CCD UBVI photometry in the field of the open cluster NGC 6231 was obtained for 1060 stars down to V=19mag. Memberships, reddening, distance and age of this cluster were investigated. Its lower sequence displays a notorious bend at V=13.5mag followed by a high number of faint stars showing a large magnitude spread at constant colour. This distribution of stars does not seem to be produced by a mere accumulation of field stars seen in the direction of the cluster but by a real star excess in the zone. The evidence suggests these stars are serious candidates to be cluster members caught in their way towards the ZAMS. Fitting them with pre main sequence isochrones an age spread of 10Myr is obtained. The luminosity and mass functions were found to be flat for -7.5<M_v_<1.5 and for 3 to 80M_{sun}_ respectively.
We have obtained CCD UBVI_KC_ photometry down to V~22.0 for the open cluster NGC 2309 and its surrounding field. Twenty three probable cluster members have been identified for the first time on the basis of sound photometric criteria. Because NGC 2309 is a relatively poor cluster projected onto a rich star field showing density fluctuations, the frequently used membership criteria based on stellar density profiles or proper motion measurements would not be enough to assess the star membership status if employed independently. We estimated a cluster radius of 7.9' and a radius at half the maximum of the cluster density profile of 1.7'. Based on the best fits of isochrones computed by the Geneva group to the cluster color-magnitude and color-color diagrams, previously shifted by the cluster reddening (E(B-V)=0.32) and distance (d=2.5kpc), we derived the cluster age (250Myr) and metallicity ([Fe/H]~0.0). If an 8.5kpc distance from the Sun to the center of the Galaxy is assumed, then NGC 2309 is found to be located just in front of the Perseus spiral arm.
We apply the continuous period search and carrier fit methods on long-term standard Johnson-Cousins V-observations from the years 1995-2010. The observations were carried out with two automated photometric telescopes, Phoenix-10 and Amadeus T7 located in Arizona. We identify complex phase behaviour in 6 of the 15 analysed data segments. We identify five flip-flop events and two cases of phase jumps, where the phase shift is {Delta}{phi}<0.4. In addition we see two mergers of spot regions and two cases where the apparent phase shifts are caused by spot regions drifting with respect to each other. Furthermore we detect variations in the rotation period corresponding to a differential rotation coefficient of |k|>0.031.
Recent progress in the seismic interpretation of field beta Cep stars has resulted in improvements of the physics in the stellar structure and evolution models of massive stars. Further asteroseismic constraints can be obtained from studying ensembles of stars in a young open cluster, which all have similar age, distance and chemical composition. To improve our comprehension of the beta Cep stars, we studied the young open cluster NGC 884 to discover new B-type pulsators, besides the two known beta Cep stars, and other variable stars. An extensive multi-site campaign was set up to gather accurate CCD photometry time series in four filters (U, B, V, I) of a field of NGC 884. The images were calibrated and reduced to transform the CCD frames into interpretable light curves. Various variability indicators and frequency analyses were applied to detect variable stars in the field.
We present the results of UBV imaging of a large sample of irregular galaxies: 94 Im systems, 24 blue compact dwarfs (BCDs), and 18 Sm galaxies. We also include JHK imaging of 42 of these galaxies. The sample spans a large range in galactic parameters. Ellipse fit axial ratios, inclinations, and position angles are derived, integrated photometry and azimuthally averaged surface photometry profiles are determined, and exponential fits give the central surface brightnesses, scale lengths, and isophotal and half-power radii.
In the course of the search for B-type pulsators in the central region of h Persei, we discovered two {beta} Cephei stars, Oo692 and Oo992, and one SPB star, Oo893. The first two stars are monoperiodic pulsators with periods of 0.1716946 and 0.1326359 d respectively, and semi-amplitudes smaller than 0.01mag. The light curve of Oo893 can be described by a single 1.193633-day period, slightly non-sinusoidal in shape, and having semi-amplitudes from 19 mmag in I to 48 mmag in U. Oo893 is the first SPB star found in h and {chi} Persei. In addition, we discovered seven other variables, including three eclipsing binaries and one {lambda} Eri star. One of the binaries is a W UMa-type star and a likely cluster member. We also present new UBV photometry for 258 stars in the field. The average reddening, estimated from the cluster colour-colour diagram, amounts to E(B-V)=0.52mag. A 0.1mag dispersion of reddenings within the cluster is also seen.
We present new optical observations of young massive star clusters in Arp 220, the nearest ultraluminous infrared galaxy, taken in UBVI with the Hubble Space Telescope ACS HRC camera. We find a total of 206 probable clusters whose spatial distribution is centrally concentrated toward the nucleus of Arp 220. We use model star cluster tracks to determine ages, luminosities, and masses for 14 clusters with complete UBVI indices or previously published near-infrared data. We estimate rough masses for 24 additional clusters with I<24mag from BVI indices alone.
We present new CCD photometry of the distant old open star cluster Berkeley 32 in Johnson V and Cousins I passbands. A total of about 3200 stars have been observed in a field of about 13'x13'. The colour-magnitude diagram (CMD) in V, (V-I) has been generated down to V=22mag. A broad but well defined main sequence is clearly visible. Some blue stragglers, a well developed subgiant branch and a Red Clump are also seen. By fitting isochrones to this CMD as well as to other CMDs available in the literature, and using the Red Clump location, the reddening, distance and age of the star cluster have been determined. The cluster has a distance of about 3.3kpc, its radius is about 2.4pc; the reddening E(B-V) is 0.08 mag and the age is about 6.3Gyr. By comparison with theoretical isochrones, a metallicity of [Fe/H]~-0.2dex has been estimated.
CCD photometry on the Johnson UBV and on the Kron-Cousins VI systems are presented for 732 stars in and near Be 31, an open cluster located towards the galactic anticenter. The resultant color-magnitude diagrams indicate that the cluster is an old object. From the color-color diagram, the reddening and metallicity parameters are determined to be E(B-V) = 0.13 mag and [Fe/H] = -0.4. By comparing the cluster C-M diagrams with the VandenBerg isochrones, and by assuming a normal reddening law, a distance of 5.2 +/- 0.5 kpc and an age of 8 Gyr are derived.