CDROM/AJ/V113/P0669     The Core of M13 (Cohen+ 1997)
================================================================================
Globular Cluster Photometry with the Hubble Space Telescope. VI. WF/PC-I
Observations of the Stellar Populations in the Core of M13 (NGC 6205)
      Randi L. Cohen, Puragra Guhathakurta, Brian Yanny, Donald P. Schneider,
      and John N. Bahcall
      <1997, AJ, 113, 669>
      =1997AJ....113..669C
================================================================================

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 (60 s) 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
  1.25 arcmin^2 region of the cluster including its core and extending
  out to r~66 arcsec (2.3 pc) from its center. The sample is complete to
  V ~= 18.3 (the main sequence turnoff) and the 1 sigma photometric error
  is about 0.1 mag. 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, 1992, ApJS, 81, 163) 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 arcsec +/- 6 arcsec (90% confidence limits) or 1.3
  pc. 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:
--------------------------------------------------------------------------------
File Name    Lrecl    Records   Explanations
--------------------------------------------------------------------------------
table1.dat      55       6436   Stars in the core of M13
--------------------------------------------------------------------------------

Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
   Bytes Format  Units   Label      Explanations
--------------------------------------------------------------------------------
   1- 4   I4     ----    ID         Identification number
   5-11   F7.2   arcsec  dRA       *R.A. J2000 offset from star #5540
  12-18   F7.2   arcsec  dDE       *Dec. J2000 offset from star #5540
  19-20   I2     ---     CCD        PC number of the CCD
  21-25   F5.1   arcsec  r          Angular distance from center of M13
  26-31   F6.2   mag     F555W      F555W magnitude
  32-37   F6.2   mag     F785LP     []? F785LP magnitude
  38-43   F6.2   mag     V          []? V magnitude
  44-49   F6.2   mag     I          []? I magnitude
  50-55   F6.2   mag     V-I        []? V-I color
--------------------------------------------------------------------------------

Notes for file: table1.dat
--------------------------------------------------------------------------------
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.
--------------------------------------------------------------------------------


================================================================================
(End)                                             Lee Brotzman [ADS] 09-Apr-1997