G. L. Baume

New Galactic star clusters discovered in the VVV survey

Context. VISTA Variables in the V´oa Lactea (VVV) is one of the six ESO Public Surveys operating on the new 4-meter Visible and Infrared Survey Telescope for Astronomy (VISTA). VVV is scanning the Milky Way bulge and an adjacent section of the disk, where star formation activity is high. One of the principal goals of the VVV Survey is to find new star clusters of different ages. Aims. In order to trace the early epochs of star cluster formation we concentrated our search in the directions to those of known star formation regions, masers, radio, and infrared sources. Methods. The disk area covered by VVV was visually inspected using the pipeline processed and calibrated KS-band tile images for stellar overdensities. Subsequently, we examined the composite JHKS and ZJKS color images of each candidate. PSF photometry of 15 × 15 arcmin fields centered on the candidates was then performed on the Cambridge Astronomy Survey Unit reduced images. After statistical field-star decontamination, color-magnitude and color-color diagrams were constructed and analyzed. Results. We report the discovery of 96 new infrared open clusters and stellar groups. Most of the new cluster candidates are faint and compact (with small angular sizes), highly reddened, and younger than 5Myr. For relatively well populated cluster candidates we derived their fundamental parameters such as reddening, distance, and age by fitting the solar- metallicity Padova isochrones to the color-magnitude diagrams.

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.

Massive open star clusters using the VVV survey - I. Presentation of the data and description of the approach

AA: U. de ConcepciA3n, U. de ValparaA-so; AB: U. de ValparaA-so, The Milky Way Millennium Nucleus; AC: U. de ValparaA-so, U. of Hertfordshire; AD: U. Federal do Rio Grande do Sul; AE: Saint Marys University; AF: U. de ConcepciA3n; AG: U. de ValparaA-so, PUC de Chile; AH: U. de ConcepciA3n; AI: U. de ValparaA-so; AJ: IALP; AK: IALP; AL: ESO; AM: U. de ConcepciA3n; AN: The Milky Way Millennium Nucleus, PUC de Chile; AO: The Milky Way Millennium Nucleus, PUC de Chile, Vatican Observatory, Princeton University; AP: U. of Hertfordshire; AQ: Kavli Institute for Astronomy and Astrophysics

Massive open star clusters using the VVV survey - II. Discovery of six clusters with Wolf-Rayet stars

Context. The ESO Public Survey “VISTA Variables in the Via Lactea” (VVV) provides deep multi-epoch infrared observations for an unprecedented 562 sq. degrees of the Galactic bulge, and adjacent regions of the disk. Nearly 150 new open clusters and cluster candidates have been discovered in this survey. Aims. This is the second in a series of papers about young, massive open clusters observed using the VVV survey. We present the first study of six recently discovered clusters. These clusters contain at least one newly discovered Wolf-Rayet (WR) star. Methods. Following the methodology presented in the first paper of the series, wide-field, deep JHKs VVV observations, combined with new infrared spectroscopy, are employed to constrain fundamental parameters for a subset of clusters. Results. We find that the six studied stellar groups are real young (2–7 Myr) and massive (between 0.8 and 2.2 × 10 3 M� ) clusters. They are highly obscured (AV ∼ 5−24 mag) and compact (1–2 pc). In addition to WR stars, two of the six clusters also contain at least one red supergiant star, and one of these two clusters also contains a blue supergiant. We claim the discovery of 8 new WR stars, and 3 stars showing WR-like emission lines which could be classified WR or OIf. Preliminary analysis provides initial masses of 30

THE PROPER MOTION OF THE MAGELLANIC CLOUDS. I. FIRST RESULTS AND DESCRIPTION OF THE PROGRAM

We present the first results of a ground-based program to determine the proper motion of the Magellanic Clouds (MCs) relative to background quasars (QSO), being carried out using the Irenee du Pont 2.5 m telescope at Las Campanas Observatory, Chile. Eleven QSO fields have been targeted in the Small Magellanic Cloud (SMC) over a time base of six years, and with seven epochs of observation. One quasar field was targeted in the Large Magellanic Cloud (LMC), over a time base of five years, and with six epochs of observation. The shorter time base in the case of the LMC is compensated by the much larger amount of high-quality astrometry frames that could be secured for the LMC quasar field (124 frames), compared to the SMC fields (an average of roughly 45 frames). In this paper, we present final results for field Q0557–6713 in the LMC and field Q0036–7227 in the SMC. From field Q0557–6713, we have obtained a measured proper motion of μαcos δ = +1.95 ± 0.13 mas yr–1, μδ = +0.43 ± 0.18 mas yr–1 for the LMC. From field Q0036–7227, we have obtained a measured proper motion of μα cosδ = +0.95 ± 0.29 mas yr–1, μδ = –1.14 ± 0.18 mas yr–1 for the SMC. Although we went through the full procedure for another SMC field (QJ0036–7225), on account of unsolvable astrometric difficulties caused by blending of the QSO image, it was impossible to derive a reliable proper motion. Current model rotation curves for the plane of the LMC indicate that the rotational velocity (V rot) at the position of LMC field Q0557–6713 can be as low as 50 km s–1, or as high as 120 km s–1. A correction for perspective and rotation effects leads to a center of mass proper motion for the LMC of μα cosδ = +1.82 ± 0.13 mas yr–1, μδ = +0.39 ± 0.15 mas yr–1 (V rot = 50 km s–1), and to μα cosδ = +1.61 ± 0.13 mas yr–1, μδ = +0.60 ± 0.15 mas yr–1 (V rot = 120 km s–1). Assuming that the SMC has a disk-like central structure, but that it does not rotate, we obtain a center of mass proper motion for the SMC of μα cosδ = +1.03 ± 0.29 mas yr–1, μδ = –1.09 ± 0.18 mas yr–1. Our results are in reasonable agreement with most previous determinations of the proper motion of the MCs, including recent Hubble Space Telescope measurements. Complemented with published values of the radial velocity of the centers of the LMC and SMC, we have used our proper motions to derive the galactocentric (gc) velocity components of the MCs. For the LMC, we obtain V gc,t = +315 ± 20 km s–1, V gc,r = +86 ± 17 km s–1 (V rot = 50 km s–1), and V gc,t = +280 ± 24 km s–1, V gc,r = +94 ± 17 km s–1 (V rot = 120 km s–1). For the SMC, we obtain V gc,t = +258 ± 50 km s–1, V gc,r = +20 ± 44 km s–1. These velocities imply a relative velocity between the LMC and SMC of 84 ± 50 km s–1, for V rot,LMC = 50 km s–1, and 62 ± 63 km s–1 for V rot,LMC = 120 km s–1. Albeit our large errors, these values are not inconsistent with the standard assumption that the MCs are gravitationally bound to each other.

A study of the Galactic plane towards l= 305°

We present optical (UBVI C ) observations of a rich and complex field in the Galactic plane towards l ~ 305° and b~ 0°. Our analysis reveals a significantly high interstellar absorption (A v ~ 10) and an abnormal extinction law in this line of sight. Availing a considerable number of colour combinations, the photometric diagrams allow us to derive new estimates of the fundamental parameters of the two open clusters Danks 1 and Danks 2. Due to the derived abnormal reddening law in this line of sight, both clusters appear much closer (to the Sun) than previously thought. Additionally, we present the optical colours and magnitudes of the WR 48a star, and its main parameters were estimated. The properties of the two embedded clusters, DBS2003 130 and 131, are also addressed. We identify a number of young stellar objects which are probable members of these clusters. This new material is then used to revisit the spiral structure in this sector of the Galaxy showing evidence of populations associated with the inner Galaxy Scutum-Crux arm.

The distance to the young open cluster Westerlund 2

A new X-ray, UBVRIc ,a ndJHKs study of the young cluster Westerlund 2 was undertaken to resolve discrepancies tied to the cluster’s distance. Existing spectroscopic observations for bright cluster members and new multi-band photometry imply a reddening relation toward Westerlund 2 described by EU−B/EB−V = 0.63 + 0.02 EB−V . Variable-extinction analyses for Westerlund 2 and nearby IC 2581 based upon spectroscopic distance moduli and ZAMS fitting yield values of RV = AV /EB−V = 3.88±0.18 and 3.77±0.19, respectively, and confirm prior assertions that anomalous interstellar extinction is widespread throughout Carina. The results were confirmed by applying the color-difference method to UBVRIcJHKs data for 19 spectroscopically observed cluster members, yielding RV = 3.85 ± 0.07. The derived distance to Westerlund 2 of d = 2.85 ± 0.43 kpc places the cluster on the far side of the Carina spiral arm. The cluster’s age is no more thanτ ∼ 2 × 10 6 yr as inferred from the cluster’s brightest stars and an X-ray (Chandra) cleaned analysis of its pre-main-sequence demographic. Four Wolf-Rayet stars in the cluster core and surrounding corona (WR20a, WR20b, WR20c, and WR20aa) are very likely cluster members, and their inferred luminosities are consistent with those of other late-WN stars in open clusters. The color–magnitude diagram for Westerlund 2 also displays a gap at spectral type B0.5 V with associated color spread at higher and lower absolute magnitudes that might be linked to close binary mergers. These features, in conjunction with the evidence for mass loss from the WR stars, may help to explain the high flux of γ-rays, cosmic rays, and X-rays from the direction toward Westerlund 2.