NGC 5385, NGC 2664 and Collinder 21: three candidate Open Cluster Remnants

We present CCD UBVI photometric and medium/high resolution spectroscopic observations obtained in the field of the previously unstudied dissolving open cluster candidates NGC 5385, NGC 2664 and Collinder~21. Our analysis stands on the discussion of star counts, photometry, radial velocity distribution, and proper motions available from the Tycho~2 catalogue. All the three aggregates clearly emerge from the mean Galactic field, but, regrettably, the close scrutiny of proper motions and radial velocities reveals that we are not facing any physical group. Instead, what we are looking at are just chance alignments of a few bright unrelated stars. Our analysis casts some doubt on the Bica et al. (2001) criterion to look for Possible Open Cluster Remnants. It seems mandatory to define a better criterion to adopt for further investigations.Comment: 12 pages, 12 eps figures (7, 9 and 11 degraded in resolution), accepted for publication in Astronomy & Astrophysic

No evidence of dark matter in the solar neighborhood

We measured the surface mass density of the Galactic disk at the solar position, up to 4 kpc from the plane,by means of the kinematics of ~400 thick disk stars. The results match the expectations for the visible mass only, and no dark matter is detected in the volume under analysis. The current models of dark matter halo are excluded with a significance higher than 5sigma, unless a highly prolate halo is assumed, very atypical in cold dark matter simulations. The resulting lack of dark matter at the solar position challenges the current models.Comment: Proceeding of the first binational Sochias-AAA meeting, held in San Juan, Argentin

Old open clusters in the outer Galactic disk

The outer parts of the Milky Way disk are believed to be one of the main arenas where the accretion of external material in the form of dwarf galaxies and subsequent formation of streams is taking place. The Monoceros stream and the Canis Major and Argo over-densities are notorious examples. VLT high resolution spectra have been acquired for five distant open clusters. We derive accurate radial velocities to distinguish field interlopers and cluster members. For the latter we perform a detailed abundance analysis and derive the iron abundance [Fe/H] and the abundance ratios of several $\alpha$ elements. Our analysis confirms previous indications that the radial abundance gradient in the outer Galactic disk does not follow the expectations extrapolated from the solar vicinity, but exhibits a shallower slope. By combining the metallicity of the five program clusters with eight more clusters for which high resolution spectroscopy is available, we find that the mean metallicity in the outer disk between 12 and 21 kpc from the Galactic center is [Fe/H] $\approx -0.35$, with only marginal indications for a radial variation. In addition, all the program clusters exhibit solar scaled or slightly enhanced $\alpha$ elements, similar to open clusters in the solar vicinity and thin disk stars. We investigate whether this outer disk cluster sample might belong to an extra-galactic population, like the Monoceros ring. However, close scrutiny of their properties - location, kinematics and chemistry - does not convincingly favor this hypothesis. On the contrary, they appear more likely genuine Galactic disk clusters. We finally stress the importance to obtain proper motion measurements for these clusters to constrain their orbits.Comment: 19 pages, 9 eps figure, in press in A&A, abstract rephrased to fit i

Metal abundances in extremely distant Galactic old open clusters. II. Berkeley 22 and Berkeley 66

We report on high resolution spectroscopy of four giant stars in the Galactic old open clusters Berkeley~22 and Berkeley~66 obtained with HIRES at the Keck telescope. We find that $[Fe/H]=-0.32\pm0.19$ and $[Fe/H]=-0.48\pm0.24$ for Berkeley~22 and Berkeley~66, respectively. Based on these data, we first revise the fundamental parameters of the clusters, and then discuss them in the context of the Galactic disk radial abundance gradient. We found that both clusters nicely obey the most updated estimate of the slope of the gradient from \citet{fri02} and are genuine Galactic disk objects.Comment: 20 pages, 6 eps figures, accepted for publication in the Astronomical Journa

Spectroscopy and BVI photometry of the young open cluster NGC 6604

BVI photometry (from South Africa Astron. Obs.), Echelle high resolution spectroscopy and AFOSC integral field spectroscopy (from Asiago, Italy) of the young open cluster NGC 6604 are presented. Age, distance, reddening, membership, radial and rotational velocities are derived and discussed. An age of 5 million years, a distance of 1.7 kpc and a reddening E(B-V)=1.02 are found. The cluster radial velocity is in agreement with the Hron (1987) model for the Galaxy disk rotation. Pre-ZAMS objects are not present down to M_V = +1.5 mag.Comment: accepted in Astron.Astrophys.Suppl. Figure 2 is degraded in resolutio

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

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&

A new look at Sco OB1 association with Gaia DR2

We present and discuss photometric optical data in the area of the OB association Sco OB1 covering about 1 squared degree. UBVI photometry is employed in tandem with Gaia DR2 data to investigate the 3 dimensional structure and the star formation history of the region. By combining parallaxes and proper motions we identify 7 physical groups located between the young open cluster NGC 6231 and the bright nebula IC4628. The most prominent group coincides with the sparse open cluster Trumpler 24. We confirm the presence of the intermediate age star cluster VdB-Hagen 202, which is unexpected in this environment, and provide for the first time estimates of its fundamental parameters. After assessing individual groups membership, we derive mean proper motion components, distances, and ages. The seven groups belong to two different families. To the younger family (family I) belong several pre-Main Sequence stars as well. These are evenly spread across the field, and also in front of VdB-Hagen 202. VdB-Hagen 202 and two smaller, slightly detached, groups of similar properties form family II, which do not belong to the association, but are caught in the act of passing through it. As for the younger population, this forms an arc-like structure from the bright nebula IC 4628 down to NGC 6231, as previously found. Moreover, the pre-Main Sequence stars density seems to increase from NGC 6231 northward to Trumpler 24