566 research outputs found
Phonons Softening in Tip-Stretched Monatomic Nanowires
It has been shown in recent experiments that electronic transport through a
gold monatomic nanowire is dissipative above a threshold voltage due to
excitation of phonons via the electron-phonon interaction. We address that data
by computing, via density functional theory, the zone boundary longitudinal
phonon frequency of a perfect monatomic nanowire during its mechanical
elongation. The theoretical frequency that we find for an ideally strained
nanowire is not compatible with experiment if a uniformly distributed stretch
is assumed. With the help of a semi-empirical Au-Au potential, we model the
realistic nanowire stretching as exerted by two tips. In this model we see that
strain tends to concentrate in the junctions, so that the mean strain of the
nanowire is roughly one half of the ideal value. With this reduced strain, the
calculated phonon softening is in much better agreement with experiment.Comment: 9 pages,3 figures, Surface Science, in pres
Asymptotic geometry of negatively curved manifolds of finite volume
We study the asymptotic behaviour of simply connected, Riemannian manifolds
of strictly negative curvature admitting a non-uniform lattice . If
the quotient manifold is asymptotically
-pinched, we prove that is divergent and has finite
Bowen-Margulis measure (which is then ergodic and totally conservative with
respect to the geodesic flow); moreover, we show that, in this case, the volume
growth of balls in is asymptotically equivalent to a purely
exponential function , where is the topological
entropy of the geodesic flow of . \linebreak This generalizes Margulis'
celebrated theorem to negatively curved spaces of finite volume. In contrast,
we exhibit examples of lattices in negatively curved spaces (not
asymptotically -pinched) where, depending on the critical exponent of the
parabolic subgroups and on the finiteness of the Bowen-Margulis measure, the
growth function is exponential, lower-exponential or even upper-exponential.Comment: 25 p. This paper replaces arXiv:1503.03971, withdrawn by the authors
due to the Theorem 1.1 whose statement is far from the main subject of the
paper; for the sake of clearness, this new version concentrates only on the
question of volume growth (theorems 1.2, 1.3 and 1.4). Theorem 1.1 of
arXiv:1503.03971 is now the subject of another paper (Signed only by 2
authors Sambusetti and Peign\'e) focused on this rigidity problem with a much
better presentation of the context and another rigidity resul
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
Stellar density distribution in the NIR on the Galactic plane at longitudes 15-27 deg. Clues for the Galactic bar ?
12 pages, 15 figures, accepted by A&AGarzon et al. (1997), Lopez-Corredoira et al. (1999) and Hammersley et al. (2000)have identified in TMGS and DENIS data a large excess of stars at l=27 deg andb=0 deg which might correspond to an in-plane bar. We compared near infraredCAIN star counts and simulations from the Besancon Model of Galaxy on 15 fieldsbetween 15 deg and 45 deg in longitude and -2 deg and 2 deg in latitude.Comparisons confirm the existence of an overdensity at longitudes lower than 27deg which is inhomogeneous and decreases very steeply off the Galactic plane.The observed excess in the star distribution over the predicted density is evenhigher than 100%. Its distance from the sun is estimated to be lower than 6kpc. If this overdensity corresponds to the stellar population of the bar, weestimate its half-length to 3.9 +/ -0.4 kpc and its angle from the Sun-centerdirection to 45 +/- 9 degrees
Asymptotic geometry of negatively curved manifolds of finite volume
We study the asymptotic behavior of simply connected Riemannian manifolds X of strictly negative curvature admitting a non-uniform lattice Γ. If the quotient manifold X = Γ\X is asymptotically 1=4-pinched, we prove that Γ is divergent and U X has finite Bowen-Margulis measure (which is then ergodic and totally conservative with respect to the geodesic flow); moreover, we show that, in this case, the volume growth of balls B(x,R) in X is asymptotically equivalent to a purely exponential function c.x/eδR, where δ is the topological entropy of the geodesic flow of X . This generalizes Margulis' celebrated theorem to negatively curved spaces of finite volume. In contrast, we exhibit examples of lattices Γ in negatively curved spaces X (not asymptotically 1/4-pinched) where, depending on the critical exponent of the parabolic subgroups and on the finiteness of the Bowen- Margulis measure, the growth function is exponential, lower-exponential or even upper-exponential
A synthetic view on structure and evolution of the Milky Way
Since the Hipparcos mission and recent large scale surveys in the optical and
the near-infrared, new constraints have been obtained on the structure and
evolution history of the Milky Way. The population synthesis approach is a
useful tool to interpret such data sets and to test scenarios of evolution of
the Galaxy. We present here new constraints on evolution parameters obtained
from the Besancon model of population synthesis and analysis of optical and
near-infrared star counts. The Galactic potential is computed
self-consistently, in agreement with Hipparcos results and the observed
rotation curve. Constraints are posed on the outer bulge structure, the warped
and flared disc, the thick disc and the spheroid populations. The model is
tuned to produce reliable predictions in the visible and the near-infrared in
wide photometric bands from U to K. Finally, we describe applications such as
photometric and astrometric simulations and a new classification tool based on
a Bayesian probability estimator, which could be used in the framework of
Virtual Observatories. As examples, samples of simulated star counts at
different wavelengths and directions are also given.Comment: 20 pages, 12 figures, latex using A&A macros, version corrected from
the original version published in A&A 409, 523 (2003) with erratum. Model
accessible at http://www.obs-besancon.fr/modele/model2003.htm
Oxygen abundances in the Galactic Bulge: evidence for fast chemical enrichment
AIMS: We spectroscopically characterize the Galactic Bulge to infer its star
formation timescale, compared to the other Galactic components, through the
chemical signature on its individual stars.
METHODS: We derived iron and oxygen abundances for 50 K giants in four fields
towards the Galactic bulge. High resolution (R=45,000) spectra for the target
stars were collected with FLAMES-UVES at the VLT.
RESULTS: Oxygen, as measured from the forbidden line at 6300 \AA, shows a
well-defined trend with [Fe/H], with [O/Fe] higher in bulge stars than in thick
disk ones, which were known to be more oxygen enhanced than thin disk stars.
CONCLUSIONS: These results support a scenario in which the bulge formed
before and more rapidly than the disk, and therefore the MW bulge can be
regarded as a prototypical old spheroid, with a formation history similar to
that of early-type (elliptical) galaxies.Comment: A&A Letters, in pres
High precision microlensing maps of the Galactic bulge
We present detailed maps of the microlensing optical depth and event density
over an area of 195 sq. deg towards the Galactic bulge. The maps are computed
from synthetic stellar catalogues generated from the Besancon Galaxy Model,
which comprises four stellar populations and a three-dimensional extinction map
calibrated against the Two-Micron All-Sky Survey. The optical depth maps have a
resolution of 15 arcminutes, corresponding to the angular resolution of the
extinction map. We compute optical depth and event density maps for all
resolved sources above I=19, for unresolved (difference image) sources
magnified above this limit, and for bright standard candle sources in the
bulge. We show that the resulting optical depth contours are dominated by
extinction effects, exhibiting fine structure in stark contrast to previous
theoretical optical depth maps. Optical depth comparisons between Galactic
models and optical microlensing survey measurements cannot safely ignore
extinction or assume it to be smooth. We show how the event distribution for
hypothetical J and K-band microlensing surveys, using existing ground-based
facilities such as VISTA, UKIRT or CFHT, would be much less affected by
extinction, especially in the K band. The near infrared provides a substantial
sensitivity increase over current I-band surveys and a more faithful tracer of
the underlying stellar distribution, something which upcoming variability
surveys such as VVV will be able to exploit. Synthetic population models offer
a promising way forward to fully exploit large microlensing datasets for
Galactic structure studies.Comment: 8 pages, submitted to MNRA
A three-dimensional Galactic extinction model
A large-scale three-dimensional model of Galactic extinction is presented
based on the Galactic dust distribution model of Drimmel and Spergel (2001).
The extinction A_V to any point within the Galactic disk can be quickly deduced
using a set of three-dimensional cartesian grids. Extinctions from the model
are compared to empirical extinction measures, including lines-of-sight in and
near the Galactic plane using optical and NIR extinction measures; in
particular we show how extinction can be derived from NIR color-magnitude
diagrams in the Galactic plane to a distance of 8 kiloparsec.Comment: 12 pages, to be published in A&
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