A star catalog for the open cluster NGC 188 (Stetson+, 2004)

Department of Physics and Astronomy, University of Victoria, Victoria, British Columbia, Canada
Publications of the Astronomical Society of the Pacific (Impact Factor: 3.5). 11/2004; 116(825). DOI: 10.1086/426122


We present new BVRI broadband photometry for the old open cluster NGC 188, based on an analysis of 299 CCD images either obtained by us, donated by colleagues, or retrieved from public archives. We compare our results on a star-by-star basis with data from 11 previous photometric data sets for the cluster. We homogenize and merge the data from all the photometric studies, and also merge membership probabilities from four previous proper-motion studies of the cluster field. Fiducial cluster sequences in the BV ( Johnson) RI ( Cousins) photometric system of Landolt ( 1993, AJ, 104, 340) represent the principal result of this paper. These are compared to reference samples defined by ( 1) Landolt's standard stars, ( 2) the old open clusters M67 and NGC 6791, and ( 3) stars within 25 pc having modern photometry and precise Hipparcos parallaxes. In a companion paper, we show that our derived cluster results agree well with the predictions of modern stellar interior and evolution theory, given reasonable estimates of the cluster chemical abundances and foreground reddening. The individual and combined data sets for NGC 188 have been made available through our Web site.

Full-text preview

Available from: ArXiv
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The color-magnitude diagram of M67 is used to constrain the value of the parameter Fover in its ~1.3 Msolar turnoff stars: Fover is effectively the fraction of the maximum possible extent of convective core overshooting predicted by the Roxburgh (1989, A&A, 211, 361) criterion. Isochrones that treat overshooting using the parameterized Roxburgh criterion are able to reproduce the morphology of the cluster turnoff, including the luminosity of the gap, if Fover~0.07 (assuming current best estimates for the reddening and metallicity). Previous studies have derived values of Fover near 0.5 in open clusters of similar metallicity but having turnoff masses >=1.55 Msolar, indicating that the overshooting parameter is a strong function of mass in the lowest mass stars that retain convective cores throughout the main-sequence phase. NGC 188 appears to be too old for core overshooting to play any role in the evolution of stars that are currently in the core H-burning phase, but the availability of well-calibrated BVRI photometry for this system, together with Landolt photometric standards, provides valuable tests of the color-Teff relations that apply to [Fe/H]~0.0 stars. Our analysis of color-color diagrams, in particular, suggests that the Castelli (1999, A&A, 346, 564) V-R and V-I transformations for lower gravity stars are more realistic than those published recently by VandenBerg & Clem (2003, AJ, 126, 778). The same color-color diagrams also indicate that the differences in recent (V-I,V) diagrams for M67 appear to be due, at least in part, to the difficulty of defining the standard VRI photometric system. The ages of M67 and NGC 188 that have been derived in this study are 4.0 and 6.8 Gyr, respectively, which is consistent with other modern determinations.
    Preview · Article · Nov 2004 · Publications of the Astronomical Society of the Pacific
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The old open cluster M67 is an ideal testbed for current cluster evolution models because of its dynamically evolved structure and rich stellar populations that show clear signs of interaction between stellar, binary and cluster evolution. Here, we present the first truly direct N-body model for M67, evolved from zero age to 4 Gyr taking full account of cluster dynamics as well as stellar and binary evolution. Our preferred model starts with 36 000 stars (12 000 single stars and 12 000 binaries) and a total mass of nearly 19 000 M⊙, placed in a Galactic tidal field at 8.0 kpc from the Galactic Centre. Our choices for the initial conditions and for the primordial binary population are explained in detail. At 4 Gyr, the age of M67, the total mass has reduced to 2000 M⊙ as a result of mass loss and stellar escapes. The mass and half-mass radius of luminous stars in the cluster are a good match to observations, although the model is more centrally concentrated than observations indicate. The stellar mass and luminosity functions (LFs) are significantly flattened by preferential escape of low-mass stars. We find that M67 is dynamically old enough that information about the initial mass function (IMF) is lost, both from the current LF and from the current mass fraction in white dwarfs (WDs). The model contains 20 blue stragglers (BSs) at 4 Gyr, which is slightly less than the 28 observed in M67. Nine are in binaries. The blue stragglers were formed by a variety of means and we find formation paths for the whole variety observed in M67. Both the primordial binary population and the dynamical cluster environment play an essential role in shaping the population. A substantial population of short-period primordial binaries (with periods less than a few days) is needed to explain the observed number of BSs in M67. The evolution and properties of two-thirds of the BSs, including all found in binaries, have been altered by cluster dynamics and nearly half would not have formed at all outside the cluster environment. On the other hand, the cluster environment is also instrumental in destroying potential BSs from the primordial binary population, so that the total number is in fact slightly smaller than what would be expected from evolving the same binary stars in isolation.
    Preview · Article · Jul 2005 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: New BVRI broad-band photometry and astrometry are presented for the globular cluster NGC 4147, based upon measurements derived from 524 ground-based CCD images mostly either donated by colleagues or retrieved from public archives. We have also reanalysed five exposures of the cluster obtained with WFPC2 on the Hubble Space Telescope in the F439W and F555W (B and V) filters. We present calibrated color-magnitude and color-color diagrams. Analysis of the color-magnitude diagram reveals morphogical properties generally consistent with published metal-abundance estimates for the cluster, and an age typical of other Galactic globular clusters of similar metallicity. We have also redetermined the periods and mean magnitudes for the RR Lyrae variables, including a new c-type variable reported here for the first time. Our data do not show clear evidence for photometric variability in candidate V18, recently reported by Arellano Ferro et al. (2004). These observations also support the non-variable status of candidates V5, V9, and V15. The union of our light-curve data with those of Newburn (1957), Mannino (1957) and Arellano Ferro et al. (op. cit.) permits the derivation of significantly improved periods. The mean periods and the Bailey period-amplitude diagrams support the classification of the cluster as Oosterhoff I despite its predominantly blue horizontal branch. The number ratio of c- to ab-type RR Lyrae stars, on the other hand, is unusually high for an Oosterhoff I cluster. The calibrated results have been made available through the first author's web site. Comment: 30 pages, 30 figures. PASP, in press. Uses 11/2004 version of emulateapj (included). For the full-resolution preprint, the reader is encouraged to download the ps file (123 pages, manuscript format) available at, or the pdf file (30 pages, preprint format) available at
    Full-text · Article · Aug 2005 · Publications of the Astronomical Society of the Pacific
Show more