865 research outputs found
Orion revisited. II. The foreground population to Orion A
Following the recent discovery of a large population of young stars in front
of the Orion Nebula, we carried out an observational campaign with the DECam
wide-field camera covering ~10~deg^2 centered on NGC 1980 to confirm, probe the
extent of, and characterize this foreground population of pre-main-sequence
stars. We confirm the presence of a large foreground population towards the
Orion A cloud. This population contains several distinct subgroups, including
NGC1980 and NGC1981, and stretches across several degrees in front of the Orion
A cloud. By comparing the location of their sequence in various color-magnitude
diagrams with other clusters, we found a distance and an age of 380pc and
5~10Myr, in good agreement with previous estimates. Our final sample includes
2123 candidate members and is complete from below the hydrogen-burning limit to
about 0.3Msun, where the data start to be limited by saturation. Extrapolating
the mass function to the high masses, we estimate a total number of ~2600
members in the surveyed region. We confirm the presence of a rich, contiguous,
and essentially coeval population of about 2600 foreground stars in front of
the Orion A cloud, loosely clustered around NGC1980, NGC1981, and a new group
in the foreground of the OMC-2/3. For the area of the cloud surveyed, this
result implies that there are more young stars in the foreground population
than young stars inside the cloud. Assuming a normal initial mass function, we
estimate that between one to a few supernovae must have exploded in the
foreground population in the past few million years, close to the surface of
Orion A, which might be responsible, together with stellar winds, for the
structure and star formation activity in these clouds. This long-overlooked
foreground stellar population is of great significance, calling for a revision
of the star formation history in this region of the Galaxy.Comment: Accepted for publication in A&
A hierarchical Bayesian model to infer PL(Z) relations using Gaia parallaxes
Aims. We aim at creating a Bayesian model to infer the coefficients of PL or
PLZ relations that propagates uncertainties in the observables in a rigorous
and well founded way. Methods. We propose a directed acyclic graph to encode
the conditional probabilities of the inference model that will allow us to
infer probability distributions for the PL and PL(Z) relations. We evaluate the
model with several semi-synthetic data sets and apply it to a sample of 200
fundamental mode and first overtone mode RR Lyrae stars for which Gaia DR1
parallaxes and literature Ks-band mean magnitudes are available. We define and
test several hyperprior probabilities to verify their adequacy and check the
sensitivity of the solution with respect to the prior choice. Results. The main
conclusion of this work is the absolute necessity of incorporating the existing
correlations between the observed variables (periods, metallicities and
parallaxes) in the form of model priors in order to avoid systematically biased
results, especially in the case of non-negligible uncertainties in the
parallaxes. The tests with the semi-synthetic data based on the data set used
in Gaia Collaboration et al. (2017) reveal the significant impact that the
existing correlations between parallax, metallicity and periods have on the
inferred parameters. The relation coefficients obtained here have been
superseded by those presented in Muraveva et al. (2018a), that incorporates the
findings of this work and the more recent Gaia DR2 measurements.Comment: 14 pages, 12 figures. Submitted to A&
Automated supervised classification of variable stars I. Methodology
The fast classification of new variable stars is an important step in making
them available for further research. Selection of science targets from large
databases is much more efficient if they have been classified first. Defining
the classes in terms of physical parameters is also important to get an
unbiased statistical view on the variability mechanisms and the borders of
instability strips. Our goal is twofold: provide an overview of the stellar
variability classes that are presently known, in terms of some relevant stellar
parameters; use the class descriptions obtained as the basis for an automated
`supervised classification' of large databases. Such automated classification
will compare and assign new objects to a set of pre-defined variability
training classes. For every variability class, a literature search was
performed to find as many well-known member stars as possible, or a
considerable subset if too many were present. Next, we searched on-line and
private databases for their light curves in the visible band and performed
period analysis and harmonic fitting. The derived light curve parameters are
used to describe the classes and define the training classifiers. We compared
the performance of different classifiers in terms of percentage of correct
identification, of confusion among classes and of computation time. We describe
how well the classes can be separated using the proposed set of parameters and
how future improvements can be made, based on new large databases such as the
light curves to be assembled by the CoRoT and Kepler space missions.Comment: This paper has been accepted for publication in Astronomy and
Astrophysics (reference AA/2007/7638) Number of pages: 27 Number of figures:
1
The Gaia Ultra-Cool Dwarf Sample -- II : Structure at the end of the main sequence
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.We identify and investigate known late M, L, and T dwarfs in the Gaia second data release. This sample is being used as a training set in the Gaia data processing chain of the ultracool dwarfs work package. We find 695 objects in the optical spectral range M8–T6 with accurate Gaia coordinates, proper motions, and parallaxes which we combine with published spectral types and photometry from large area optical and infrared sky surveys. We find that 100 objects are in 47 multiple systems, of which 27 systems are published and 20 are new. These will be useful benchmark systems and we discuss the requirements to produce a complete catalogue of multiple systems with an ultracool dwarf component. We examine the magnitudes in the Gaia passbands and find that the G BP magnitudes are unreliable and should not be used for these objects. We examine progressively redder colour–magnitude diagrams and see a notable increase in the main-sequence scatter and a bivariate main sequence for old and young objects. We provide an absolute magnitude – spectral subtype calibration for G and G RP passbands along with linear fits over the range M8–L8 for other passbands.Peer reviewedFinal Published versio
The Seven Sisters DANCe III: Projected spatial distribution
Methods. We compute Bayesian evidences and Bayes Factors for a set of
variations of the classical radial models by King (1962), Elson et al. (1987)
and Lauer et al. (1995). The variations incorporate different degrees of model
freedom and complexity, amongst which we include biaxial (elliptical) symmetry,
and luminosity segregation. As a by-product of the model comparison, we obtain
posterior distributions and maximum a posteriori estimates for each set of
model parameters. Results. We find that the model comparison results depend on
the spatial extent of the region used for the analysis. For a circle of 11.5
parsecs around the cluster centre (the most homogeneous and complete region),
we find no compelling reason to abandon Kings model, although the Generalised
King model, introduced in this work, has slightly better fitting properties.
Furthermore, we find strong evidence against radially symmetric models when
compared to the elliptic extensions. Finally, we find that including mass
segregation in the form of luminosity segregation in the J band, is strongly
supported in all our models. Conclusions. We have put the question of the
projected spatial distribution of the Pleiades cluster on a solid probabilistic
framework, and inferred its properties using the most exhaustive and least
contaminated list of Pleiades candidate members available to date. Our results
suggest however that this sample may still lack about 20% of the expected
number of cluster members. Therefore, this study should be revised when the
completeness and homogeneity of the data can be extended beyond the 11.5
parsecs limit. Such study will allow a more precise determination of the
Pleiades spatial distribution, its tidal radius, ellipticity, number of objects
and total mass.Comment: 39 pages, 31 figure
Cluster membership probabilities from proper motions and multiwavelength photometric catalogues: I. Method and application to the Pleiades cluster
We present a new technique designed to take full advantage of the high
dimensionality (photometric, astrometric, temporal) of the DANCe survey to
derive self-consistent and robust membership probabilities of the Pleiades
cluster. We aim at developing a methodology to infer membership probabilities
to the Pleiades cluster from the DANCe multidimensional astro-photometric data
set in a consistent way throughout the entire derivation. The determination of
the membership probabilities has to be applicable to censored data and must
incorporate the measurement uncertainties into the inference procedure.
We use Bayes' theorem and a curvilinear forward model for the likelihood of
the measurements of cluster members in the colour-magnitude space, to infer
posterior membership probabilities. The distribution of the cluster members
proper motions and the distribution of contaminants in the full
multidimensional astro-photometric space is modelled with a
mixture-of-Gaussians likelihood. We analyse several representation spaces
composed of the proper motions plus a subset of the available magnitudes and
colour indices. We select two prominent representation spaces composed of
variables selected using feature relevance determination techniques based in
Random Forests, and analyse the resulting samples of high probability
candidates. We consistently find lists of high probability (p > 0.9975)
candidates with 1000 sources, 4 to 5 times more than obtained in the
most recent astro-photometric studies of the cluster.
The methodology presented here is ready for application in data sets that
include more dimensions, such as radial and/or rotational velocities, spectral
indices and variability.Comment: 14 pages, 4 figures, accepted by A&
CORRELATION BETWEEN FUNCTIONAL CLASSIFICATION AND KINEMATICAL VARIABLES IN ELITE WHEELCHAIR RUGBY PLAYERS
Wheelchair rugby is a
Paralympic team sport for athletes with disabilities affecting the four limbs. Players are classified according to their functional level from 0.5 (lowest function) to 3.5 (highest function). A player’s classification is based on muscle tests designed to evaluate the strength and range of motion of the upper limbs and trunk and also includes observation of the athlete on court (IWRF, 2008). Although the sport class is based on movement potential associated with neuromuscular function and performance of tasks related to the sport, it is not well known how functional classification in rugby correlates with variables strongly related to performance such as distance covered. In a previous investigation (Sarro et al., 2010), kinematical variables were analyzed in an international rugby competition and suggested a relation between functional classification and distance covered during the game. To further examine this relationship, this project aimed to investigate the correlation between functional classification and player physical performance as measured by distance covered during a game. In addition, the correlation was examined for each game quarter and as a function of velocity range
Corona-Australis DANCe I. Revisiting the census of stars with Gaia-DR2 data
Context. Corona-Australis is one of the nearest regions to the Sun with recent and ongoing star formation, but the current picture of its stellar (and substellar) content is not complete yet.
Aims. We take advantage of the second data release of the Gaia space mission to revisit the stellar census and search for additional members of the young stellar association in Corona-Australis.
Methods. We applied a probabilistic method to infer membership probabilities based on a multidimensional astrometric and photometric data set over a field of 128 deg(2) around the dark clouds of the region.
Results. We identify 313 high-probability candidate members to the Corona-Australis association, 262 of which had never been reported as members before. Our sample of members covers the magnitude range between G greater than or similar to 5 mag and G less than or similar to 20 mag, and it reveals the existence of two kinematically and spatially distinct subgroups. There is a distributed "off-cloud" population of stars located in the north of the dark clouds that is twice as numerous as the historically known "on-cloud" population that is concentrated around the densest cores. By comparing the location of the stars in the HR-diagram with evolutionary models, we show that these two populations are younger than 10 Myr. Based on their infrared excess emission, we identify 28 Class II and 215 Class III stars among the sources with available infrared photometry, and we conclude that the frequency of Class II stars (i.e. "disc-bearing" stars) in the on-cloud region is twice as large as compared to the off-cloud population. The distance derived for the Corona-Australis region based on this updated census is d = 149.4(-0.4)(+0.4) pc, which exceeds previous estimates by about 20 pc.
Conclusions. In this paper we provide the most complete census of stars in Corona-Australis available to date that can be confirmed with Gaia data. Furthermore, we report on the discovery of an extended and more evolved population of young stars beyond the region of the dark clouds, which was extensively surveyed in the past
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