17,551 research outputs found
Search for star clusters close to the Galactic plane with DENIS
An automated search for star clusters close to the Galactic plane (|b| < 5
deg) was carried out on the Point Source Catalogue of the DENIS survey. 44% of
the Galactic plane have been observed and calibrated. The method allowed to
retrieve 22 known star clusters and to identify two new ones, not published yet
although previously presented in the 2MASS web site as embedded clusters in HII
regions. Extinction in the field and in front of the clusters are estimated
using a model of population synthesis. We present the method and give the
properties of these clusters.Comment: 5 pages, 3 figures, accepted for publication in A&
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 microlensing rate and distribution of free-floating planets towards the Galactic bulge
Ground-based optical microlensing surveys have provided tantalising, if
inconclusive, evidence for a significant population of free-floating planets
(FFPs). Both ground and space-based facilities are being used and developed
which will be able to probe the distrubution of FFPs with much better
sensitivity. It is vital also to develop a high-precision microlensing
simulation framework to evaluate the completeness of such surveys. We present
the first signal-to-noise limited calculations of the FFP microlensing rate
using the Besancon Galactic model. The microlensing distribution towards the
Galactic centre is simulated for wide-area ground-based optical surveys such as
OGLE or MOA, a wide-area ground-based near-IR survey, and a targeted
space-based near-IR survey which could be undertaken with Euclid or WFIRST. We
present a calculation framework for the computation of the optical and
near-infrared microlensing rate and optical depth for simulated stellar
catalogues which are signal-to-noise limited, and take account of extinction,
unresolved stellar background light and finite source size effects, which can
be significant for FFPs. We find that the global ground-based I-band yield over
a central 200 deg^2 region covering the Galactic centre ranges from 20
Earth-mass FFPs year^-1 up to 3,500 year^-1 for Jupiter FFPs in the limit of
100% detection efficiency, and almost an order of magnitude larger for a K-band
survey. For ground-based surveys we find that the inclusion of finite source
and the unresolved background reveals a mass-dependent variation in the spatial
distribution of FFPs. For a space-based H-band covering 2 deg^2, the yield
depends on the target field but maximizes close to the Galactic centre with
around 76 Earth through to 1,700 Jupiter FFPs year^-1. For near-IR space-based
surveys the spatial distribution of FFPs is found to be largely insensitive to
the FFP mass scale.Comment: 14 pages, submitted to A&A and accepte
The Galactic bulge as seen in optical surveys
The bulge is a region of the Galaxy of tremendous interest for understanding
galaxy formation. However measuring photometry and kinematics in it raises
several inherent issues, such as severe crowding and high extinction in the
visible. Using the Besancon Galaxy model and a 3D extinction map, we estimate
the stellar density as a function of longitude, latitude and apparent magnitude
and we deduce the possibility of reaching and measuring bulge stars with Gaia.
We also present an ongoing analysis of the bulge using the Canada-France-Hawaii
Telescope.Comment: In SF2A-2008: Proceedings of the Annual meeting of the French Society
of Astronomy and Astrophysic
The visibility of the Galactic bulge in optical surveys. Application to the Gaia mission
The bulge is a region of the Galaxy which is of tremendous interest for
understanding Galaxy formation. However, measuring photometry and kinematics in
it raises several inherent issues, like high extinction in the visible and
severe crowding. Here we attempt to estimate the problem of the visibility of
the bulge at optical wavelengths, where large CCD mosaics allow to easily cover
wide regions from the ground, and where future astrometric missions are
planned. Assuming the Besancon Galaxy model and high resolution extinction
maps, we estimate the stellar density as a function of longitude, latitude and
apparent magnitude and we deduce the possibility of reaching and measuring
bulge stars. The method is applied to three Gaia instruments, the BBP and MBP
photometers, and the RVS spectrograph. We conclude that, while in the BBP most
of the bulge will be accessible, in the MBP there will be a small but
significant number of regions where bulge stars will be detected and accurately
measured in crowded fields. Assuming that the RVS spectra may be extracted in
moderately crowded fields, the bulge will be accessible in most regions apart
from the strongly absorbed inner plane regions, because of high extinction, and
in low extinction windows like the Baades's window where the crowding is too
severe.Comment: 11 pages, 9 figures, accepted for publication in A&A, latex using A&A
macro
Crystal structure, thermodynamics, magnetics and disorder properties of Be-Fe-Al intermetallics
The elastic and magnetic properties, thermodynamical stability, deviation
from stoichiometry and order/disorder transformations of phases that are
relevant to Be alloys were investigated using density functional theory
simulations coupled with phonon density of states calculations to capture
temperature effects. A novel structure and composition were identified for the
Be-Fe binary {\epsilon} phase. In absence of Al, FeBe_5 is predicted to form at
equilibrium above ~ 1250 K, while the {\epsilon} phase is stable only below ~
1650 K, and FeBe_2 is stable at all temperatures below melting. Small additions
of Al are found to stabilise FeBe_5 over FeBe_2 and {\epsilon}, while at high
Al content, AlFeBe_4 is predicted to form. Deviations from stoichiometric
compositions are also considered and found to be important in the case of
FeBe_5 and {\epsilon}. The propensity for disordered vs ordered structures is
also important for AlFeBe_4 (which exhibits complete Al-Fe disordered at all
temperatures) and FeBe_5 (which exhibits an order-disorder transition at ~ 950
K).Comment: 14 pages, 10 figures, accepted for publication in J. Alloy Compd. on
14 March 201
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