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We present the results of CCD UBVRI observations of the open cluster NGC 6811 obtained on 18th July 2012 with the 1m telescope at the TUB.ITAK National Observatory (TUG). Using these photometric results, we determine the structural and astrophysical parameters of the cluster. The mean photometric uncertainties are better than 0.02 mag in the V magnitude and B-V, V-R, and V-I colour indices to about 0.03 mag for U-B among stars brighter than magnitude V=18. Cluster member stars were separated from the field stars using the Galaxia model of Sharma et al. (2011) together with other techniques. The core radius of the cluster is found to be $r_{c}$=3.60 arcmin. The astrophysical parameters were determined simultaneously via Bayesian statistics using the colour-magnitude diagrams V versus B-V, V versus V-I, V versus V-R, and V versus R-I of the cluster. The resulting most likely parameters were further confirmed using independent methods, removing any possible degeneracies. The colour excess, distance modulus, metallicity and the age of the cluster are determined simultaneously as E(B-V)=0.05$pm$0.01 mag, $mu=10.06pm0.08$ mag, [M/H]=-0.10$pm$0.01 dex and t=1.00$pm$0.05 Gyr, respectively. Distances of five red clump stars which were found to be members of the cluster further confirm our distance estimation.
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From $uvby-beta$ photometry of the open clusters NGC 6811 (75 stars), and NGC 6830 (19 stars) we were able to determine membership of the stars to each cluster, and fix the age and reddening for each. Since several short period stars have recently been found, we have carried out a study of these variables.
We present the results of CCD $UBV$ photometric and spectroscopic observations of the open cluster NGC 225. In order to determine the structural parameters of NGC 225, we calculated the stellar density profile in the clusters field. We estimated the probabilities of the stars being physical members of the cluster using the existing astrometric data. The most likely members of the cluster were used in the determination of the astrophysical parameters of the cluster. We calculated the mean radial velocity of the cluster as $V_{r}=-8.3pm 5.0$ km s$^{-1}$ from the optical spectra of eight stars in the clusters field. Using the U-B vs B-V two-colour diagram and UV excesses of the F-G type main-sequence stars, the reddening and metallicity of NGC 225 were inferred as $E(B-V)=0.151pm 0.047$ mag and $[Fe/H]=-0.11pm 0.01$ dex, respectively. We fitted the colour-magnitude diagrams of NGC 225 with the PARSEC isochrones and derived the distance modulus, distance and age of the cluster as $mu_{V}=9.3pm 0.07$ mag, d=585$pm$20 pc and $t=900pm 100$ Myr, respectively. We also estimated the galactic orbital parameters and space velocity components of the cluster and found that the cluster has a slightly eccentric orbit of $e=0.07pm 0.01$ and an orbital period of $P_{orb}= 255pm 5$ Myr.
We report on a UV-oriented imaging survey in the fields of the old, metal-rich open clusters, NGC 6791, NGC 6819 and NGC 7142. These three clusters represent both very near and ideal stellar aggregates to match the distinctive properties of the evolved stellar populations, as in elliptical galaxies and bulges of spirals. The CMD of the three clusters is analyzed in detail, with special emphasis to the hot stellar component. We report, in this regard, one new extreme horizontal-branch star candidate in NGC 6791. For NGC 6819 and 7142, the stellar luminosity function points to a looser radial distribution of faint lower Main Sequence stars, either as a consequence of cluster dynamical interaction with the Galaxy or as an effect of an increasing fraction of binary stars toward the cluster core, as actually observed in NGC 6791 too.
We studied solar-like oscillations in 115 red giants in the three open clusters NGC 6791, NGC 6811, and NGC 6819, based on photometric data covering more than 19 months with NASAs Kepler space telescope. We present the asteroseismic diagrams of the asymptotic parameters delta u_02, delta u_01 and epsilon, which show clear correlation with fundamental stellar parameters such as mass and radius. When the stellar populations from the clusters are compared, we see evidence for a difference in mass of the red giant branch stars, and possibly a difference in structure of the red clump stars, from our measurements of the small separations delta u_02 and delta u_01. Ensemble {e}chelle diagrams and upper limits to the linewidths of l = 0 modes as a function of Delta u of the clusters NGC 6791 and NGC 6819 are also shown, together with the correlation between the l = 0 ridge width and the T_eff of the stars. Lastly, we distinguish between red giant branch and red clump stars through the measurement of the period spacing of mixed dipole modes in 53 stars among all the three clusters to verify the stellar classification from the color-magnitude diagram. These seismic results also allow us to identify a number of special cases, including evolved blue stragglers and binaries, as well as stars in late He-core burning phases, which can be potentially interesting targets for detailed theoretical modeling.
UBVI CCD photometry is obtained for the open clusters NGC 4609 and Hogg 15 in Crux. For NGC 4609, CCD data are presented for the first time. From new photometry we derive the reddening, distance modulus and age of each cluster - NGC 4609 : E(B-V) = 0.37 +/- 0.03, V_0 - M_V = 10.60 +/- 0.08, log tau = 7.7 +/- 0.1; Hogg 15 : E(B-V) = 1.13 +/- 0.11, V_0 - M_V = 12.50 +/- 0.15, log tau <= 6.6. The young age of Hogg 15 strongly implies that WR 47 is a member of the cluster. We have also determined the mass function of these clusters and have obtained a normal slope (Gamma = -1.2 +/- 0.3) for NGC 4609 and a somewhat shallow slope (Gamma = -0.95 +/- 0.5) for Hogg 15.
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