3,201 research outputs found
A short note on the nested-sweep polarized traces method for the 2D Helmholtz equation
We present a variant of the solver in Zepeda-N\'u\~nez and Demanet (2014),
for the 2D high-frequency Helmholtz equation in heterogeneous acoustic media.
By changing the domain decomposition from a layered to a grid-like partition,
this variant yields improved asymptotic online and offline runtimes and a lower
memory footprint. The solver has online parallel complexity that scales
\emph{sub linearly} as , where is
the number of volume unknowns, and is the number of processors, provided
that . The variant in Zepeda-N\'u\~nez and Demanet
(2014) only afforded . Algorithmic scalability is a
prime requirement for wave simulation in regimes of interest for geophysical
imaging.Comment: 5 pages, 5 figure
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
New membership determination and proper motions of NGC 1817. Parametric and non-parametric approach
We have calculated proper motions and re-evaluated the membership
probabilities of 810 stars in the area of two NGC objects, NGC 1817 and NGC
1807. We have obtained absolute proper motions from 25 plates in the reference
system of the Tycho-2 Catalogue. The plates have a maximum epoch difference of
81 years; and they were taken with the double astrograph at Zo-Se station of
Shanghai Observatory, which has an aperture of 40 cm and a plate scale of 30
arcsec/mm. The average proper motion precision is 1.55 mas/yr. These proper
motions are used to determine the membership probabilities of stars in the
region, based on there being only one very extended physical cluster: NGC 1817.
With that aim, we have applied and compared parametric and non-parametric
approaches to cluster/field segregation. We have obtained a list of 169
probable member stars.Comment: 11 pages, 8 figures, A&A in pres
Pressure effects in the triangular layered cobaltites NaxCoO2
We have measured transport properties as a function of temperature and
pressure up to 30GPa in the NaxCoO2 system. For the x=0.5 sample the transition
temperature at 53K increases with pressure, while paradoxically the sample
passes from an insulating to a metallic ground state. A similar transition is
observed in the x=0.31 sample under pressure. Compression on the x=0.75 sample
transforms the sample from a metallic to an insulating state. We discuss our
results in terms of interactions between band structure effects and Na+ order.Comment: 18 pages, 5 figure
PREFACE: The Milky Way Unravelled by Gaia: GREAT Science from the Gaia Data Releases
International audiencePREFAC
Mott-Hubbard quantum criticality in paramagnetic CMR pyrochlores
We present a correlated {\it ab initio} description of the paramagnetic phase
of TlMnO, employing a combined local density approximation (LDA)
with multiorbital dynamical mean field theory (DMFT) treatment. We show that
the insulating state observed in this colossal magnetoresistance (CMR)
pyrochlore is determined by strong Mn intra- and inter-orbital local
electron-electron interactions. Hybridization effects are reinforced by the
correlation-induced spectral weight transfer. Our result coincides with optical
conductivity measurements, whose low energy features are remarkably accounted
for by our theory. Based on this agreement, we study the disorder-driven
insulator-metal transition of doped compounds, showing the proximity of
TlMnO to quantum phase transitions, in agreement with recent
measurements.Comment: 4 pages, 4 figure
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&
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