UPMASK: unsupervised photometric membership assignment in stellar clusters

We develop a method for membership assignment in stellar clusters using only photometry and positions. The method, UPMASK, is aimed to be unsupervised, data driven, model free, and to rely on as few assumptions as possible. It is based on an iterative process, principal component analysis, clustering algorithm, and kernel density estimations. Moreover, it is able to take into account arbitrary error models. An implementation in R was tested on simulated clusters that covered a broad range of ages, masses, distances, reddenings, and also on real data of cluster fields. Running UPMASK on simulations showed that it effectively separates cluster and field populations. The overall spatial structure and distribution of cluster member stars in the colour-magnitude diagram were recovered under a broad variety of conditions. For a set of 360 simulations, the resulting true positive rates (a measurement of purity) and member recovery rates (a measurement of completeness) at the 90% membership probability level reached high values for a range of open cluster ages ($10^{7.1}-10^{9.5}$ yr), initial masses ($0.5-10\times10^3$M_{\sun}) and heliocentric distances ($0.5-4.0$ kpc). UPMASK was also tested on real data from the fields of the open cluster Haffner~16 and of the closely projected clusters Haffner~10 and Czernik~29. These tests showed that even for moderate variable extinction and cluster superposition, the method yielded useful cluster membership probabilities and provided some insight into their stellar contents. The UPMASK implementation will be available at the CRAN archive.Comment: 12 pages, 13 figures, accepted for publication in Astronomy and Astrophysic

NGC 2401: A template of the Norma-Cygnus Arm's young population in the Third Galactic Quadrant

Based on a deep optical CCD (UBV(RI)_C) photometric survey and on the Two-Micron All-Sky-Survey (2MASS) data we derived the main parameters of the open cluster NGC 2401. We found this cluster is placed at 6.3 $\pm$ 0.5 kpc (V_O - M_V = 14.0 \pm 0.2) from the Sun and is 25 Myr old, what allows us to identify NGC 2401 as a member of the young population belonging to the innermost side of the extension of the Norma-Cygnus spiral--arm in the Third Galactic Quadrant. A spectroscopic study of the emission star LSS 440 that lies in the cluster area revealed it is a B0Ve star; however, we could not confirm it is a cluster member. We also constructed the cluster luminosity function (LF) down to $V \sim 22$ and the cluster initial mass function (IMF) for all stars with masses above M \sim 1-2 M_{\sun}. It was found that the slope of the cluster IMF is $x \approx 1.8 \pm 0.2$. The presence of a probable PMS star population associated to the cluster is weakly revealed.Comment: 10 paginas, 11 eps figures, accepted for publication in MNRA

Evidence of tidal distortions and mass loss from the old open cluster NGC 6791

We present the first evidence of clear signatures of tidal distortions in the density distribution of the fascinating open cluster NGC 6791. We used deep and wide-field data obtained with the Canada-France-Hawaii-Telescope covering a 2x2 square degrees area around the cluster. The two-dimensional density map obtained with the optimal matched filter technique shows a clear elongation and an irregular distribution starting from ~300" from the cluster center. At larger distances, two tails extending in opposite directions beyond the tidal radius are also visible. These features are aligned to both the absolute proper motion and to the Galactic center directions. Moreover, other overdensities appear to be stretched in a direction perpendicular to the Galactic plane. Accordingly to the behaviour observed in the density map, we find that both the surface brightness and the star count density profiles reveal a departure from a King model starting from ~600" from the center. These observational evidence suggest that NGC 6791 is currently experiencing mass loss likely due to gravitational shocking and interactions with the tidal field. We use this evidence to argue that NGC 6791 should have lost a significant fraction of its original mass. A larger initial mass would in fact explain why the cluster survived so long. Using available recipes based on analytic studies and N-body simulations, we derived the expected mass loss due to stellar evolution and tidal interactions and estimated the initial cluster mass to be M_ini=(1.5-4) x 10^5 M_sun.Comment: Accepted for publication in the MNRAS (9 pages, 8 Figures

The edge of the young Galactic disc

In this work we report and discuss the detection of two distant diffuse stellar groups in the third Galactic quadrant. They are composed of young stars, with spectral types ranging from late O to late B, and lie at galactocentric distances between 15 and 20 kpc. These groups are located in the area of two cataloged open clusters (VdB-Hagen~04 and Ruprecht~30), projected towards the Vela-Puppis constellations, and within the core of the Canis Major over-density. Their reddening and distance has been estimated analyzing their color-color and color-magnitude diagrams, derived from deep $UBV$ photometry. The existence of young star aggregates at such extreme distances from the Galactic center challenges the commonly accepted scenario in which the Galactic disc has a sharp cut-off at about 14 kpc from the Galactic center, and indicates that it extends to much greater distances (as also supported by recent detection of CO molecular complexes well beyond this distance). While the groups we find in the area of Ruprecht~30 are compatible with the Orion and Norma-Cygnus spiral arms, respectively, the distant group we identify in the region of VdB-Hagen~4 lies in the external regions of the Norma-Cygnus arm, at a galactocentric distance ($\sim$20 kpc) where no young stars had been detected so far in the optical.Comment: 45 pages, 11 eps figure, accepted for publication in the Astrophysical Journa

Blue Straggler Stars in Galactic Open Clusters and the effect of field star contamination

We investigate the distribution of Blue Straggler stars in the field of three open star clusters. The main purpose is to highlight the crucial role played by general Galactic disk fore-/back-ground field stars, which are often located in the same region of the Color Magnitude Diagram as Blue Straggler stars. We analyze photometry taken from the literature of 3 open clusters of intermediate/old age rich in Blue Straggler stars, and which are projected in the direction of the Perseus arm, and study their spatial distribution and the Color Magnitude Diagram. As expected, we find that a large portion of the Blue Straggler population in these clusters are simply young field stars belonging to the spiral arm. This result has important consequences on the theories of the formation and statistics of Blue Straggler stars in different population environments: open clusters, globular clusters or dwarf galaxies. As previously emphasized by many authors, a detailed membership analysis is mandatory before comparing the Blue Straggler population in star clusters against theoretical models. Moreover, these sequences of young field stars (blue plumes) are potentially powerful tracers of Galactic structure which require further consideration.Comment: 11 pages, 4 figurs, in press as Research Note in A&

The long bar as seen by the VVV Survey: II. Star counts

Context: There is still some debate about the presence and the morphological properties of the long bar in the inner Galaxy. Aims: We investigate the morphological properties of the long Galactic bar using the VVV survey extending star counts at least 3 mag deeper than 2MASS. Our study covers the relatively unexplored negative longitudes of the Galactic bar. We obtain a detailed description of the spatial distribution of star counts towards the long Galactic bar as well as to measure its parameters. Methods: We performed star counts towards -20<l<0 deg., |b|< 2 deg. using VVV, 2MASS, and GLIMPSE data. We applied an average interstellar extinction correction. We also adjusted latitudinal profiles to obtain the centroid variation and bar thickness. Results: We probe the structure of long Galactic bar, as well as its far edge at l=-14 deg. The differences between counts with and without extinction correction allow us to produce a crude extinction map showing regions with high extinction, mainly beyond the end of long Galactic bar. The latitudinal profiles show evidence of the centroid vertical variation with Galactic longitude reaching a minimum at l=-13.8 deg. The bar has an inclination angle 43+/-5 deg with respect to the line Sun-Galactic center. In addition, we have determined the bar parameters, such as thickness, length, and stellar distribution.Comment: Accepted for publication at Astronomy \& Astrophysics (17 pages, 17 figures). Second version: 1- Only minor changes on the abstract of the manuscript. 2- Abstract of arXiv.org modified to be in accordingly to Astronomy & Astrophysics abstract structur

Photometry of a Galactic field at l = 232, b = -6. The old open cluster Auner 1, the Norma-Cygnus spiral arm and the signature of the warped Galactic Thick Disk

We perform a detailed photometric study of the stellar populations in a Galactic Field at l = 232, b = -6 in the Canis Major (CMa) constellation. We present the first U,B,V,I photometry of the old open cluster Auner1 and determine it to be 3.25 Gyr old and to lie at 8.9 kpc from the Sun. In the background of the cluster, at more than 9 kpc, we detect a young population most probably associated to the Norma Cygnus spiral arm. Furthermore, we detect the signature of an older population and identify its Turn Off and Red Giant Branch. This population is found to have a mean age of 7 Gyrs and a mean metallicity of Z = 0.006 . We reconstruct the geometry of the stellar distribution and argue that this older population - often associated to the Canis Major {\it galaxy}- belongs in fact to the warped old thin/thick disk component along this line of sight.Comment: 19 pages, 7 eps figures (some degraded), accepted for publication in the Astronomical Journa

Funções Executivas (Frontal Assessment Battery), Capacidade Visuo Construtiva e Memória (Rey Complex Figure) numa Amostra de Idosos sob Resposta Social

Introdução: As funções executivas (FE) têm sido sistematicamente associadas ao funcionamento dos lobos frontais e sabemos que o declínio cognitivo se associa a piores resultados em provas que avaliam estas funções. Para além de pretendermos analisar se um teste que avalia as FE (Frontal Assessment Battery/FAB) discrimina idosos com/sem declínio cognitivo, avaliado através do Montreal Cognitive Assessment (MoCA) e se existem associações entre os resultados obtidos com a Figura Complexa de Rey-Osterreith/FCR-O (qualidade da cópia, memória de 3 e de 20 minutos) e a presença/ausência de declínio cognitivo, queremos sobretudo analisar se a qualidade e exactidão da cópia (capacidade visuo construtiva) e a memória de 3 minutos na FCR-O se associam ao resultado obtido com a FAB, dado que as duas provas estão supostamente associadas às FE e ao funcionamento dos lobos frontais avaliados com o FAB, por oposição à memória de 20 minutos (supostamente associada à área temporal, não avaliada pela FAB). Não deixámos de considerar, ainda, a associação entre as variáveis sociodemográficas e os resultados na FAB, na FCR-O e no MoCA. Metodologia: A amostra total incluiu 556 idosos (média de idades, M = 80,2; Desvio-padrão, DP = 5,23; variação = 60-100) sob resposta social em diferentes instituições do Concelho de Coimbra que aceitou responder voluntariamente (ou cujos familiares/cuidadores concederam consentimento) a uma bateria de testes (incluindo questões sociodemográficas, a FCR-O, o MoCA e a FAB). Estas variáveis foram estratificadas de acordo com a idade e escolaridade dos idosos e dicotomizadas. Para testar os nossos objetivos recorremos a diferentes sub amostras compostas pelos sujeitos que tinham resultados nas provas cujas associações queríamos testar. Resultados: De acordo com o MoCA, 59,7% dos idosos apresentavam declínio cognitivo, com 73,9% a apresentar défice executivo ligeiro, de acordo com a FAB. Quanto à FCR-O, 24,0% dos idosos apresentavam défice práxico ligeiro a moderado, 73,9% défice mnésico visual a curto prazo leve (3 minutos) e 60.9% défice mnésico visual a longo prazo leve a moderado (20 minutos). Não se verificaram associações estatisticamente significativas entre o género, estado civil e tipo de resposta social e as três variáveis centrais do estudo (MoCA, FAB e FCR-O). Quer a FAB, quer a FCR-O (as três provas: qualidade e exatidão da cópia, memória de 3 e 20 minutos) revelaram associações com a ausência/presença de declínio cognitivo. Considerando as variáveis estratificadas pela idade e escolaridade dos idosos e dicotomizadas, um teste do qui quadrado para a independência mostrou que a prova qualidade da FCR-O não estava associada ao resultado na FAB (com/sem défice executivo), ao contrário da prova memória de 3 minutos da FCR-O, que se mostrou associada a este resultado. A prova memória de 20 minutos da FCR-O voltou a não estar associada à ausência/presença de défice executivo (usando o mesmo teste). Analisando as diferentes provas da FCR-O e a FAB, sem estratificação, correlações de Spearman confirmaram as associações encontradas, mas também entre a prova qualidade da cópia da FCR-O e a FAB. Conclusão/Discussão: Os resultados seguem a literatura quanto à associação entre a prova da FCR-O memória a curto prazo/3 minutos e as funções executivas (associadas aos lobos frontais e testadas através da FAB), por oposição com a memória a longo prazo que aparece, na literatura, como mais associada/ou envolvendo à/a área temporal e que, de facto, não se mostrou associada ao resultado na FAB. Os resultados não são tão claros/consensuais no que toca à prova qualidade da cópia da FCR-O