We present Stroemgren and near-infrared (NIR) photometry of the bulge cluster NGC 6528 and its surrounding field in Baade's Window. uvby images were collected with EFOSC2 on the New Technology Telescope (NTT, La Silla, ESO). The NIR catalogs are based on J,K-band VIRCAM at VISTA (Paranal, ESO) and SOFI at NTT photometry. We matched the aforementioned data sets with Hubble Space Telescope photometry to obtain proper-motion-cleaned samples of NGC 6528 and bulge stars. Furthermore, we were able to correct the Stroemgren-NIR photometry for differential reddening. The huge color sensitivity of the Stroemgren-NIR color-magnitude-diagrams (CMDs) helped us in separating age and metallicity effects.
A sample of 26 dwarf novae was observed in the Stroemgren photometric system. The observations were performed on 3 different seasons in 1986 January, February and July, with a 6-channel photometer attached to the 1.5m telescope if the Observatorio Astronomicao Nacional at San Pedro Martir, Baja California, Mexico. A number of standard stars were observed, and a nearby comparison star was observed for each object.
We present a photometric study of M13 multiple stellar populations over a wide field of view, covering approximately 6.5 half-light radii, using archival Isaac Newton Telescope observations to build an accurate multiband Stroemgren catalogue. The use of the Stroemgren index cy permits us to separate the multiple populations of M13 on the basis of their position on the red giant branch. The comparison with medium and high resolution spectroscopic analysis confirms the robustness of our selection criterion. To determine the radial distribution of stars in M13, we complemented our data set with Hubble Space Telescope observations of the cluster core, to compensate for the effect of incompleteness affecting the most crowded regions. From the analysis of the radial distributions, we do not find any significant evidence of spatial segregation. Some residuals may be present in the external regions where we observe only a small number of stars. This finding is compatible with the short dynamical time-scale of M13 and represents, to date, one of the few examples of fully spatially mixed multiple populations in a massive globular cluster.
Several large scale photometric and spectroscopic surveys are being undertaken to provide a more detailed picture of the Milky Way. Given the necessity of generalisation in the determination of, e.g., stellar parameters when tens and hundred of thousands of stars are considered it remains important to provide independent, detailed studies to verify the methods used in the surveys.
We have investigated the two open clusters NGC 6192 and NGC 6451 for which widely different reddening values and thus ages and distances via Stroemgren uvby photometry can be found in the literature. Our measurements allow us to disentangle the apparent discrepancies in the literature and to derive new accurate values. From appropriate calibrations we find that the overall abundance for NGC 6192 is about solar whereas a subsolar value for NGC 6451 was estimated. From two previous reported photometrically candidate CP stars, one within NGC 6192 shows Stroemgren indices typical for a B8Si star whereas the other object of NGC 6451 is most probably a foreground G-type star.
We have observed (with vby filters) a field north of the core of the most massive globular cluster in our galaxy, {omega} Centauri. We have found a correlation of age and metallicity in a region that avoids the dense core and the inhomogeneous foreground dust emission shown by IRAS. Our observations show that the comparatively metal-rich stars (as defined by the b-y and m_1_ colors) are younger than the metal-poor stars by at least 3 Gyr.
RR Lyrae stars have been observed to improve the insight into processes at work in their atmospheres. Simultaneous Stroemgren-photometry allows to obtain a rapid sequence of measurements in which photometric indices are unaffected by non-optimum observing conditions. The indices y, b-y, and c1 are used with an established calibration to derive Teff and to determine the gravity, logg_BJ_ from the Balmer jump, throughout the pulsation cycle. By employing the equations for stellar structure, additional parameters can be derived.
We present the results of simultaneous uvby{beta} photometry carried out from 1999 to 2001 of the variable V2109 Cyg together with a spectroscopic analysis based on one high resolution spectrum obtained in 2000. From this study, the star is definitively classified as an evolved {delta} Sct-type variable with solar metal abundances. This conclusion is also supported by the detected multiperiodic pulsational behaviour and the observed variation of the m1 index over the pulsation cycle.
CCD photometry on the intermediate-band uvbyCaH{beta} system is presented for the open cluster, NGC 3680. Restricting the data to probable cluster members using the color-magnitude diagram (CMD) and the photometric indices alone defines a sample of 34 stars at the cluster turnoff that imply E(b-y)=0.042+/-0.002(s.e.m.) or E(B-V)=0.058+/-0.003(s.e.m.), where the errors refer to internal errors alone. With this reddening, [Fe/H] is derived from both m_1_ and hk using both b-y and H{beta} as the temperature indices.
By studying young open clusters, the mechanisms important for star formation over several Myr can be examined. For example, accretion rate as a function of rotational velocity can be investigated. Similarly, sequential star formation triggered by massive stars with high mass-loss rates can be studied in detail. We identified and characterized probable members of NGC 6383, as well as determined cluster parameters. New Stromgren uvby CCD photometry, obtained by us, is presented. This new data, together with Johnson UBV and 2MASS data in the NIR, was used to investigate characteristics of pre- as well as zero age main sequence cluster members. We present Stromgren uvby CCD photometry for 272 stars in the field of NGC 6383 and derive its reddening, E(b-y)=0.21(4)mag, as well as distance, d=1.7(3)kpc from the Sun. Several stars with NIR excess and objects in the domain of the classical Herbig Ae/Be and T Tauri stars were detected. Two previously known variables were identified as rapidly-rotating PMS stars. The field population is clearly separated from the probable members in the color-magnitude diagram. NGC 6383 is a young open cluster, with an age of less than 4Myr, undergoing continuous star formation. True pre-main sequence members might be found down to absolute magnitudes of +6mag, with a variety of rotational velocities and stellar activities.
