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 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:
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RA (2000) DE Designation(s)
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20 06 04.8 -21 55 16 M 75 = NGC 6864
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
m75phot.dat 51 2681 Photometric data for 2681 stars in M75
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Byte-by-byte Description of file: m75phot.dat
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Bytes Format Units Label Explanations
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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
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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
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Acknowledgements:
Valery Kravtsov, vkravtsov(at)ucn.cl
(End) Patricia Vannier [CDS] 04-Jun-2007