A total of 363 CCD frames in the B, V, and I passbands have been used to study the color-magnitude diagram (CMD) and the variable-star population of the metal-poor galactic globular cluster M68 (NGC 4590, C1236-264). Light curves have been prepared for 40 RR Lyrae variables, five of which are new discoveries, and for two probable SX Phe stars. The RR Lyraes have <V>=15.64+/-0.01, and via several methods E(B-V)=0.07+/-0.01. [Fe/H]=-2.1 is adopted. From a comparison with Sandage's data for M3 (NGC 5272, C1339+286), a period shift of Delta(logP)/Delta([Fe/H])=-0.11+/-0.02 is derived, and the mass-to-light parameter is found to be <A>=1.90. RR Lyrae masses are derived from Fourier fits to the light curves, from the RRd variables, and by comparison with HB (horizontal branch) evolutionary models calculated by Dorman. The light-curve structure agrees qualitatively with recent theoretical models. From short exposures, CMDs have been prepared that are complete from the tip of the red giant branch (RGB) to below the level of the HB, while from the long exposures a CMD to V~23 is provided for 7298 stars more than 158 arcsec distant from the cluster center. The HB is skewed to the blue with the eight bluest stars clearly separated from the remainder of the HB stars. This HB gap is at the identical color to that in M15 (NGC 7078, C2127+119), but M68 lacks the extensive population of the very blue HB stars present in M15. The total number of HB stars is deficient with respect to the RGB population, with the ratio of RGB to HB stars R=0.99. There are several likely blue stragglers identified on CMDs of inner annuli, but near to the cluster center photometric errors scatter stars off the main sequence (MS) into this region. The deep CMDs are in excellent agreement with existing CCD CMDs for M68, with the MS turnoff Delta(V)=3.42+/-0.10mag below the HB. Matching the M68 CMD to that for M15 in the subgiant-MS turnoff region shows that the age of M68 is identical to that for M15 to within +/-0.4Gyr.
File table1 contains various identifications the master list of 977 stars that were included in the photometric variability survey, and which were cross-identified with stars in the study of Fan et al. (1996, Cat. <J/AJ/112/628>). Another 13 stars included in the study, but not cross-identified with the study of Fan et al. can be found in Table 2, which is published in the paper. These 977 stars are those that were included in a roughly square region, 23 arcminutes on each side, centered approximately 5 arcminutes north of the cluster center. The stars have magnitudes in the approximate range 12.5<B<18.5, 12.5<V<18.5, and 12<I<18. File table3 contains BVI photometry for all 990 stars (from Tables 1 and 2) included in the photometric variability study.
We have compiled the asymptotic giant, horizontal, and upper red giant branch (AGB, HB, and RGB) stars in the globular cluster M55 (NGC 6809). Using the star counts and the R-parameter we compute the initial helium abundance Y=0.274+/-0.016.
The observation of young stars with circumstellar disks suggests that the disks are dissipated, starting from the inner region, by the radiation of the central star and eventually by the formation of rocky planetesimals, over a time scale of several million years. It was also shown that strong UV radiation emitted by nearby massive stars can heat a circumstellar disk up to some thousand degrees, inducing the photoevaporation of the gas. This process strongly reduces the dissipation time scale. We study whether there exists a correlation between the spatial distribution of stars with circumstellar disks and the position of massive stars with spectral class earlier than B5, in the open cluster NGC 6611. We created a multiband catalog of the cluster, down to V~23mag, using optical data from a WFI observation at 2.2m of ESO in the BVI bands, the 2MASS public point source catalog and an archival X-ray observation made with CHANDRA/ACIS. We selected the stars with infrared excess (due to the emission of a circumstellar disk) using suitable color indices independent of extinction, and studied their spatial distribution.
We used HST-WFPC2 and the 1.54 Danish ESO-telescope at La Silla (Chile) to obtain photometry for stars located within 200 arcsec from the center of the cluster NGC 6101. HST-WFPC2 data for 1 field centered on the cluster, have been collected during HST-cycle 6. The resulting V, V-I photometry reach a limiting magnitude of V~25.5. Ground observations have been collected on July 1995. The resulting photometry containing the stars simultaneously detected in the V, B and I filters, is characterized by a limiting magnitude of V~22.5.
Based on 13 nights of observations of four fields in NGC 300, we have set up an extensive sequence of stars with accurate BVI photometry covering a relatively large (25'x25') region centered on this galaxy. This sequence of standard stars is very useful for calibrating the photometry of variable stars and other objects in NGC 300 and other galaxies obtained from wide-field mosaic images. Our standard-star list contains B, V, and I measurements for 390 stars. The accuracy of the zero points in the V filter and B-V color is better than 0.02mag, and about 0.03mag for the V-I color. We found very good agreement between our measurements and those previously obtained by Walker for 26 stars near NGC 300.
A photometric sequence consisting of 53 stars measured in the V, B, and I bands has been set up in the vicinity of the rich, young Large Magellanic Cloud cluster, NGC 1866. Many of these stars are relatively isolated and are thus suitable for use as local standard stars for photometric studies of the cluster. Color magnitude diagrams to a limit of V=20.5 for various parts of the 13x13arcmin field, centered on the cluster, show that NGC 1866 is embedded in a mix of populations which includes stars younger than the cluster itself, the dominant 0.5-3Gyr population, plus some very old stars represented by four RR Lyrae variables.
We report on a multiwavelength study of the relationship between young star clusters in the Antennae galaxies (NGC 4038/9) and their interstellar environment, with the goal of understanding the formation and feedback effects of star clusters in merging galaxies.
We present broadband BVI photometry for the open cluster NGC 6791, based upon analysis of 1764 individual CCD images. Data tables listing measured magnitudes and standard errors, image-quality indices, a variability index, and equinox J2000.0 equatorial coordinates for 14,342 stars to V~24 have been made available to the public through the services of the Canadian Astronomy Data Centre. Equatorial coordinates only have been provided for a further 1916 stars, the photometry for which we were unable to calibrate because of lack of color information.
The aim of this paper is to present the results of photometric investigations of the central cluster of the W5 E HII region as well as a follow-up study of the triggered star formation in and around bright-rimmed clouds (BRCs). We have carried out wide-field UBVIc and deep VIc photometry of the W5 E HII region. A distance of ~2.1kpc and a mean age of ~1.3Myr have been obtained for the central cluster. The young stellar objects (YSOs) associated with the region are identified on the basis of near-infrared and mid-infrared observations.
