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J/AJ/108/555        M68 galactic globular clusters photometry     (Walker, 1994)
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BVI CCD photometry of galactic globular clusters. II. M68
     Walker A.R.
     <Astron. J. 108, 555 (1994)>
     =1994AJ....108..555W      (SIMBAD/NED Reference)
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ADC_Keywords: Clusters, globular; Photometry; Colors

Abstract:
   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.

Addresses:
    Walker A.R.                        awalker@noao.edu
       Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile

File Summary:
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 FileName    Lrecl    Records   Explanations
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ReadMe          80          .   This file
table4.dat      37       4680   Photometry of the RR Lyrae variables
table12.dat     41        562   Photometry of the brighter M68 stars
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Byte-by-byte Description of file: table4.dat
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   Bytes Format  Units    Label      Explanations
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   1-  3  A3     ---      Name       Star name
   5- 15  F11.3  d        HJD        Heliocentric Julian day (1)
  18- 23  F6.3    mag     Vmag       ?=99.999 V band magnitude
  25- 30  F6.3    mag     Bmag       ?=99.999 B band magnitude
  32- 37  F6.3    mag     Imag       ?=99.999 I band magnitude
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Note (1): Heliocentric Julian dates correspond to the start of the V-band
           exposures.  The midpoints of the V, B, and I exposures are,
           respectively, 2.5, 11.0, and 18.5 min later than this HJD
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Byte-by-byte Description of file: table12.dat
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   Bytes Format  Units    Label      Explanations
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   1- 3   I3      ---     Name       Star number
   5-10   F6.1    ---     X          X coordinate
  12-17   F6.1    ---     Y          Y coordinate
  20-25   F6.3    mag     Vmag       V magnitude
  28-33   F6.3    mag     B-V        B-V color
  36-41   F6.3    mag     V-I        []? V-I color
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Origin: AAS CD-ROM series, Volume 3, 1995
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(End)          Lee Brotzman [ADS] 10-Oct-199, Patricia Bauer [CDS]   16-Feb-1995