arXiv subjects

W. S. Dias

Publications and source records attributed to W. S. Dias.

32 records · Page 2Linked to original sources

arXiv2018

Catching Galactic open clusters in advanced stages of dynamical evolution

During their dynamical evolution, Galactic open clusters (OCs) gradually lose their stellar content mainly because of internal relaxation and tidal forces. In this context, the study of dynamically evolved OCs is necessary to properly understand such processes. We present a comprehensive Washington $CT_1$ photometric analysis of six sparse OCs, namely: ESO 518-3, Ruprecht 121, ESO 134-12, NGC 6573, ESO 260-7 and ESO 065-7. We employed Markov chain Monte-Carlo simulations to robustly determine the central coordinates and the structural parameters and $T_1\times(C-T_1)$ colour-magnitude diagrams (CMDs) cleaned from field contamination were used to derive the fundamental parameters. ESO 518-03, Ruprecht 121, ESO 134-12 and NGC 6573 resulted to be of nearly the same young age (8.2 $\leq\textrm{log}(t \textrm{yr}^{-1})\leq$ 8.3); ESO 260-7 and ESO065-7 are of intermediate age (9.2 $\leq\textrm{log}(t \textrm{yr}^{-1})\leq$ 9.4). All studied OCs are located at similar Galactocentric distances (R$_{G}\sim6-6.9 $kpc), considering uncertainties, except for ESO 260-7 ($R_{G}=8.9 $kpc). These OCs are in a tidally filled regime and are dynamically evolved, since they are much older than their half-mass relaxation times ($t/t_{rh}\gtrsim30$) and present signals of low-mass star depletion. We distinguished two groups: those dynamically evolving towards final disruptions and those in an advanced dynamical evolutionary stage. Although we do not rule out that the Milky Way potential could have made differentially faster their dynamical evolutions, we speculate here with the possibility that they have been mainly driven by initial formation conditions.

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arXiv2017

A Multi-Membership Catalogue for 1876 Open Clusters using UCAC4 data

The main objective of this work is to determine the cluster members of 1876 open clusters, using positions and proper motions of the astrometric catalogue UCAC4. For this purpose we apply three different methods, all them based on a Bayesian approach, but with different formulations: a purely parametric method, another completely non-parametric algorithm, and a third, recently developed by Sampedro & Alfaro, using both formulations at different steps of the whole process. The first and second statistical moments of the members phase-space subspace, obtained after applying the three methods, are compared for every cluster. Although, on average, the three methods yield similar results, specific differences between them, as well as for some particular clusters, are also present. The comparison with other published catalogues shows good agreement. We have also estimated for the first time the mean proper motion for a sample of 18 clusters. The results are organized in a single catalogue formed by two main files, one with the most relevant information for each cluster, partially including that in UCAC4, and the other showing the individual membership probabilities for each star in the cluster area. The final catalogue, with an interface design that enables an easy interaction with the user, is available in electronic format at SSG-IAA (http://ssg.iaa.es/en/content/sampedro-cluster-catalog) website.

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arXiv2017

Unidirectional quantum walk of two correlated particles: Separating bound-pair and unbound wavepacket components

We study the unidirectional transport of two-particle quantum wavepackets in a regular one-dimensional lattice. We show that the bound-pair state component behaves differently from unbound states when subjected to an external pulsed electric field. Thus, strongly entangled particles exhibit a quite distinct dynamics when compared to a single particle system. With respect to centroid motion, our numerical results are corroborated with an analytical expression obtained using a semi-classical approach. The wavefunction profile reveals that the particle-particle interaction induces the splitting of the initial wavepacket into two branches that propagate with specific directions and drift velocities. With a proper external field tunning, the wavepacket components can perform an unidirectional transport on the same or opposite directions. The amplitude of each mode is related to the degree of entanglement betweem particles, which presents a non-monotonic dependence on the interaction strength.

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arXiv2016

Models for the 3-D axisymmetric gravitational potential of the Milky Way Galaxy - A detailed modelling of the Galactic disk

Aims. Galaxy mass models based on simple and analytical functions for the density and potential pairs have been widely proposed in the literature. Disk models constrained by kinematic data alone give information on the global disk structure only very near the Galactic plane. We attempt to circumvent this issue by constructing disk mass models whose three-dimensional structures are constrained by a recent Galactic star counts model in the near-infrared and also by observations of the hydrogen distribution in the disk. Our main aim is to provide models for the gravitational potential of the Galaxy that are fully analytical but also with a more realistic description of the density distribution in the disk component. Methods. From the disk model directly based on the observations (here divided into the thin and thick stellar disks and the HI and H$_2$ disks subcomponents), we produce fitted mass models by combining three Miyamoto-Nagai disk profiles of any "model order" (1, 2, or 3) for each disk subcomponent. The Miyamoto-Nagai disks are combined with models for the bulge and "dark halo" components and the total set of parameters is adjusted by observational kinematic constraints. A model which includes a ring density structure in the disk, beyond the solar Galactic radius, is also investigated. Results. The Galactic mass models return very good matches to the imposed observational constraints. In particular, the model with the ring density structure provides a greater contribution of the disk to the rotational support inside the solar circle. The gravitational potential models and their associated force-fields are described in analytically closed forms, and in addition, they are also compatible with our best knowledge of the stellar and gas distributions in the disk component. The gravitational potential models are suited for investigations of orbits in the Galactic disk.

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arXiv2015

Quantum entanglement and drifting generated by an AC field resonant with frequency-doubled Bloch oscillations of correlated particles

We show that initially localized and uncorrelated two-particles quantum wavepackets evolving in a one-dimensional discrete lattice become strongly entangled while drifting under the action of an harmonic AC field resonant with doubled Bloch oscillations promoted by a static DC field. Although partial entanglement is achieved when the AC field is resonant with the single-particle Bloch oscillations, it is strongly limited by the survival of anti-correlated unbounded states. We further show that the phase dependence of the wavepacket centroid velocity is similar to the semiclassical behavior depicted by a single-particle. However, the drift velocity exhibits a non-trivial non-monotonic dependence on the interaction strength, vanishing in the limit of uncorrelated particles, that unveils its competing influence on unbounded and bounded states.

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arXiv2015

Photometric and proper motion study of neglected open cluster NGC 2215

Optical UBVRI photometric measurements using the Faulkes Telescope North were taken in early 2011 and combined with 2MASS JHK$_s$ and WISE infrared photometry as well as UCAC4 proper motion data in order to estimate the main parameters of the galactic open cluster NGC 2215 of which large uncertainty exists in the current literature. Fitting a King model we estimate a core radius of 1.12$'\pm$0.04$'$ (0.24$\pm$0.01pc) and a limiting radius of $4.3'\pm$0.5$'$ (0.94$\pm$0.11pc) for the cluster. The results of isochrone fits indicates an age of $log(t)=8.85\pm0.10$ with a distance of $d=790\pm90$pc, a metallicity of $[Fe/H]=-0.40\pm0.10$ dex and a reddening of $E(B-V)=0.26\pm0.04$. A proportion of the work in this study was undertaken by Australian and Canadian upper secondary school students involved in the Space to Grow astronomy education project, and is the first scientific publication to have utilized our star cluster photometry curriculum materials.

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arXiv2013

Fitting isochrones to open cluster photometric data III. Estimating metallicities from UBV photometry

The metallicity is a critical parameter that affects the correct determination fundamental characteristics stellar cluster and has important implications in Galactic and Stellar evolution research. Fewer than 10 % of the 2174 currently catalog open clusters have their metallicity determined in the literature. In this work we present a method for estimating the metallicity of open clusters via non-subjective isochrone fitting using the cross-entropy global optimization algorithm applied to UBV photometric data. The free parameters distance, reddening, age, and metallicity simultaneously determined by the fitting method. The fitting procedure uses weights for the observational data based on the estimation of membership likelihood for each star, which considers the observational magnitude limit, the density profile of stars as a function of radius from the center of the cluster, and the density of stars in multi-dimensional magnitude space. We present results of [Fe/H] for nine well-studied open clusters based on 15 distinct UBV data sets. The [Fe/H] values obtained in the ten cases for which spectroscopic determinations were available in the literature agree, indicating that our method provides a good alternative to determining [Fe/H] by using an objective isochrone fitting. Our results show that the typical precision is about 0.1 dex.

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arXiv2011

Overlapping abundance gradients and azimuthal gradients related to the spiral structure of the Galaxy

The connection between some features of the metallicity gradient in the Galactic disk, best revealed by Open Clusters and Cepheids, and the spiral structure, is explored. The step-like abrupt decrease in metallicity at 8.5 kpc (with R_0= 7.5 kpc, or at 9.5 kpc if R_0 = 8.5 kpc is adopted) is well explained by the corotation ring-shaped gap in the density of gas, which isolates the internal and external regions of the disk one from the other. This solves a long standing problem of understanding the different chemical characteristics of the inner and outer parts of the disk. The time required to build up the metallicity difference between the two sides of the step is a measure of the minimal life-time of the present grand-design spiral pattern structure, of the order of 3 Gyr. The plateaux observed on each side of the step are interpreted in terms of the large scale radial motion of the stars and of the gas flow induced by the spiral structure. The star-formation rate revealed by the density of open clusters is maximum in the Galactic radial range from 6 to 12 kpc (with an exception of a narrow gap at corotation), coinciding with the region where the 4-arms mode is allowed to exist. We argue that most of the old open clusters situated at large galactocentric radii were born in this inner region where conditions more favorable to star-formation are found. The ratio of $α$-elements to Fe of the sample of Cepheids does not vary appreciably with the Galactic radius, which reveals an homogeneous history of star formation. Different arguments are given showing that usual approximations of chemical evolution models, which assume fast mixing of metallicity in the azimuthal direction and ignore the existence of the spiral arms, are a poor ones.

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arXiv2010

Fitting Isochrones to Open Cluster photometric data: A new global optimization tool

We present a new technique to fit color-magnitude diagrams of open clusters based on the Cross-Entropy global optimization algorithm. The method uses theoretical isochrones available in the literature and maximizes a weighted likelihood function based on distances measured in the color-magnitude space. The weights are obtained through a non parametric technique that takes into account the star distance to the observed center of the cluster, observed magnitude uncertainties, the stellar density profile of the cluster among others. The parameters determined simultaneously are distance, reddening, age and metallicity. The method takes binary fraction into account and uses a Monte-Carlo approach to obtain uncertainties on the determined parameters for the cluster by running the fitting algorithm many times with a re-sampled data set through a bootstrapping procedure. We present results for 9 well studied open clusters, based on 15 distinct data sets, and show that the results are consistent with previous studies. The method is shown to be reliable and free of the subjectivity of most previous visual isochrone fitting techniques.

astro-ph.IM↗

arXiv2009

UBV(RI)_C photometry of the open clusters Be 15, Be 80, and NGC 2192

The three open clusters Be 15, Be 80, and NGC 2192 have been observed using CCD UBV(RI)_C photometry at the San Pedro Mártir Observatory, México within the framework of our open-cluster survey. The fundamental parameters of interstellar reddening, distance, and age have been derived, and also the metallicity for NGC 2192 (solar metallicity has been assumed for the other two). By shifting the colours of Schmidt-Kaler in the (U-B, B-V) two-colour diagram along the appropriate reddening vector, the interstellar reddenings have been derived as E(B-V)=0.23+/-0.03 mag for Be 15, 1.31+/-0.05 for Be 80, and 0.16+/-0.03 for NGC 2192. Evidence is shown for a variable interstellar extinction across the cluster Be 80. For NGC 2192 a nicely consistent fit is obtained for both the interstellar reddening and the metallicity ([Fe/H]=-0.31) using simultaneously the F-type and red-clump stars. By fitting isochrones to the observed sequences of these three clusters in various colour-magnitude diagrams of different colour indices, (B-V, V-I, or V-R) the averages of distance moduli and heliocentric distances ((V-M_V)_o (mag); d(pc)) are the following: (10.74+/-0.01; 1202) for Be 15, (10.75+/-0.01; 1413) for Be 80, and (12.7+/-0.01; 3467) for NGC 2192, and the averages of the inferred best ages (log(age); age (Gyr)) are (8.6+/-0.05; 0.4) for Be 80, and (9.15+/-0.05; 1.4) for NGC 2192; for Be 15 there are two distinct possibilities for the age fit, depending on the membership of three brighter stars, (9.35 or 9.95+/-0.05; 2.2 or 8.9). The need for spectroscopic observations in Be 15 is emphasized to select between alternate reddening and age solutions, and for deeper UBV observations in Be 80 to study in greater detail the variable interstellar, or intracluster, reddening across this cluster.

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arXiv2005

Direct determination of the spiral pattern rotation speed of the Galaxy

The rotation velocity of the spiral pattern of the Galaxy is determined by direct observation of the birthplaces of open clusters of stars in the galactic disk as a function of their age. Our measurement does not depend on any specific model of the spiral structure, like the existence of a given number of spiral arms, or the presence of a bar in the central regions. This study became possible due to the recent completion of a large database on open clusters by our group. The birthplaces of the clusters are determined by two methods, one that assumes that the orbits are circular, and the other by integrating the orbits in the Galactic potential for a time equal to the age of the clusters. We selected in the database a sample of 212 clusters for which proper motions, radial velocities, distances and ages are available, or of 612 clusters that have ages and distances available. We tested different assumptions concerning the rotation curve and the radius $R_0$ of the solar orbit. Our results confirm that a dominant fraction of the open clusters are formed in spiral arms, and that the spiral arms rotate like a rigid body, as predicted by the classical theory of spiral waves. We find that the corotation radius $R_c$ is close to the solar galactic orbit ($R_c/R_0 = 1.08 \pm 0.08$). This proximity has many potentially interesting consequences, like a better preservation of life on the Earth, and a new understanding of the history of star formation in the solar neighborhood, and of the evolution of the abundance of elements in the galactic disk.

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arXiv2003

Searching for unknown open clusters in the Tycho-2 catalog

We present 11 new open cluster candidates found in a systematic search for unknown star clusters using the astrometric and photometric data included in the Tycho2 catalog. The possible existence of these stellar aggregates is supported by the analysis of proper motions, color-magnitude diagrams, stellar density distributions, and by the visual inspection of the Digitized Sky Survey (DSS) plates. With these tools we were able to determine mean absolute proper motions as well as preliminary reddenings, distances and ages for the majority of the candidates. We found that most of them are possibly nearby (closer than about 600 pc) open clusters never studied before.

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arXiv2002

New catalogue of optically visible open clusters and candidates

We have compiled a new catalogue of open clusters in the Galaxy which updates the previous catalogues of Lynga (1987) and of Mermilliod (1995) (included in the WEBDA database). New objects and new data, in particular, data on kinematics (proper motions) that were not present in the old catalogues, have been included. Virtually all the clusters (1537) presently known were included, which represents an increment of about 347 objects relative to the Lynga (1987) catalogue. The catalogue is presented in a single table containing all the important data, which makes it easy to use. The catalogue can be accessed on line either at http://www.iagusp.usp.br/~wilton/ or as an electronic table which will be made available at the CDS.

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arXiv2000

Proper motion and membership determination in the young open cluster NGC1662

Relative proper motions and cluster membership probabilities of 30 stars within an area of 14' of diameter centered in the open cluster NGC1662 (α=04^{\rm h}48^{\rm m}, δ=+10^{\rm o}56') are determined by combining positions of stars observed by the Valinhos CCD meridian circle with those from other astrometric catalogues. Our basic source for first epoch of stars brighter than B=13 magnitudes is the AC2000 Catalogue, which provides a time baseline of about 90 years, whereas the USNO-A2.0 Catalogue is used for fainter stars, providing a time baseline of about 40 years in this region. Average accuracies in proper motion of 2 mas/yr and 7 mas/yr are achieved, respectively, when the AC2000 or the USNO-A2.0 is used. Membership determination is obtained by applying the Zhao & He (1990) method. The cluster proper motion was found to be (-3.2\pm 0.6) mas/yr in right ascension and (-1.6\pm 0.6) mas/yr in declination.

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