R. A. Vazquez

Spiral structure of the Third Galactic Quadrant and the solution to the Canis Major debate

ABSTRACT With the discovery of the Sagittarius dwarf spheroidal (Ibata et al. 1994), a galaxycaught in the process of merging with the Milky Way, the hunt for other such ac-cretion events has become a very active field of astrophysical research. The identifi-cation of a stellar ring-like structure in Monoceros, spanning more than 100 degrees(Newberg et al. 2002), and the detection of an overdensity of stars in the directionof the constellation of Canis Major (CMa; Martin et al. 2004), apparently associatedto the ring, has led to the widespread belief that a second galaxy being cannibalisedby the Milky Way had been found. In this scenario, the overdensity would be theremaining core of the disrupted galaxy and the ring would be the tidal debris leftbehind. However, unlike the Sagittarius dwarf, which is well below the Galactic planeand whose orbit, and thus tidal tail, is nearly perpendicular to the plane of the MilkyWay, the putative CMa galaxy and ring are nearly co-planar with the Galactic disk.This severely complicates the interpretation of observations. In this letter, we showthat our new description of the Milky Way leads to a completely different picture. Weargue that the Norma-Cygnus spiral arm defines a distant stellar ring crossing Mono-ceros and the overdensity is simply a projection effect of looking along the nearbylocal arm. Our perspective sheds new light on a very poorly known region, the thirdGalactic quadrant (3GQ), where CMa is located.Key words: Galaxy: structure — open clusters and associations: general — Galaxy:stellar content — galaxies: dwarf

SPIRAL STRUCTURE IN THE OUTER GALACTIC DISK. I. THE THIRD GALACTIC QUADRANT

We combine optical and radio observations to trace the spiral structure in the third quadrant of the Milky Way. The optical observations consist of a large sample of young open clusters and associations, whereas the radio observations consist of a surveyofnearbyand distant clouds observedinCO. Boththe optical and radio samples are the largest ones thus far presented in the literature. We use this unique material to analyze the behavior of interstellar extinction and totracethedetailedstructureofthethirdGalacticquadrant(TGQ).Wefindthattheouter(Cygnus)granddesignspiral arm is traced by stellar and CO components, while the Perseus arm is traced solely by CO and is possibly being disrupted by the crossing of the Local (Orion) arm. The Local arm is traced by CO and young stars toward l ¼ 240 � and extends for over 8 kpc along the line of sight reaching the outer arm. Finally, we characterize the Galactic warp and compare the geometries implied by the young stellar and CO components.

Photometric study of the young open cluster NGC 3293

Deep and extensive CCD photometric observations at UB V(RI)CH were carried out in the area of the open cluster NGC 3293. The new data set allows to see the entire cluster sequence down to MV +4:5, revealing that stars with MV + 2a re placed above it. According to our analysis, the cluster distance is d= 2750 250 pc (V0 MV = 12:2 0:2) and its nuclear age is 8 1 Myr. NGC 3293 contains an important fraction of pre-main sequence (PMS) stars distributed along a parallel band to the ZAMS with masses from 1 to 2:5M and a mean contraction age of 10 Myr. This last value does not dier too much from the nuclear age estimate. If we take into account the many factors that may aect the PMS star positions on the colour- magnitude diagram, both ages can be perfectly reconciled. The star formation rate, on the other hand, suggests that NGC 3293 stars formed surely in one single event, therefore favouring a coeval process of star formation. Using the H data, we detected nineteen stars with signs of H emission in the region of NGC 3293, another indication that the star formation process is still active in the region. The computed initial mass function for the cluster has a slope of x= 1:2 0:2, a bit flatter than the typical slope for field stars and similar to the values found for other young open clusters.

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.

THE EDGE OF THE YOUNG GALACTIC DISK

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 toward the Vela-Puppis constellations, and within the core of the Canis Major overdensity. Their reddening and distances have been estimated by 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 disk has a sharp cutoff at about 14 kpc from the Galactic center and indicates that it extends to much greater distances (as also supported by the 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 04 lies in the external regions of the Norma-Cygnus arm, at a galactocentric distance (~20 kpc) where no young stars have been detected so far in the optical.

ASteCA: Automated Stellar Cluster Analysis

Fil: Perren, Gabriel Ignacio. Universidad Nacional de la Plata. Facultad de Ciencias Astronomicas y Geofisicas; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico La Plata. Instituto de Astrofisica de La Plata; Argentina; Argentina

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 10 8 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.

Stellar populations in the Canis Major overdensity

We performed a photometric multicolour survey of the core of the Canis Major overdensity at I ≈ 244°, b ≈ -8°0, reaching V ∼ 22 and covering 0.3 x 1.0 arcmin 2 . 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 of the Milky Way, we recognize a young stellar population compatible with the expected structure and extension of the Local (Orion) and Outer (Norma-Cygnus) 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.

Blue straggler stars in Galactic open clusters and the effect of field star contamination

Context. We investigate the distribution of blue straggler stars in the field of three open star clusters. Aims. 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. Methods. We analyze photometry taken from the literature of 3 open clusters of intermediate/old age rich in blue straggler stars, which are projected in the direction of the Perseus arm, and study their spatial distribution and the color magnitude diagram. Results. 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. Conclusions. As previously emphasized by many authors, a detailed membership analysis is mandatory before comparing the blue straggler population in star clusters to theoretical models. Moreover, these sequences of young field stars (blue plumes) are potentially powerful tracers of Galactic structure and they require further consideration.

Revisiting the area of the open cluster Stock¿16

Deep CCD UBV(RI)C photometry has been carried out in the area of the open cluster Stock 16. The different photo- metric diagrams indicate the presence of three superimposed stellar groups, all composed of early type stars. The first of them, the cluster Stock 16, suffers from slightly variable reddening with a mean color excessEB−V � = 0.51 . The second group, reddened byEB−V � = 0.84 , includes the Wolf-Rayet star WR 50 (a WC7+OB), and the third one, with a meanEB−V � = 1.18 , also includes another Wolf-Rayet star, WR 51 (a WN4+OB?). Our results confirm the distance and reddening already stated for Stock 16 but include new members. Concerning the other two highly reddened groups, we show that the first one - located at a distance of 3600 pc - is probably related to an anonymous OB association behind Cen OB1 also at 3600 pc already mentioned in the literature; the other seems to be a distant OB association - at more than 9000 pc - which may belong to the Scutum-Crux spiral arm. In these two groups we find evidence that the absorption law, AV /EB−V = R amounts to 4.0 approximately. The ages of the different star populations in the zone range from 5 × 10 6 yr to 6.4 × 10 6 yr for Stock 16, 8 × 10 6 for the second most reddened group and 5 × 10 6 for the probable association in Scutum-Crux. A determination of the IMF slope of Stock 16 was also carried out for stars in the mass range 1.8 < M < 14 M� , giving a normal value x = 1.3.