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WIYN Open Cluster Study. XXXII. Stellar Radial Velocities in the Old Open Cluster NGC 188
We present the results of our ongoing radial-velocity (RV) survey of theold (7 Gyr) open cluster NGC 188. Our WIYN 3.5 m data set spans a timebaseline of 11 years, a magnitude range of 12 <= V <= 16.5(1.18-0.94 M sun), and a 1° diameter region on the sky.With the addition of a Domain Astrophysical Observatory data set weextend our bright limit to V = 10.8 and, for some stars, extend our timebaseline to 35 years. Our magnitude limits include solar-massmain-sequence stars, subgiants, giants, and blue stragglers (BSs), andour spatial coverage extends radially to 17 pc (~13 core radii). For theWIYN data we present a detailed description of our data reductionprocess and a thorough analysis of our measurement precision of 0.4 kms-1 for narrow-lined stars. We have measured radialvelocities for 1046 stars in the direction of NGC 188, and havecalculated RV membership probabilities for stars with >=3measurements, finding 473 to be likely cluster members. We detect 124velocity-variable cluster members, all of which are likely to bedynamically hard-binary stars. Using our single member stars, we find anaverage cluster radial velocity of -42.36 ± 0.04 kms-1. We use our precise RV and proper-motion membership datato greatly reduce field-star contamination in our cleanedcolor-magnitude diagram, from which we identify six stars of note thatlie far from a standard single-star isochrone. We present a detailedstudy of the spatial distribution of cluster-member populations, andfind the binaries to be centrally concentrated, providing evidence forthe presence of mass segregation in NGC 188. We observe the BSs topopulate a bimodal spatial distribution that is not centrallyconcentrated, suggesting that we may be observing two populations of BSsin NGC 188, including a centrally concentrated distribution as well as ahalo population. Finally, we find NGC 188 to have a global RV dispersionof 0.64 ± 0.04 km s-1, which may be inflated by up to0.23 km s-1 from unresolved binaries. When corrected forunresolved binaries, the NGC 188 RV dispersion has a nearly isothermalradial distribution. We use this mean-corrected velocity dispersion toderive a virial mass of 2300 ± 460 M sun .
| A Star Catalog for the Open Cluster NGC 188
We present new BVRI broadband photometry for the old open cluster NGC188, based on an analysis of 299 CCD images either obtained by us,donated by colleagues, or retrieved from public archives. We compare ourresults on a star-by-star basis with data from 11 previous photometricdata sets for the cluster. We homogenize and merge the data from all thephotometric studies, and also merge membership probabilities from fourprevious proper-motion studies of the cluster field. Fiducial clustersequences in the BV (Johnson) RI (Cousins) photometric system of Landolt(1993, AJ, 104, 340) represent the principal result of this paper. Theseare compared to reference samples defined by (1) Landolt's standardstars, (2) the old open clusters M67 and NGC 6791, and (3) stars within25 pc having modern photometry and precise Hipparcos parallaxes. In acompanion paper, we show that our derived cluster results agree wellwith the predictions of modern stellar interior and evolution theory,given reasonable estimates of the cluster chemical abundances andforeground reddening. The individual and combined data sets for NGC 188have been made available through our Web site.
| WIYN Open Cluster Study. XVII. Astrometry and Membership to V=21 in NGC 188
We present techniques for obtaining precision astrometry using oldphotographic plates from assorted large-aperture reflectors incombination with recent CCD Mosaic Imager frames. At the core of thisapproach is a transformation of plate/CCD coordinates into a previouslyconstructed astrometric reference frame around the open cluster NGC 188.This allows us to calibrate independently the optical field angledistortion for all telescopes and field correctors used in this study.Particular attention is paid to computing the differential colorrefraction, which has a marked effect in the case of NGC 188 as a resultof the large zenith distances at which this cluster has been observed.Our primary result is a new catalog of proper motions and positions for7812 objects down to V=21 in the 0.75 deg2 area around NGC188. The precision for well-measured stars is 0.15 mas yr-1for proper motions and 2 mas for positions on the system of the Tycho-2catalog. In total, 1490 stars have proper-motion membershipprobabilities Pμ>=10%. The sum of membershipprobabilities indicates that NGC 188 contains ~1050 stars down to V=21.Comprehensive lists of the candidate blue stragglers and red giant starssubstantially enlarge the number of such stars known in NGC 188. We havealso obtained a small correction to the proper motions from the mean``motion'' of background galaxies. Thus, the absolute proper motion ofNGC 188 is μabsx=-2.56+/-0.2 andμabsy=+0.18+/-0.2 mas yr-1.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| A Proper-Motion Membership Study of the Old Open Cluster NGC 188
Relative proper motions have been determined for 1127 stars in the fieldof the open cluster NGC 188 from plates spanning a 60-year interval,taken with the 76-cm Thaw refractor of the Allegheny Observatory.Special attention has been paid to the corrections for systematic errorsas a function of magnitude and color, and to the treatment of membershipprobabilities for stars with different proper-motion errors. The studyis complete to a magnitude B = 16.2. Also some stars brighter than B =12.5 were eliminated from the study because of the large uncertainty inestimating the magnitude equation at the bright end. The final relativeproper motions have an accuracy of 0.5 mas/yr for well-exposed stars.From the cluster membership probabilities based on proper motions weobtain 360 cluster members, enabling us to better define thecolor-magnitude diagram. We provide a list of highly probable candidateblue stragglers in addition to the sample proposed by Eggen &Sandage (1969). In order to investigate mass segregation, we haveanalyzed the spatial distribution of the possible blue stragglers. Basedon our proper-motion probabilities the spatial cumulative distributionsshow that the blue stragglers are not more centrally concentrated thanthe red giants.
| Possible abundance difference among giant stars in NGC 188
Abstract image available at:http://adsabs.harvard.edu/abs/1974ApJ...194..355M
| Proper motions, membership, and stellar content of the old cluster NGC 188.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972AJ.....77...74U&db_key=AST
| Photometric Data for the Old Galactic Cluster NGC 188.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1962ApJ...135..333S&db_key=AST
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Observation and Astrometry data
| Constellation: | Kepheus |
| Right ascension: | 00h55m45.36s |
| Declination: | +85°12'20.9" |
| Apparent magnitude: | 10.968 |
| Proper motion RA: | -1.4 |
| Proper motion Dec: | 1.7 |
| B-T magnitude: | 12.762 |
| V-T magnitude: | 11.117 |
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