes, simultanément sur les données et sur le modÚle, apporte une convergence rapide aux plus hautes résolutions. La solution obtenue est plus robuste et plus précise en présence de bruit que celle obtenue par un ajustement de contour actif non multirésolution
A New Association of Post-T Tauri Stars Near The Sun
Observing ROSAT sources in 20 x 25 deg centered at the high latitude active
star ER Eri, we found evidences for a new young nearby association (~30Myr
at~60pc), the Horologium Association (HorA), formed by at least 10 probable and
6 possible members, some being Post-T Tauri stars. We examine several
requirements that characterize a young association and they, together, create a
strong evidence for the reality of the HorA. In fact, the Li line intensities
are between those of the oldest classical T Tauri stars and the ones of the
Local Association stars. The space velocities of the HorA relative to the Sun,
U= -9.5+/-1.0, V = -20.9 +/- 1.1, W = -2.1 +/- 1.9, are not far from those of
the Local Association. We suggest that some hotter and non-X-ray active stars,
with similar space velocities, could be massive members of the HorA, among
them, the nearby Be star Achernar. The maximum of the mass distribution
function of the HorA is around 0.8 solar masses. At its distance, the projected
size of the HorA, ~50 pc, would be larger than our surveyed area and many other
members could have been missed. We also observed 3 control regions, two at
northern and southern galactic latitudes and a third one in the known TW Hya
Association (TWA), and the properties and distribution of their young stars
strengthen the reality of the HorA. Contrary to the TWA, the only known
binaries in the HorA are 2 very wide systems. The HorA is much more isolated
from clouds and older than the TWA and could give some clues about the lifetime
of the disks around T Tauri stars. Actually, none of the proposed members is an
IRAS source indicating an advanced stage of the evolution of their accreting
disks. ER Eri itself was found to be a RS CVn-like system.Comment: 25 pages, 5 eps figures, to appear in Astron.
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