24 research outputs found
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
Kinematics of Stars in Kapteyn Selected Area 71: Sampling the Monoceros and Sagittarius Tidal Streams
We present a 3D kinematical analysis of stars located in Kapteyn Selected
Area 71 (l = 167.1, b = -34.7), where previously a stellar excess was found
(Dinescu et al. 2002, Newberg et al. 2002). Previous findings indicated that
the stellar excess has a cold kinematical signature as inferred from proper
motions, and was initially associated with debris from the Sagittarius dwarf
galaxy (Sgr) -- namely the southern trailing tail. We have obtained radial
velocities using the Hydra multiobject spectrograph on the WIYN 3.5 m
telescope. Results for 183 proper-motion selected stars indicate that the
dominant population in this stellar excess is not debris from Sgr, but rather a
population that kinematically belongs to the ring-like stream that is now known
as the Monoceros stream. The orbit determined for this population agrees very
well with the predictions for the Monoceros stream from Penarrubia et al.
(2005). The radial-velocity dispersion of this population is between 20 and 30
km/s, lower than that of the Galactic field. Also, the shape of the
radial-velocity distribution shows a sharp cut-off on one side, which is more
in line with model predictions of the disruption of a satellite rather than
with the distribution of the Galactic field. Despite the fact that we now
believe most of the stars in the stellar excess to be part of Monoceros, about
ten stars in this stellar excess have highly negative radial velocities, which
is a clear indication of their membership to the Sgr trailing tidal tail.Comment: Accepted for publication in the Astronomical Journal - 26 pages, 8
figures, (Figures 7 and 8 in lower resolution
The Milky Way's external disc constrained by 2MASS star counts
Context. Thanks to recent large scale surveys in the near infrared such as
2MASS, the galactic plane that most suffers from extinction is revealed and its
overall structure can be studied. Aims. This work aims at constraining the
structure of the Milky Way external disc as seen in 2MASS data, and in
particular the warp. Methods. We use the Two Micron All Sky Survey (hereafter
2MASS) along with the Stellar Population Synthesis Model of the Galaxy,
developed in Besancon, to constrain the external disc parameters such as its
scale length, its cutoff radius, and the slope of the warp. In order to
properly interpret the observations, the simulated stars are reddened using a
three dimensional extinction map. The shape of the stellar warp is then
compared with previous results and with similar structures in gas and dust.
Results. We find new constraints on the stellar disc, which is shown to be
asymmetrical, similar to observations of HI. The positive longitude side is
found to be easily modelled with a S shape warp but with a slope significantly
smaller than the slope seen in the HI warp. At negative longitudes, the disc
presents peculiarities which are not well reproduced by any simple model.
Finally, comparing with the warp seen in the dust, it seems to follow a slope
intermediate between the gas and the stars.Comment: 9 pages. Accepted for publication in Astronomy and Astrophysic
Probabilistic multi-catalogue positional cross-match
[Context]: Catalogue cross-correlation is essential to building large sets of multi-wavelength data, whether it be to study the properties of populations of astrophysical objects or to build reference catalogues (or timeseries) from survey observations. Nevertheless, resorting to automated processes with limited sets of information available on large numbers of sources detected at different epochs with various filters and instruments inevitably leads to spurious associations. We need both statistical criteria to select detections to be merged as unique sources, and statistical indicators helping in achieving compromises between completeness and reliability of selected associations. [Aims]: We lay the foundations of a statistical framework for multi-catalogue cross-correlation and cross-identification based on explicit simplified catalogue models. A proper identification process should rely on both astrometric and photometric data. Under some conditions, the astrometric part and the photometric part can be processed separately and merged a posteriori to provide a single global probability of identification. The present paper addresses almost exclusively the astrometrical part and specifies the proper probabilities to be merged with photometric likelihoods. [Methods]: To select matching candidates in n catalogues, we used the Chi (or, indifferently, the Chi-square) test with 2(n-1) degrees of freedom. We thus call this cross-match a χ-match. In order to use Bayes' formula, we considered exhaustive sets of hypotheses based on combinatorial analysis. The volume of the χ-test domain of acceptance-a 2(n-1)-dimensional acceptance ellipsoid-is used to estimate the expected numbers of spurious associations. We derived priors for those numbers using a frequentist approach relying on simple geometrical considerations. Likelihoods are based on standard Rayleigh, χ and Poisson distributions that we normalized over the χ-test acceptance domain. We validated our theoretical results by generating and cross-matching synthetic catalogues. [Results]: The results we obtain do not depend on the order used to cross-correlate the catalogues. We applied the formalism described in the present paper to build the multi-wavelength catalogues used for the science cases of the Astronomical Resource Cross-matching for High Energy Studies (ARCHES) project. Our cross-matching engine is publicly available through a multi-purpose web interface. In a longer term, we plan to integrate this tool into the CDS XMatch Service.A large part of this work was supported by the ARCHES project. ARCHES (No. 313146) was funded by the 7th Framework of the European Union and coordinated by the University of Strasbourg. . F. J. Carrera also acknowledges financial support through grant AYA2015-64346-C2-1-P (MINECO/FEDER).Peer Reviewe
Cross-correlation of the 2XMMi catalogue with Data Release 7 of the Sloan Digital Sky Survey
The Survey Science Centre of the XMM-Newton satellite released the first
incremental version of the 2XMM catalogue in August 2008 . With more than
220,000 X-ray sources, the 2XMMi was at that time the largest catalogue of
X-ray sources ever published and thus constitutes an unprecedented resource for
studying the high-energy properties of various classes of X-ray emitters such
as AGN and stars. The advent of the 7th release of the Sloan Digital Sky Survey
offers the opportunity to cross-match two major surveys and extend the spectral
energy distribution of many 2XMMi sources towards the optical bands. We here
present a cross-matching algorithm based on the classical likelihood ratio
estimator. The method developed has the advantage of providing true
probabilities of identifications without resorting to Monte-Carlo simulations.
Over 30,000 2XMMi sources have SDSS counterparts with individual probabilities
of identification higher than 90%. Using spectroscopic identifications from the
SDSS DR7 catalogue supplemented by extraction from other catalogues, we build
an identified sample from which the way the various classes of X-ray emitters
gather in the multi dimensional parameter space can be analysed. We investigate
two scientific use cases. In the first example we show how these
multi-wavelength data can be used to search for new QSO2s. Although no specific
range of observed properties allows us to identify Compton Thick QSO2s, we show
that the prospects are much better for Compton Thin AGN2 and discuss several
possible multi-parameter selection strategies. In a second example, we confirm
the hardening of the mean X-ray spectrum with increasing X-ray luminosity on a
sample of over 500 X-ray active stars and reveal that on average X-ray active M
stars display bluer colour indexes than less active ones (abridged).Comment: Accepted for publication in A&A. The corresponding fits file can be
downloaded from the XCat-DB home page (http://xcatdb.u-strasbg.fr/) (tools
and data). The file also contains line information for all SDSS spectroscopic
entries matching a 2XMM source. Results from the cross-correlation with the
2XMM DR3 are also available at the same location. 22 pages and 14 figure
The SPECFIND V2.0 catalogue of radio cross-identifications and spectra. SPECFIND meets the Virtual Observatory
The new release of the SPECFIND radio cross-identification catalogue,
SPECFIND V2.0, is presented. It contains 107488 cross-identified objects with
at least three radio sources observed at three independent frequencies.
Compared to the previous release the number of entry radio catalogues is
increased from 20 to 97 containing 115 tables. This large increase was only
made possible by the development of four tools at CDS which use the standards
and infrastructure of the Virtual Observatory (VO). This was done in the
framework of the VO-TECH European Design Study of the Sixth Framework Program.
We give an overview of the different classes of radio sources that a user can
encounter. Due to the increase of frequency coverage of the input radio
catalogues, this release demonstrates that the SPECFIND algorithm is able to
detect spectral breaks around a frequency of ~1 GHz.Comment: 11 pages, 10 figures, accepted for publication in A&
Units in Astronomy and Astrophysics
This paper has been prepared by the authors as the International Astronomical Union’s (IAU) contribution to the Task Group on Digital Representation of Units of Measure (DRUM) of the Committee on Data (CODATA) of the International Science Council (ISC).
It describes the standards for use of units in astronomy and astrophysics and their development over the last 3+ decades: the IAU Style Manual, the FITS standard, and the IVOA recommendation on the use of units. In addition, it highlights special units in use in the field (units of length, time, and flux density, as well as celestial coordinates); caveats (on the importance of reference positions, linear velocities versus Doppler velocities, time, and polarization); and special quirks
Units in the VO
This document describes a recommended syntax for writing the string representation of unit labels ("VOUnits"). In addition, it describes a set of recognised and deprecated units, which is as far as possible consistent with other relevant standards (BIPM, ISO/IEC and the IAU). The intention is that units written to conform to this specification will likely also be parsable by other well-known parsers. To this end, we include machine-readable grammars for other units syntaxes