515 research outputs found
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
Accretion by the Galaxy
Cosmology requires at least half of the baryons in the Universe to be in the
intergalactic medium, much of which is believed to form hot coronae around
galaxies. Star-forming galaxies must be accreting from their coronae. HI
observations of external galaxies show that they have HI halos associated with
star formation. These halos are naturally modelled as ensembles of clouds
driven up by supernova bubbles. These models can fit the data successfully only
if clouds exchange mass and momentum with the corona. As a cloud orbits, it is
ablated and forms a turbulent wake where cold high-metallicity gas mixes with
hot coronal gas causing the prompt cooling of the latter. As a consequence the
total mass of HI increases. This model has recently been used to model the
Leiden-Argentina-Bonn survey of Galactic HI. The values of the model's
parameters that are required to model NGC 891, NGC 2403 and our Galaxy show a
remarkable degree of consistency, despite the very different natures of the two
external galaxies and the dramatic difference in the nature of the data for our
Galaxy and the external galaxies. The parameter values are also consistent with
hydrodynamical simulations of the ablation of individual clouds. The model
predicts that a galaxy that loses its cool-gas disc for instance through a
major merger cannot reform it from its corona; it can return to steady star
formation only if it can capture a large body of cool gas, for example by
accreting a gas-rich dwarf. Thus the model explains how major mergers can make
galaxies "red and dead."Comment: Invited review at "Assembling the Puzzle of the Milky Way", Grand
Bornand, April 2011; 6 page
Early galaxy evolution from deep wide field star counts. II. First estimate of the thick disc mass function
Star counts at high and intermediate galactic latitudes, in the visible and
the near infrared, are used to determine the density law and the initial mass
function of the thick disc population. The combination of shallow fields
dominated by stars at the turnoff with deep fields allows the determination of
the thick disc mass function in the mass range 0.2-0.8 Msun. Star counts are
compared with simulations of a synthesis population model. The fit is based on
a maximum likelihood criterion. The best fit model gives a scale height of 800
pc, a scale length of 2500 pc and a local density of 10e-3 stars/pc3 or 7.1e-4
Msun/pc3 for Mv<8. The IMF is found to follow a power law with a slope
alpha=-0.5. This is the first determination of the thick disc mass function.Comment: 10 pages, 7 figures, accepted for publication in Astron. Astrophy
Close encounters involving RAVE stars beyond the 47 Tucanae tidal radius
The most accurate 6D phase-space information from the Radial Velocity
Experiment (RAVE) was used to integrate the orbits of 105 stars around the
galactic globular cluster 47 Tucanae, to look for close encounters between them
in the past, with a minimum distance approach less than the cluster tidal
radius. The stars are currently over the distance range 3.0 kpc d 5.5
kpc. Using the uncertainties in the current position and velocity vector for
both, star and cluster, 105 pairs of star-cluster orbits were generated in a
Monte Carlo numerical scheme, integrated over 2 Gyr and considering an
axisymmetric and non-axisymmetric Milky-Way-like Galactic potential,
respectively. In this scheme, we identified 20 potential cluster members that
had close encounters with the globular cluster 47 Tucanae, all of which have a
relative velocity distribution (V) less than 200 km s at the
minimum distance approach. Among these potential members, 9 had close
encounters with the cluster with velocities less than the escape velocity of 47
Tucanae, therefore a scenario of tidal stripping seems likely. These stars have
been classified with a 93\% confidence level, leading to the identification of
extratidal cluster stars. For the other 11 stars, V exceeds the escape
velocity of the cluster, therefore they were likely ejected or are unassociated
interlopers.Comment: 10 pages, 6 figures, 2 table, Accepted for publication in MNRA
Integration of the atmospheric fluctuations in a dual-field optical interferometer: the short exposure regime
Spatial phase-referencing in dual-field optical interferometry is
reconsidered. Our analysis is based on the 2-sample variance of the
differential phase between target and reference star. We show that averaging
over time of the atmospheric effects depends on this 2-sample phase variance
(Allan variance) rather than on the true variance. The proper expression for
fringe smearing beyond the isoplanatic angle is derived. With simulations of
atmospheric effects, based on a Paranal turbulence model, we show how the
performances of a dual-field optical interferometer can be evaluated in a
diagram 'separation angle' versus 'magnitude of faint object'. In this diagram,
a domain with short exposure is found to be most useful for interferometry,
with about the same magnitude limits in the H and K bands. With star counts
from a Galaxy model, we evaluate the sky coverage for differential astrometry
and detection of exoplanets, i.e. likelihood of faint reference stars in the
vicinity of a bright target. With the 2mass survey, we evaluate sky coverage
for phase-referencing, i.e. avaibility of a bright enough star for main delay
tracking in the vicinity of any target direction.Comment: 9 pages, 8 figures, accepted for publication in A&
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
Dark halo baryons not in ancient halo white dwarfs
Having ruled out the possibility that stellar objects are the main
contributor of the dark matter embedding galaxies, microlensing experiments
cannot exclude the hypothesis that a significant fraction of the Milky Way dark
halo might be made of MACHOs with masses in the range 0.5-0.8 \msun. Ancient
white dwarfs are generally considered the most plausible candidates for such
MACHOs. We report the results of a search for such white dwarfs in a proper
motion survey covering a 0.16 sqd field at three epochs at high galactic
latitude, and 0.938 sqd at two epochs at intermediate galactic latitude (VIRMOS
survey), using the CFH telescope. Both surveys are complete to I = 23, with
detection efficiency fading to 0 at I = 24.2. Proper motion data are suitable
to separate unambiguously halo white dwarfs identified by belonging to a non
rotating system. No candidates were found within the colour-magnitude-proper
motion volume where such objects can be safely discriminated from any standard
population as well as from possible artefacts. In the same volume, we estimate
the maximum white dwarf halo fraction compatible with this observation at
different significance levels if the halo is at least 14 gigayears old and
under different ad hoc initial mass functions. Our data alone rules out a halo
fraction greater than 14% at 95% confidence level. Combined with two previous
investigations exploring comparable volumes pushes the limit below 4 % (95%
confidence level) or below 1.3% (64% confidence), this implies that if baryonic
dark matter is present in galaxy halos, it is not, or it is only marginally in
the form of faint hydrogen white dwarfs.Comment: accepted in Astronomy and Astrophysics (19-05-2004
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
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