The first large-format CCD color-magnitude diagram (CMD) in the B and V passbands is presented for the Galactic globular cluster M53 (NGC 5024). The observations consist of 40 pairs of B and V CCD frames with FWHM<1.2" in a pattern covering a 12'x13' region of M53. These frames were used to produce color-magnitude data for more than 20,000 stars from the tip of the red giant branch (RGB) to about 2 mag below the main-sequence turnoff (V~22). The CMD reveals a classic example of a metal-poor stellar system, with the horizontal branch (HB) predominantly blueward of the RR Lyrae instability strip and the relatively steep RGB. The blue HB of M53 does not show the blue tail phenomenon that is observed in M15. The relative age dating based on the color difference between the turnoff and the base of RGB reveals no significant age difference ({Delta}t<1Gyr) between M53 and M92. We have discovered 114 new blue straggler stars (BSSs) in the field of M53. The analysis of bright (V<19.39) BSSs clearly shows a bimodal radial distribution with a high frequency in the inner and outer regions but a distinct dip in the intermediate region. The distribution is similar to that found in M3, a globular cluster with a similar central density and concentration.
Broad band photometry (in Johnson B and V) has been obtained in a region (34'x34') at the North-Eastern quadrant of M31 over 5 years (1999-2003) at the Isaac Newton Telescope (La Palma, Spain). The data, containing more than 250 observations per filter, have been reduced by means of the so-called difference image analysis technique and the DAOPHOT program. A catalog with 236238 objects with photometry in both B and V passbands has been obtained. The catalog is the deepest (V<25.5mag) obtained so far in the studied region and contains 3964 identified variable stars, with 437 eclipsing binaries and 416 Cepheids.
A new CCD BV color-magnitude diagram (CMD) has been derived for the intermediate age open cluster NGC 3680. The quality of photometry coupled with the best-to-date knowledge of cluster memberships allows for a detailed isochrone fit to the CMD. The theoretical isochrones have been constructed using the Yale Rotating Evolution Code (YREC) in its non-rotating mode and the OPAL opacities (circa 1991). Four sets of isochrones have been calculated: one for the standard stellar model calibrated to the Sun and three others for models with different amounts of convective overshoot at the edge of the convective core, namely, 0.15, 0.20, and 0.25H_p_, where H_p_ is the pressure scale height at the core edge. All four sets of theoretical isochrones were adjusted to an adopted distance modulus of V_0_-M_V_=10.20 and reddening E(B-V)=0.075 which leads to ages of 1.3+/-0.15, 1.5+/-0.15, 1.6+/-0.15, and 1.7+/-0.15Gyr, respectively The uncertainties in age mainly reflect a subjective decision in differentiating a good fit from a poorer one The model with a convective overshoot of 0.20H_p_ seems to yield the best fit to the sharply curved upper main sequence. Hence, the estimated age of NGC 3680 is 1.6+/-0.15Gyr. The adopted overshoot parameter ostensibly is uncertain by +/-0.05H_p_. For comparison, the same stellar models with overshoot of 0.25H_p_ were fit to the color-magnitude diagram of the cluster NGC 752, which is similar in age and composition to NGC 3680 The age estimate for NGC 752 is 1.6+/-0.2Gyr, assuming V_0_-M_V_=8.20 and E(B-V)=0.030 for the cluster.
We present a new deep (down to V~24) photometry of a wide region (~6'x6') around the Large Magellanic Cloud globular cluster NGC 1866. Our sample is much larger (by more than a factor of 3) than any previous photometry and with a main sequence (MS) that may be considered complete, down to at least 3 mag below the brightest MS star; such an occurrence allows a meaningful and robust comparison with various theoretical scenarios produced by means of models computed with the evolutionary code FRANEC.
New BV CCD photometry for the bright stars in cluster fields centered on the core of NGC 7099 is presented. The morphology of the LF though the turnoff region is similar to that found in other metal-poor clusters in that the relative number of turnoff to giant stars is lower than predicted by canonical models. Comparisons with evolutionary sequences of horizontal branch stars suggest a reddening E(B-V)=0.06 and a distance modulus (m-M)_V=14.83. Isochrone fits to [Fe/H]=-2.03, [O/Fe]=+0.70 models are consistent with an age of at least 14 Gyr. The model LF for this age accurately predicts the brightness of the tip of the red giant branch (RGB) as well as the location of the break which signals the transition from the base of the RGB to the main sequence. There is an indication that the evolutionary pause on the RGB, which manifests itself as a bump in the LF, occurs one magnitude fainter than the models predict. The excess of HB stars with respect to giants is confirmed and implies a helium abundance Y=0.28, which is higher than that found in other metal-poor globular clusters from the R method. From a comparison with the metal-rich, blue horizontal branch cluster NGC 288, it is argued that age cannot by itself be the solution to the second parameter problem.
CCD BV photometry to V=20.5 of stars in the old open cluster NGC 6791 is presented. Comparison with the theoretical isochrones of VandenBerg gives a cluster age in the range of 10 to 12.5Gyr, and an apparent distance modulus of 13.45mag, adopting a solar metallicity. This cluster is only marginally younger than Palomar 12, which is the youngest globular cluster known. A search for short-period variables in the cluster field led to the discovery of only one low-amplitude variable, probably belonging to the FK Com-type stars. A very blue star was detected which may be an extremely young white dwarf formed in the cluster.
In this paper we examine the age and internal dynamics of the young binary LMC cluster NGC 1850 using BV CCD images and echelle spectra of 52 supergiants.
Deep B, V time-series photometry obtained with the ESO Very Large Telescope has been used to identify variable stars in the dwarf irregular galaxy NGC 6822. We surveyed a 6.8x6.8 arcmin area of the galaxy and detected a total number of 390 candidate variables with the optimal image subtraction technique (Alard C., 2000A&AS..144..363A). Light curves on a magnitude scale were obtained for 262 of these variables. Differential flux light curve are available for the remaining sample. We were able to determine periods and pulsation characteristics for 69 out of the 262 variables with light curves in magnitude scale, they include: 18 RR Lyrae stars (16 ab-type and 2 c-type), 45 Cepheids and 6 binary systems. We also tentatively calibrated 8 of the 128 objects with light curves on a differential flux scale on the assumption that they are ab-type RR Lyrae stars.
We have performed a search for variable stars in the Sculptor Group spiral galaxy NGC 300 using wide-field multi-epoch BV photometry up to a limiting magnitude V~23. Apart from the Cepheid variables in this galaxy already reported in an earlier paper by Pietrzynski et al. (2002, Cat. <J/AJ/123/789>) and the variable blue supergiants reported by Bresolin et al. (2002ApJ...567..277B), we have identified 26 additional periodic variables, with periods ranging from 0.2 to 190d, and 121 slow variables. Two of these newly discovered variables are Galactic foreground stars (one of them is a W UMa system), and three correspond to previously identified Wolf-Rayet stars. 16 stars are new Cepheids, and the other objects span a huge range in colours and represent a mixture of different types of luminous variables.
