Access to Astronomical Catalogues

J/A+A/469/529       UBVI photometry of NGC 6864              (Kravtsov+, 2007)
Multi-color photometry of the Galactic globular cluster M75 = NGC 6864. A new
sensitive metallicity indicator and the position of the horizontal branch in UV.
    Kravtsov, V., Alcaino, G., Marconi, G., Alvarado F.
   <Astron. Astrophys. 469, 529 (2007)>
   =2007A&A...469..529K
ADC_Keywords: Clusters, globular ; Photometry, CCD ; Photometry, UBVRI

Keywords: Galaxy: globular clusters: general -
          Galaxy: globular clusters: individual: M75 - stars: population II

Abstract:
    We carry out and analyze new multi-color photometry of the Galactic
    globular cluster (GC) M75 in UBVI and focus on the brighter sequences
    of the color-magnitude diagram (CMD), with particular emphasis on
    their location in U-based CMD. Specifically, we study the level both
    of the horizontal (HB) and red giant branches (RGB) relative to the
    main-sequence turnoff (TO) in the U magnitude. Along with the
    presented photometry of M75, we use our collection of photometric data
    on GCs belonging to the metal-poor range, [Fe/H]zw←1.1dex,
    obtained from observations with different equipment, but calibrated by
    standard stars situated in the observed cluster fields. We confirm our
    earlier finding, and extend it to a larger magnitude range. We
    demonstrate that Delta(UTOBHB) expressing the difference in U
    magnitude between the TO point and the level of the blue HB, near its
    red boundary, of the metal-poor GCs observed with the EMMI camera of
    the NTT/ESO telescope is about 0.4-0.5mag smaller as compared to GCs
    observed with the 100" telescope and 1.3m Warsaw telescope of the Las
    Campanas Observatory. At the same time, Delta(UTORGB), the
    difference in U magnitude between the TO and RGB inflection
    (brightest) points, does not show such an apparent dependence on the
    characteristics of U filters used, but it depends on cluster
    metallicity. We have shown, for the first time, the dependence of the
    parameter Delta(UTORGB) on [Fe/H] and have estimated its
    analytical expression, by assuming a linear relation between the
    parameter and metallicity. Its slope,
    Delta(UTORGB)/Delta[Fe/H]∼1.2mag/dex, is approximately a factor of
    two steeper than that of the dependence of the RGB bump position in
    the V magnitude on metallicity. The asymptotic giant branch (AGB)
    clump and features of the RGB luminosity function (LF) of M75 are also
    discussed.

    The observations were made on 3 nights, 9-12 October 1998, with the
    1.3m Warsaw telescope.
Objects:
    ------------------------------------------
       RA   (2000)   DE      Designation(s)
    ------------------------------------------
    20 06 04.8   -21 55 16   M 75 = NGC 6864
    ------------------------------------------

File Summary:

       FileName      Lrecl  Records   Explanations

ReadMe            80        .   This file
m75phot.dat       51     2681   Photometric data for 2681 stars in M75


Byte-by-byte Description of file: m75phot.dat

   Bytes Format Units   Label     Explanations

   1-  4  I4    ---     Seq       Sequential number
   6- 12  F7.2  pix     Xpos      x coordinate (1)
  14- 20  F7.2  pix     Ypos      y coordinate (1)
  22- 27  F6.3  mag     Vmag      V magnitude
  30- 36  F7.3  mag     Umag      ?=-99.999 U magnitude (2)
  38- 44  F7.3  mag     Bmag      ?=-99.999 B magnitude (2)
  46- 51  F6.3  mag     Imag      I magnitude

Note (1): x increases to the east, y increases to the north.
          The center of the measured field (x∼1025pix, y∼1025pix) is
          approximatly 50" to the east and 10" to the south of the
          cluster center. The array scale is 0.417arcsec/pixel.
Note (2): Number -99.999 means lack of photometric measurement



Acknowledgements:
    Valery Kravtsov, vkravtsov(at)ucn.cl
(End)                                        Patricia Vannier [CDS]  04-Jun-2007