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J/AJ/113/669                         Stars in M13 core (Cohen+ 1997)
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Globular Cluster Photometry with the Hubble Space Telescope. VI. WF/PC-I
Observations of the Stellar Populations in the Core of M13 (NGC 6205)
     Cohen R.L., Guhathakurta P., Yanny B., Schneider D.P., Bahcall J.N.
    <Astron. J. 113, 669 (1997)>
    =1997AJ....113..669C
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ADC_Keywords: Clusters, globular ; Photometry
Mission_Name: HST

Abstract:
    We study the dense core of the globular cluster Messier 13 (NGC 6205)
    using pre-refurbishment Planetary Camera-I images obtained with the
    Hubble Space Telescope. Short exposures (60s) through the F555W and
    F785LP filters (similar to Johnson V and I, respectively) have been
    used to obtain V and I photometry of 2877 stars brighter than V~20 in
    a 1.25arcmin^2^ region of the cluster including its core and extending
    out to r~66arcsec (2.3pc) from its center. The sample is complete to
    V~=18.3 (the main sequence turnoff) and the 1{sigma} photometric error
    is about 0.1mag. We find 15 blue straggler star candidates and 10
    other possible blue stragglers in this region of M13. Their specific
    frequency is in the range F_(BSS)=0.04-0.07, comparable to what is
    observed near the centers of other dense clusters. A comparison
    between M13's observed V band stellar luminosity function and a
    theoretical model (Bergbusch & Vandenberg, 1992ApJS...81..163B) for
    the luminosity function of an old, metal-poor cluster shows that the
    model predicts too few of the brightest red giants (V~12.5-15) by a
    factor of two relative to subgiants/turnoff stars (>6{sigma} effect).
    The radial distributions of red giants, blue stragglers, and subgiants
    are consistent with one another, and are well fit by a King profile of
    core radius r_(core)_=38+/-6arcsec (90% confidence limits) or
    1.3pc. Stars in the blue horizontal branch of M13, however, appear to
    be centrally depleted relative to other stellar types. We combine data
    from three dense ``King model clusters,'' M13, M3, and 47 Tuc, and two
    post core collapse clusters, M30 and M15, and compare the
    distributions of various stellar types as a function of
    (r/r_(half light)_) and (r/r_(core)_). The horizontal branch stars in
    the combined sample appear to be centrally depleted relative to the
    giants (97% significance), this depletion is only a 1-2{sigma} effect
    in each of the clusters taken individually. The blue stragglers in the
    combined sample are centrally concentrated relative to the giants.
    (Copyright) 1997 American Astronomical Society.

File Summary:
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 FileName    Lrecl    Records   Explanations
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ReadMe          80          .   This file
table1.dat      55       6436   Stars in the core of M13
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See also:
   J/AJ/107/1745 : HST UVI photometry of M15. II. (Yanny+, 1994)
   J/AJ/108/1786 : Blue stragglers + variable stars in M3 (Guhathakurta+, 1994)


Byte-by-byte Description of file: table1.dat
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   Bytes Format  Units   Label      Explanations
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   1-  4  I4     ----    ID         Identification number
   6- 11  F6.2   arcsec  dRA       *R.A. J2000 offset from star #5540
  13- 18  F6.2   arcsec  dDE       *Dec. J2000 offset from star #5540
      20  I1     ---     CCD        PC number of the CCD
  22- 25  F4.1   arcsec  r          Angular distance from center of M13
  27- 31  F5.2   mag     F555W      F555W magnitude
  33- 37  F5.2   mag     F785LP     ? F785LP magnitude
  39- 43  F5.2   mag     Vmag       ? V magnitude
  45- 49  F5.2   mag     Imag       ? I magnitude
  51- 55  F5.2   mag     V-I        ? V-I color
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Note on dRA, dDE:
  All positions are on the equinox J2000 coordinate system, and are measured
  relative to the reference star U (#5540) whose coordinates are:
  RA = 16h 41m 41.61s, DE = +36d 27' 42.06", accurate to about 0.25 arcsec.
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References:
   Guhathakurta et al., Paper I.    1992AJ....104.1790G
   Yanny et al., Paper II.          1994AJ....107.1745Y, cat. <J/AJ/107/1745>
   Guhathakurta et al., Paper III.  1994AJ....108.1786G, Cat. <J/AJ/108/1786>
   Guhathakurta et al., Paper V.    1996AJ....111..267G

Origin: AAS CD-ROM series, Volume 8, 1997         Lee Brotzman [ADS] 09-Apr-1997
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(End)                                                         [CDS]  03-Jul-1997