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Register a new userNew Galactic Star Clusters Discovered in the Disk Area of the VVVX Survey
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Radostin Georgiev Kurtev
Publication date
2018
fields
Physics
and research's language is
English
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The VISTA Variables in the Via Lactea eXtended (VVVX) ESO Public Survey is a near-infrared photometric sky survey that covers nearly 1700 sq. deg towards the Galactic disk and bulge. It is well-suited to search for new open clusters, hidden behind dust and gas. The pipeline processed and calibrated Ks-band tile images of 40% of the disk area covered by VVVX was visually inspected for stellar over-densities. Then, we identified cluster candidates by examination of the composite JHKs color images. The color-magnitude diagrams of the cluster candidates are constructed. Whenever possible the Gaia DR2 parameters are used to calculate the mean proper motions, radial velocities, reddening and distances. We report the discovery of 120 new infrared clusters and stellar groups. Approximately, half of them (47%) are faint, compact, highly reddened, and they seem to be associated with other indicators of recent star formation, such as nearby Young Stellar Objects, Masers, H II regions or bubbles. The preliminary distance determinations allow us to trace the clusters up to 4.5 kpc, but most of the cluster candidates are centered at 2.2 kpc. The mean proper motions of the clusters, show that in general, they follow the disk motion of the Galaxy.
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VISTA Variables in the V{i}a 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. 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. The disk area covered by VVV was visually inspected using the pipeline processed and calibrated $K_{rm S}$-band tile images for stellar overdensities. Subsequently, we examined the composite $JHK_{rm S}$ and $ZJK_{rm S}$ color images of each candidate. PSF photometry of $15times15$ 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. 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 5,Myr. 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.
Open clusters are unique tracers of the history of our own Galaxys disk. According to our membership analysis based on textit{Gaia} astrometry, out of the 226 potential clusters falling in the footprint of GALAH or APOGEE, we find that 205 have secure members that were observed by at least one of the survey. Furthermore, members of 134 clusters have high-quality spectroscopic data that we use to determine their chemical composition. We leverage this information to study the chemical distribution throughout the Galactic disk of 21 elements, from C to Eu. The radial metallicity gradient obtained from our analysis is $-$0.076$pm$0.009 dex kpc$^{-1}$, which is in agreement with previous works based on smaller samples. Furthermore, the gradient in the [Fe/H] - guiding radius (r$_{rm guid}$) plane is $-$0.073$pm$0.008 dex kpc$^{-1}$. We show consistently that open clusters trace the distribution of chemical elements throughout the Galactic disk differently than field stars. In particular, at given radius, open clusters show an age-metallicity relation that has less scatter than field stars. As such scatter is often interpreted as an effect of radial migration, we suggest that these differences are due to the physical selection effect imposed by our Galaxy: clusters that would have migrated significantly also had higher chances to get destroyed. Finally, our results reveal trends in the [X/Fe]$-$r$_{rm guid}$$-$age space, which are important to understand production rates of different elements as a function of space and time.
An earlier analysis of the Milky Way Star Cluster (MWSC) catalogue revealed an apparent lack of old (> 1 Gyr) open clusters in the solar neighbourhood (< 1 kpc). To fill this gap we undertook a search for hitherto unknown star clusters, assuming that the missing old clusters reside at high Galactic latitudes |b|> 20{deg}. We were looking for stellar density enhancements using a star count algorithm on the 2MASS point source catalogue. To increase the contrast between potential clusters and the field, we applied filters in colour-magnitude space according to typical colour-magnitude diagrams of nearby old open clusters. The subsequent comparison with lists of known objects allowed us to select thus far unknown cluster candidates. For verification they were processed with the standard pipeline used within the MWSC survey for computing cluster membership probabilities and for determining structural, kinematic, and astrophysical parameters. In total we discovered 782 density enhancements, 522 of which were classified as real objects. Among them 139 are new open clusters with ages 8.3 < log (t [yr]) < 9.7, distances d < 3 kpc, and distances from the Galactic plane 0.3 < Z < 1 kpc. This new sample has increased the total number of known high latitude open clusters by about 150%. Nevertheless, we still observe a lack of older nearby clusters up to 1 kpc from the Sun. This volume is expected to still contain about 60 unknown clusters that probably escaped our detection algorithm, which fails to detect sparse overdensities with large angular size.
In this work, we present a spectroscopic study of very massive stars found outside the center of the massive stellar cluster NGC3603. From the analysis of SOAR spectroscopic data and related optical-NIR photometry, we confirm the existence of several very massive stars in the periphery of NGC 3603. The first group of objects (MTT58, WR42e and RFS7) is compound by three new Galactic exemplars of the OIf*/WN type, all of them with probable initial masses well above 100 Msun and estimated ages of about 1 Myr. Based on Goodman blue-optical spectrum of MTT68, we can confirm the previous finding in the NIR of the only other Galactic exemplar (besides HD93129A) of the O2If* type known to date. Based on its position relative to a set of theoretical isochrons in a Hertzprung-Russel diagram, we concluded that the new O2If* star could be one of the most massive (150 Msun) and luminous (Mv=-7.3) O-star in the Galaxy. Also, another remarkable result is the discovery of a new O2V star (MTT31) that is the first exemplar of the class so far identified in the Milk Way. From its position in the Hertzprung-Russel diagram it is found that this new star probably had an initial mass of 80 Msun, as well as an absolute magnitude Mv=-6.0 corresponding to a luminosity similar to other known O2V stars in the LMC. We also communicate the discovery of a new Galactic O3.5If* star (RFS8) which case is quite intriguing. Indeed, It is located far to the south of the NGC 3603 center, in apparent isolation at a large radial projected linear distance of 62 pc. Its derived luminosity is similar to that of the other O3.5If* (Sh18) found in the NGC 3603s innermost region. The fact that a such high mass star is observed far isolated in the field led us to speculate that perhaps it could have been expelled from the innermost parts of the complex by a close fly-by dynamical encounter with a very massive hard binary system.
Until recently it was thought that high Galactic latitude clouds were a non-star-forming ensemble. However, in a previous study we reported the discovery of two embedded clusters (ECs) far away from the Galactic plane ($sim5$ kpc). In our recent star cluster catalogue we provided additional high and intermediate latitude cluster candidates. This work aims to clarify if our previous detection of star clusters far away from the disc represents just an episodic event or if the star cluster formation is currently a systematic phenomenon in the Galactic halo. We analyse the nature of four clusters found in our recent catalogue and report the discovery of three new ECs with unusually high latitude and distance from the Galactic disc midplane. All of these clusters are younger than 5 Myr. The high-latitude ECs C 932, C 934, and C 939 appear to be related to a cloud complex about 5 kpc below the Galactic disc, under the Local arm. The other clusters are above the disc, C 1074 and C 1100 with a vertical distance of $sim3$ kpc, C 1099 with $sim2$ kpc, and C 1101 with $sim1.8$ kpc. According to the derived parameters there occur ECs located below and above the disc, which is an evidence of widespread star cluster formation throughout the Galactic halo. Thus, this study represents a paradigm shift, in the sense that a sterile halo becomes now a host of ongoing star formation. The origin and fate of these ECs remain open. There are two possibilities for their origin, Galactic fountain or infall. The discovery of ECs far from the disc suggests that the Galactic halo is more actively forming stars than previously thought and since most ECs do not survive the textit{infant mortality} it may be raining stars from the halo into the disc, and/or the halo harbours generations of stars formed in clusters like those hereby detected.
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