841 research outputs found
Hunting for open clusters in \textit{Gaia} DR2: the Galactic anticentre
The Gaia Data Release 2 (DR2) provided an unprecedented volume of precise
astrometric and excellent photometric data. In terms of data mining the Gaia
catalogue, machine learning methods have shown to be a powerful tool, for
instance in the search for unknown stellar structures. Particularly, supervised
and unsupervised learning methods combined together significantly improves the
detection rate of open clusters. We systematically scan Gaia DR2 in a region
covering the Galactic anticentre and the Perseus arm and
, with the goal of finding any open clusters that may
exist in this region, and fine tuning a previously proposed methodology
successfully applied to TGAS data, adapting it to different density regions.
Our methodology uses an unsupervised, density-based, clustering algorithm,
DBSCAN, that identifies overdensities in the five-dimensional astrometric
parameter space that may correspond
to physical clusters. The overdensities are separated into physical clusters
(open clusters) or random statistical clusters using an artificial neural
network to recognise the isochrone pattern that open clusters show in a colour
magnitude diagram. The method is able to recover more than 75% of the open
clusters confirmed in the search area. Moreover, we detected 53 open clusters
unknown previous to Gaia DR2, which represents an increase of more than 22%
with respect to the already catalogued clusters in this region. We find that
the census of nearby open clusters is not complete. Different machine learning
methodologies for a blind search of open clusters are complementary to each
other; no single method is able to detect 100% of the existing groups. Our
methodology has shown to be a reliable tool for the automatic detection of open
clusters, designed to be applied to the full Gaia DR2 catalogue.Comment: 8 pages, accepted by Astronomy and Astrophysics (A&A) the 14th May,
2019. Tables 1 and 2 available at the CD
A ring in a shell: the large-scale 6D structure of the Vela OB2 complex
The Vela OB2 association is a group of 10 Myr stars exhibiting a complex
spatial and kinematic substructure. The all-sky Gaia DR2 catalogue contains
proper motions, parallaxes (a proxy for distance) and photometry that allow us
to separate the various components of Vela OB2. We characterise the
distribution of the Vela OB2 stars on a large spatial scale, and study its
internal kinematics and dynamic history. We make use of Gaia DR2 astrometry and
published Gaia-ESO Survey data. We apply an unsupervised classification
algorithm to determine groups of stars with common proper motions and
parallaxes. We find that the association is made up of a number of small
groups, with a total current mass over 2330 Msun. The three-dimensional
distribution of these young stars trace the edge of the gas and dust structure
known as the IRAS Vela Shell across 180 pc and shows clear signs of expansion.
We propose a common history for Vela OB2 and the IRAS Vela Shell. The event
that caused the expansion of the shell happened before the Vela OB2 stars
formed, imprinted the expansion in the gas the stars formed from, and most
likely triggered star formation.Comment: Accepted by A&A (02 November 2018), 13 pages, 9+2 figure
Can we identify non-stationary dynamics of trial-to-trial variability?"
Identifying sources of the apparent variability in non-stationary scenarios is a fundamental problem in many biological data analysis settings. For instance, neurophysiological responses to the same task often vary from each repetition of the same experiment (trial) to the next. The origin and functional role of this observed variability is one of the fundamental questions in neuroscience. The nature of such trial-to-trial dynamics however remains largely elusive to current data analysis approaches. A range of strategies have been proposed in modalities such as electro-encephalography but gaining a fundamental insight into latent sources of trial-to-trial variability in neural recordings is still a major challenge. In this paper, we present a proof-of-concept study to the analysis of trial-to-trial variability dynamics founded on non-autonomous dynamical systems. At this initial stage, we evaluate the capacity of a simple statistic based on the behaviour of trajectories in classification settings, the trajectory coherence, in order to identify trial-to-trial dynamics. First, we derive the conditions leading to observable changes in datasets generated by a compact dynamical system (the Duffing equation). This canonical system plays the role of a ubiquitous model of non-stationary supervised classification problems. Second, we estimate the coherence of class-trajectories in empirically reconstructed space of system states. We show how this analysis can discern variations attributable to non-autonomous deterministic processes from stochastic fluctuations. The analyses are benchmarked using simulated and two different real datasets which have been shown to exhibit attractor dynamics. As an illustrative example, we focused on the analysis of the rat's frontal cortex ensemble dynamics during a decision-making task. Results suggest that, in line with recent hypotheses, rather than internal noise, it is the deterministic trend which most likely underlies the observed trial-to-trial variability. Thus, the empirical tool developed within this study potentially allows us to infer the source of variability in in-vivo neural recordings
The extended halo of NGC 2682 (M 67) from Gaia DR2
Context: NGC 2682 is a nearby open cluster, approximately 3.5 Gyr old.
Dynamically, most open clusters should dissolve on shorter timescales, of ~ 1
Gyr. Having survived until now, NGC 2682 was likely much more massive in the
past, and is bound to have an interesting dynamical history. Aims: We
investigate the spatial distribution of NGC 2682 stars to constrain its
dynamical evolution, especially focusing on the marginally bound stars in the
cluster outskirts. Methods: We use Gaia DR2 data to identify NGC 2682 members
up to a distance of ~150 pc (10 degrees). Two methods (Clusterix and UPMASK)
are applied to this end. We estimate distances to obtain three-dimensional
stellar positions using a Bayesian approach to parallax inversion, with an
appropriate prior for star clusters. We calculate the orbit of NGC 2682 using
the GRAVPOT16 software. Results: The cluster extends up to 200 arcmin (50 pc)
which implies that its size is at least twice as previously believed. This
exceeds the cluster Hill sphere based on the Galactic potential at the distance
of NGC 2682. Conclusions: The extra-tidal stars in NGC 2682 may originate from
external perturbations such as disk shocking or dynamical evaporation from
two-body relaxation. The former origin is plausible given the orbit of NGC
2682, which crossed the Galactic disk ~40 Myr ago.Comment: 9 pages, 5 figures, accepted for publication on A&
Using street based metrics to characterize urban typologies
[EN] Urban spatial structures reflect local particularities produced during the development of a city. High spatial resolution imagery and LiDAR data are currently used to derive numerical attributes to describe in detail intra-urban structures and morphologies. Urban block boundaries have been frequently used to define the units for extracting metrics from remotely sensed data. In this paper, we propose to complement these metrics with a set of novel descriptors of the streets surrounding the urban blocks under consideration. These metrics numerically describe geometrical properties in addition to other distinctive aspects, such as presence and properties of vegetation and the relationship between the streets and buildings. For this purpose, we also introduce a methodology for partitioning the street area related to an urban block into polygons from which the street urban metrics are derived. We achieve the assessment of these metrics through application of a one-way ANOVA procedure, the winnowing technique, and a decision tree classifier. Our results suggest that street metrics, and particularly those describing the street geometry, are suitable for enhancing the discrimination of complex urban typologies and help to reduce the confusion between certain typologies. The overall classification accuracy increased from 72.7% to 81.1% after the addition street of descriptors. The results of this study demonstrate the usefulness of these metrics for describing street properties and complementing information derived from urban blocks to improve the description of urban areas. Street metrics are of particular use for the characterization of urban typologies and to study the dynamics of cities.The authors appreciate the financial support provided by the Spanish Ministry of Science and Innovation in the framework of
the project CGL2010-19591/BTE, and the data made available by the Spanish Instituto GeogrĂĄfico Nacional (IGN)Hermosilla, T.; Palomar-VĂĄzquez, J.; Balaguer Beser, ĂA.; Balsa Barreiro, J.; Ruiz FernĂĄndez, LĂ. (2014). Using street based metrics to characterize urban typologies. Computers, Environment and Urban Systems. 44:68-79. https://doi.org/10.1016/j.compenvurbsys.2013.12.002S68794
NGC 6705 a young -enhanced Open Cluster from OCCASO data
The stellar [/Fe] abundance is sometimes used as a proxy for stellar
age, following standard chemical evolution models for the Galaxy, as seen by
different observational results. In this work we show that the Open Cluster
NGC6705/M11 has a significant -enhancement [/Fe] dex,
despite its young age (300 Myr), challenging the current paradigm. We use
high resolution (R) high signal-to-noise (70) spectra of 8 Red
Clump stars, acquired within the OCCASO survey. We determine very accurate
chemical abundances of several elements, using an equivalent width
methodology (Si, Ca and Ti), and spectral synthesis fits (Mg and O). We obtain
[Si/Fe]=, [Mg/Fe]=, [O/Fe]=,
[Ca/Fe]= and [Ti/Fe]=. Our results place these
cluster within the group of young [/Fe]-enhanced field stars recently
found by several authors in the literature. The ages of our stars have an
uncertainty of around 50 Myr, much more precise than for field stars. By
integrating the cluster's orbit in several non-axisymmetric Galactic
potentials, we establish the M11's most likely birth radius to lie between
6.8-7.5 kpc from the Galactic center, not far from its current position. With
the robust Open Cluster age scale, our results prove that a moderate
[/Fe]-enhancement is no guarantee for a star to be old, and that not
all -enhanced stars can be explained with an evolved blue straggler
scenario. Based on our orbit calculations, we further argue against a Galactic
bar origin of M11.Comment: 10 pages, 6 figures, accepted in A&
Some comments on "The Mathematical Universe"
I discuss some problems related to extreme mathematical realism, focusing on
a recently proposed "shut-up-and-calculate" approach to physics
(arXiv:0704.0646, arXiv:0709.4024). I offer arguments for a moderate
alternative, the essence of which lies in the acceptance that mathematics is
(at least in part) a human construction, and discuss concrete consequences of
this--at first sight purely philosophical--difference in point of view.Comment: 11 page
The multiplicity fraction in 202 open clusters from Gaia
In this study, we estimate the fraction of binaries with high mass ratios for
202 open clusters in the extended solar neighbourhood (closer than 1.5 kpc from
the Sun). This is one of the largest homogeneous catalogues of multiplicity
fractions in open clusters to date, including the unresolved and total
(close-binary) multiplicity fractions of main-sequence systems with mass ratio
larger than . The unresolved multiplicity fractions are
estimated applying a flexible mixture model to the observed Gaia
colour-magnitude diagrams of the open clusters. Then we use custom Gaia
simulations to account for the resolved systems and derive the total
multiplicity fractions. The studied open clusters have ages between 6.6 Myr and
3.0 Gyr and total high-mass-ratio multiplicity fractions between 6% and 80%,
with a median of 18%. The multiplicity fractions increase with the mass of the
primary star, as expected. The average multiplicity fraction per cluster
displays an overall decreasing trend with the open cluster age up to ages about
100 Myr, above which the trend increases. Our simulations show that most of
this trend is caused by complex selection effects (introduced by the mass
dependence of the multiplicity fraction and the magnitude limit of our sample).
Furthermore, the multiplicity fraction is not significantly correlated with the
clusters' position in the Galaxy. The spread in multiplicity fraction decreases
significantly with the number of cluster members (used as a proxy for cluster
mass). We also find that the multiplicity fraction decreases with metallicity,
in line with recent studies using field stars.Comment: 17 pages, 13 figures, resubmitted to A&A following referee comment
The Ontology of Intentional Agency in Light of Neurobiological Determinism: Philosophy Meets Folk Psychology
The moot point of the Western philosophical rhetoric about free will
consists in examining whether the claim of authorship to intentional, deliberative
actions fits into or is undermined by a one-way causal framework of determinism.
Philosophers who think that reconciliation between the two is possible are known as
metaphysical compatibilists. However, there are philosophers populating the other
end of the spectrum, known as the metaphysical libertarians, who maintain that claim
to intentional agency cannot be sustained unless it is assumed that indeterministic
causal processes pervade the action-implementation apparatus employed by the agent.
The metaphysical libertarians differ among themselves on the question of whether the
indeterministic causal relation exists between the series of intentional states and
processes, both conscious and unconscious, and the action, making claim for what has
come to be known as the event-causal view, or between the agent and the action,
arguing that a sort of agent causation is at work. In this paper, I have tried to propose
that certain features of both event-causal and agent-causal libertarian views need to be
combined in order to provide a more defendable compatibilist account accommodating
deliberative actions with deterministic causation. The ââagent-executed-eventcausal
libertarianismââ, the account of agency I have tried to develop here, integrates
certain plausible features of the two competing accounts of libertarianism turning
them into a consistent whole. I hope to show in the process that the integration of these
two variants of libertarianism does not challenge what some accounts of metaphysical
compatibilism proposeâthat there exists a broader deterministic relation between the
web of mental and extra-mental components constituting the agentâs dispositional
systemâthe agentâs beliefs, desires, short-term and long-term goals based on them,
the acquired social, cultural and religious beliefs, the general and immediate and
situational environment in which the agent is placed, etc. on the one hand and the
decisions she makes over her lifetime on the basis of these factors. While in the
ââIntroductionââ the philosophically assumed anomaly between deterministic causation
and the intentional act of deciding has been briefly surveyed, the second section is
devoted to the task of bridging the gap between compatibilism and libertarianism. The
next section of the paper turns to an analysis of folk-psychological concepts and
intuitions about the effects of neurochemical processes and prior mental events on the
freedom of making choices. How philosophical insights can be beneficially informed
by taking into consideration folk-psychological intuitions has also been discussed,
thus setting up the background for such analysis. It has been suggested in the end that
support for the proposed theory of intentional agency can be found in the folk-psychological intuitions, when they are taken in the right perspective
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