J. Alonso-García

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.

THE EDGE OF THE MILKY WAY STELLAR DISK REVEALED USING CLUMP GIANT STARS AS DISTANCE INDICATORS

We use the clump giants of the disk as standard candles calibrated from Hipparcos parallaxes in order to map their distribution with two new near-infrared surveys of the Galactic plane: UKIDSS-GPS and VISTA Variables in the Via Lactea (VVV). We explore different selection cuts of clump giants. We conclude that there is an edge of the stellar disk of the Milky Way at R = 13.9 ± 0.5 kpc along various lines of sight across the Galaxy. The effect of the warp is considered, taking fields at different longitudes and above and below the plane. We demonstrate that the edge of the stellar disk of the Milky Way can now be mapped in the near-infrared in order to test different models, and to establish our own place within the Galaxy.

Milky Way demographics with the VVV survey - I. The 84-million star colour–magnitude diagram of the Galactic bulge

Context. The Milky Way (MW) bulge is a fundamental Galactic componentfor understanding the formation and evolution of galaxies, in particular our own. The ESO Public Survey VISTA Variables in the V´ ia Lactea is a deep near-IR survey mapping the Galactic bulge and southern plane. Particularly for the bulge area, VVV is c overing∼ 315 deg 2 . Data taken during 2010 and 2011 covered the entire bulge area in the JHKs bands. Aims. We used VVV data for the whole bulge area as a single and homogeneous data set to build for the first time a single colour- magnitude diagram (CMD) for the entire Galactic bulge. Methods. Photometric data in the JHKs bands were combined to produce a single and huge data set containing 173, 150, 467 sources in the three bands, for the∼ 315 deg 2 covered by VVV in the bulge. Selecting only the data points fla gged as stellar, the total number of sources is 84, 095, 284. Results. We built the largest colour-magnitude diagrams published up to date, containing 173.1+ million sources for all data points, and more than 84.0 million sources accounting for the stellar sources only. The CMD has a complex shape, mostly owing to the complexity of the stellar population and the effects of extinction and reddening towards the Galactic centr e. The red clump (RC) giants are seen double in magnitude at b∼−8 ◦ − 10 ◦ , while in the inner part (b∼−3 ◦ ) they appear to be spreading in colour, or even splitting into a secondary peak. Stellar population models show the predominance of main-sequence and giant stars. The analysis of the outermost bulge area reveals a well-defined sequence of l ate K and M dwarfs, seen at (J− Ks)∼ 0.7− 0.9 mag and Ks& 14 mag. Conclusions. The interpretation of the CMD yields important information about the MW bulge, showing the fingerprint of its structure and content. We report a well-defined red dwarf sequence in th e outermost bulge, which is important for the planetary tran sit searches of VVV. The double RC in magnitude seen in the outer bulge is the signature of the X-shaped MW bulge, while the spreading of the RC in colour, and even its splitting into a secondary peak, ar e caused by reddening effects. The region around the Galactic centre is harder to interpret because it is strongly affected by reddening and extinction.

Stellar density profile and mass of the Milky Way bulge from VVV data

We present the first stellar density profile of the Milky Way bulge that reaches latitude b = 0◦. The profile was derived by counting red clump stars within the colour-magnitude diagram that was constructed using the new PSF-fitting photometry from VISTA Variables in the Via Lactea (VVV) survey data. The new stellar density map covers the area between |l| ≤ 10◦ and |b| ≤ 4.5◦ with unprecedented accuracy, allowing the stellar kinematics from the Giraffe Inner Bulge Spectroscopic Survey (GIBS) to be linked to the stellar mass density distribution. In particular, the location of the central velocity-dispersion peak from GIBS matches a high over-density in the VVV star count map. By scaling the total luminosity function (LF) obtained from all VVV fields to the LF from Zoccali et al.(2003), we obtain the first fully empirical estimate of the mass in stars and in remnants of the Galactic bulge. Within (|b| < 9.5◦, |l| < 10◦), the Milky Way bulge stellar mass is 2.0 ± 0.3 × 1010 M .

Discovery of VVV CL001. A Low-Mass Globular Cluster Next to UKS~1 in the Direction of the Galactic Bulge

Context. It is not known how many globular clusters may have been left undetected towards the Galactic bulge. Aims. One of the aims of the VISTA Variables in the Via Lactea (VVV) Survey is to accurately measure the physical parameters of the known globular clusters in the inner regions of the Milky Way and to search for new ones, hidden in regions of large extinction. Methods. Deep near infrared images give deep JHKS-band photometry of a region surrounding the known globular cluster UKS 1 and reveal a new low-mass globular cluster candidate that we name VVV CL001. Results. We use the horizontal branch red clump in order to measure E(B-V) 2.2 mag, (m M)0 = 16:01 mag, and D=15.9 kpc for the globular cluster UKS 1. Based on the near-infrared colour magnitude diagrams, we also measure that VVV CL001 has E(B-V) 2.0, and that it is at least as metal-poor as UKS 1, however, its distance remains uncertain. Conclusions. Our finding confirms the previous projection that the central region of the Milky Way harbors more globular clusters. VVV CL001 and UKS 1 are good candidates for a physical cluster binary, but follow-up observations are needed to decide if they are located at the same distance and have similar radial velocities.

Variable stars in the VVV globular clusters. I. 2MASS-GC 02 and Terzan 10

The VISTA Variables in the V?a L?ctea (VVV) ESO Public Survey is opening a new window to study inner Galactic globular clusters (GCs) using their variable stars. These GCs have been neglected in the past due to the difficulties caused by the presence of elevated extinction and high field stellar densities in their lines of sight. However, the discovery and study of any present variables in these clusters, especially RR Lyrae stars, can help to greatly improve the accuracy of their physical parameters. It can also help to shed some light on the questions raised by the intriguing Oosterhoff dichotomy in the Galactic GC system. In a series of papers we plan to explore variable stars in the GCs falling inside the field of the VVV survey. In this first paper, we search for and study the variables present in two highly reddened, moderately metal-poor, faint, inner Galactic GCs: 2MASS-GC 02 and Terzan 10. We report the discovery of sizable populations of RR Lyrae stars in both GCs. We use near-infrared period?luminosity relations to determine the color excess of each RR Lyrae star, from which we obtain both accurate distances to the GCs and the ratios of the selective-to-total extinction in their directions. We find the extinction toward both clusters to be elevated, non-standard, and highly differential. We also find both clusters to be closer to the Galactic center than previously thought, with Terzan 10 being on the far side of the Galactic bulge. Finally, we discuss their Oosterhoff properties, and conclude that both clusters stand out from the dichotomy followed by most Galactic GCs.

THE VVV SURVEY REVEALS CLASSICAL CEPHEIDS TRACING A YOUNG AND THIN STELLAR DISK ACROSS THE GALAXY’S BULGE

Solid insight into the physics of the inner Milky Way is key to understanding our Galaxys evolution, but extreme dust obscuration has historically hindered efforts to map the area along the Galactic mid-plane. New comprehensive near-infrared time-series photometry from the VVV Survey has revealed 35 classical Cepheids, tracing a previously unobserved component of the inner Galaxy, namely a ubiquitous inner thin disk of young stars along the Galactic mid-plane, traversing across the bulge. The discovered period (age) spread of these classical Cepheids implies a continuous supply of newly formed stars in the central region of the Galaxy over the last 100 million years.

Chemical abundances in bright giants of the globular cluster M62 (NGC 6266)

With the exception of Terzan 5, all the Galactic globular clusters that possess significant metallicity spreads, such as ω Cen and M22, are preferentially the more luminous clusters with extended horizontal branches. Here we present radial velocities and chemical abundances for seven bright giants in the globular cluster M62, a previously little-studied cluster. With MV = −9.18, M62 is the ninth most luminous Galactic globular cluster and has an extended horizontal branch. Within our sample, we find (i) no evidence for a dispersion in metallicity, [Fe/H], beyond the measurement uncertainties, (ii) star-to-star abundance variations for C, O, Na and Al with the usual correlations between these elements as seen in other globular clusters, and (iii) a global enrichment for the elements Zr, Ba and La at the level [X/Fe] ≃ +0.4 dex. For elements heavier than La, the abundance ratios are consistent with the scaled-solar r-process distribution. Below La, the abundances are anomalous when compared to the scaled-solar s-process or r-process distributions. For these elements, the abundance signature in M62 is in agreement with predictions of the s-process from fast-rotating massive stars, although the high [Rb/Y] ratio we measure may be a challenge to this scenario.

CONSTRAINTS ON HELIUM ENHANCEMENT IN THE GLOBULAR CLUSTER M3 (NGC 5272): THE HORIZONTAL BRANCH TEST*

It has recently been suggested that the presence of multiple populations showing various amounts of helium enhancement is the rule, rather than the exception, among globular star clusters. An important prediction of this helium enhancement scenario is that the helium-enhanced blue horizontal branch (HB) stars should be brighter than the red HB stars which are not helium enhanced. In this Letter, we test this prediction in the case of the Galactic globular cluster M3 (NGC 5272), for which the helium-enhancement scenario predicts helium enhancements of 0.02 in virtually all blue HB stars. Using high-precision Stromgren photometry and spectroscopic gravities for blue HB stars, we find that any helium enhancement among most of the clusters blue HB stars is very likely less than 0.01, thus ruling out the much higher helium enhancements that have been proposed in the literature.

The VVV Templates Project Towards an automated classification of VVV light-curves - I. Building a database of stellar variability in the near-infrared

Context. The Vista Variables in the Via Lactea (VVV) ESO Public Survey is a variability survey of the Milky Way bulge and an adjacent section of the disk carried out from 2010 on ESO Visible and Infrared Survey Telescope for Astronomy (VISTA). The VVV survey will eventually deliver a deep near-IR atlas with photometry and positions in five passbands (ZY JHKS) and a catalogue of 1−10 million variable point sources – mostly unknown – that require classifications. Aims. The main goal of the VVV Templates Project, which we introduce in this work, is to develop and test the machine-learning algorithms for the automated classification of the VVV light-curves. As VVV is the first massive, multi-epoch survey of stellar variability in the near-IR, the template light-curves that are required for training the classification algorithms are not available. In the first paper of the series we describe the construction of this comprehensive database of infrared stellar variability. Methods. First, we performed a systematic search in the literature and public data archives; second, we coordinated a worldwide observational campaign; and third, we exploited the VVV variability database itself on (optically) well-known stars to gather high-quality infrared light-curves of several hundreds of variable stars. Results. We have now collected a significant (and still increasing) number of infrared template light-curves. This database will be used as a training-set for the machine-learning algorithms that will automatically classify the light-curves produced by VVV. The results of such an automated classification will be covered in forthcoming papers of the series.Context. The Vista Variables in the Via Lactea (VVV) ESO Public Survey is a variability survey of the Milky Way bulge and an adjacent section of the disk carried out from 2010 on ESO Visible and Infrared Survey Telescope for Astronomy (VISTA). The VVV survey will eventually deliver a deep near-IR atlas with photometry and positions in five passbands (ZYJHKS) and a catalogue of 1−10 million variable point sources – mostly unknown – that require classifications. Aims. The main goal of the VVV Templates Project, which we introduce in this work, is to develop and test the machine-learning algorithms for the automated classification of the VVV light-curves. As VVV is the first massive, multi-epoch survey of stellar variability in the near-IR, the template light-curves that are required for training the classification algorithms are not available. In the first paper of the series we describe the construction of this comprehensive database of infrared stellar variability. Methods. First, we performed a systematic search in the literature and public data archives; second, we coordinated a worldwide observational campaign; and third, we exploited the VVV variability database itself on (optically) well-known stars to gather high-quality infrared light-curves of several hundreds of variable stars. Results. We have now collected a significant (and still increasing) number of infrared template light-curves. This database will be used as a training-set for the machine-learning algorithms that will automatically classify the light-curves produced by VVV. The results of such an automated classification will be covered in forthcoming papers of the series.