Taking the census of star clusters in the Milky Way

With the advent of Gaia and its all-sky accurate astrometry and photometry for almost two billion sources, a broad interest in open clusters (OCs) has resurged, having become the subject of active research. While in pre-Gaia times, there were two main reference catalogues of open clusters (Dias et al. 2002, and Kharchenko et al. 2013), the wealth of Gaia data together with recent developments in machine learning techniques, availability of open source software, compute power, and a new generation of researchers trained in these methods has brought an explosion of reported OC discoveries. These reported discoveries are usually published in catalogues, in which the cross-identification with previously known OCs is done with various degrees of rigour, leading to "discoveries" of clusters that were in fact known and other situations illustrated in this contribution. Indeed, it is a delicate task to cross-identify OCs, which are often sparse and discrete stellar groups with irregular shapes, different sizes, and without clear boundaries. The universally employed method of relying on a reported cluster centre and (highly uncertain) radius for cross-identification is clearly not producing high-quality compilations of OCs. Moreover, nowadays when hardly a month passes without the publication of a new catalogue of reported discoveries, it has become extremely hard to veto and integrate new discoveries into a carefully curated compilation of all known OCs. To address the issues above, we present a framework being developed for cross-identifying OCs and for building a master list of the known OCs and candidates. Noting that OCs are defined by their members, we develop member-based approaches for cross-identification instead of employing cluster coordinates and radii-based matches. To support the storage and analysis of the growing number of catalogues, we have built a system of two databases. The first is a data warehouse with the original tables of clusters and/or members from the literature (most of which are automatically retrieved from the CDS), and whose formats, fields and other descriptors are quite heterogeneous. The other one is our primary analysis database where cluster identifiers, parameters and stellar memberships are stored in a structured and homogeneous way, allowing an easy integration and analysis of new catalogues as they arrive. In addition to the generated master list, statistical reports are generated which can be used for tuning the matching criteria. Finally, the framework supports optional filters for validating/checking the reality of catalogued OCs

Stellar populations in the Canis Major overdensity

We performed a photometric multicolour survey of the core of the Canis Major overdensity at l ≈ 244°, b ≈ —8°.0, reaching V ~ 22 and covering 0.3 x 1.0 arcmin2. The main aim is to unravel the complex mixture of stellar populations toward this Galactic direction, where in the recent past important signatures of an accretion event have been claimed to be detected. While our previous investigations were based on disjointed pointings aimed at revealing the large-scale structure of the third Galactic Quadrant, we now focus on a complete coverage of a smaller field centred on the Canis Major overdensity. A large wavelength baseline, in the UBVRI bands, allows us to build up a suite of colour-colour and colour-magnitude diagrams, providing a much better diagnostic tool to disentangle the stellar populations of the region. In fact, the simple use of one colour-magnitude diagram, widely employed in all the previous studies defending the existence of the Canis Major galaxy, does not allow one to separate the effects of the different parameters (reddening, age, metallicity and distance) involved in the interpretation of data, forcing to rely on heavy modelling. In agreement with our previous studies, in the same general region ofthe Milky Way, we recognize a young stellar population compatible with the expected structure and extension of the Local (Orion) and Outer (NormaCygnus) spiral arms in the Third Galactic Quadrant. Moreover, we interpret the conspicuous intermediate-age metal-poor population as belonging to the Galactic thick disc, distorted by the effect of strong disc warping at this latitude, and to the Galactic halo.Facultad de Ciencias Astronómicas y Geofísica

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

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

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

Detection of a young stellar population in the background of open clusters in the third galactic quadrant

We report the detection of a young stellar population (≤100 Myr) in the background of nine young open clusters belonging to a homogenous sample of 30 star clusters in the third Galactic quadrant (at 217° ≤ l ≤ 260°). Deep and accurate UBVRI photometry allows us to measure model-independent age and distance for the clusters and the background population with high confidence. This population is exactly the same population (the blue plume) recently detected in three intermediate-age open clusters and suggested to be a ≤1-2 Gyr old population belonging to the Canis Major (CMa) overdensity (Bellazzini et al.; Martinez-Delgado et al.). However, we find that the young population in those three clusters and in six clusters of our sample follows the pattern of the Norma-Cygnus spiral arm as defined by CO clouds remarkably well, while in the other three program clusters it lies in the Perseus arm. We finally provide one example (out of 21) of a cluster that does not show any background population, demonstrating that this population is not ubiquitous toward CMa.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Open clusters in the Third Galactic Quadrant. III: alleged binary clusters

Aims. We aim to determine accurate distances and ages of eight open clusters in order to: (1) assess their possible binarity (2) provide probes to trace the structure of the Third Galactic Quadrant. Methods. Cluster reddenings, distances, ages and metallicities are derived from ZAMS and isochrone fits in UBVRI photometric diagrams. Field contamination is reduced by restricting analysis to stars within the cluster limits derived from star counts. Further membership control is done by requiring that stars have consistent positions in several diagrams and by using published spectral types. Results. The derived distances, ages and metallicities have shown that none of the analysed clusters compose binary/double systems. Of the four candidate pairs, only NGC 2383/NGC 2384 are close to each other, but have different metallicities and ages. Ruprecht 72 and Ruprecht 158 are not clusters but fluctuations of the field stellar density. Haffner 18 is found to be the superposition of two stellar groups at different distances: Haffner 18(1) at 4.5 kpc and Haffner 18(2) between 9.5 and 11.4 kpc from the Sun. The derived distances and ages have been used to situate the clusters in the Galactic context. In particular, young stellar groups trace spiral structure at large Galactocentric radii. At least two clusters formed during the last few 108 yr in an interstellar medium with less than solar abundances. Conclusions. In contrast with the LMC, double clusters are apparently rare, or even non existent, in the undisturbed environment of the Third Galactic Quadrant. This leaves open the question of whether binary clusters form more easily toward denser and more violent regions of the Milky Way such as the inner Galaxy.Fil: Vazquez, Ruben Angel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Astrofísica de La Plata; ArgentinaFil: Moitinho, André. Universidade de Lisboa. Faculdade de Ciencias; PortugalFil: Carraro, Giovanni. European Southern Observatory; ChileFil: Dias, Wilton S.. Universidade Federal de Itajubá. UNIFEI Instituto de Ciencias Exatas; Brasi

Observational properties of the open cluster system of the Milky Way and what they tell us about our Galaxy

Almost 80 years have passed since Trumpler's analysis of the Galactic open cluster system laid one of the main foundations for understanding the nature and structure of the Milky Way. Since then, the open cluster system has been recognised as a key source of information for addressing a wide range of questions about the structure and evolution of our Galaxy. Over the last decade, surveys and individual observations from the ground and space have led to an explosion of astrometric, kinematic and multiwavelength photometric and spectroscopic open cluster data. In addition, a growing fraction of these data is often time-resolved. Together with increasing computing power and developments in classification techniques, the open cluster system reveals an increasingly clearer and more complete picture of our Galaxy. In this contribution, I review the observational properties of the Milky Way's open cluster system. I discuss what they can and cannot teach us now and in the near future about several topics such as the Galaxy's spiral structure and dynamics, chemical evolution, large-scale star formation, stellar populations and more.Comment: 11 pages, 3 figures, to appear in proc. IAUS 266 "Star clusters: basic galactic building blocks "(eds. R. de Grijs, J. R. D. Lepine

OCAAT: Automated analysis of star cluster colour-magnitude diagrams for gauging the local distance scale

Star clusters are among the fundamental astrophysical objects used in setting the local distance scale. Despite its crucial importance, the accurate determination of the distances to the Magellanic Clouds (SMC/LMC) remains a fuzzy step in the cosmological distance ladder. The exquisite astrometry of the recently launched ESA Gaia mission is expected to deliver extremely accurate statistical parallaxes, and thus distances, to the SMC/LMC. However, an independent SMC/LMC distance determination via main sequence fitting of star clusters provides an important validation check point for the Gaia distances. This has been a valuable lesson learnt from the famous Hipparcos Pleiades distance discrepancy problem. Current observations will allow hundreds of LMC/SMC clusters to be analyzed in this light. Today, the most common approach for star cluster main sequence fitting is still by eye. The process is intrinsically subjective and affected by large uncertainties, especially when applied to poorly populated clusters. It is also, clearly, not an efficient route for addressing the analysis of hundreds, or thousands, of star clusters. These concerns, together with a new attitude towards advanced statistical techniques in astronomy and the availability of powerful computers, have led to the emergence of software packages designed for analyzing star cluster photometry. With a few rare exceptions, those packages are not publicly available. Here we present OCAAT (Open Cluster Automated Analysis Tool), a suite of publicly available open source tools that fully automatises cluster isochrone fitting. The code will be applied to a large set of hundreds of open clusters observed in the Washington system, located in the Milky Way and the Magellanic Clouds. This will allow us to generate an objective and homogeneous catalog of distances up to ~ 60 kpc along with its associated reddening, ages and metallicities and uncertainty estimates.Facultad de Ciencias Astronómicas y Geofísica