Issue |
A&A
Volume 469, Number 2, July II 2007
|
|
---|---|---|
Page(s) | 529 - 537 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361:20066721 | |
Published online | 24 April 2007 |
Multi-color photometry of the Galactic globular cluster M 75 = NGC 6864. A new sensitive metallicity indicator and the position of the horizontal branch in UV *,**
1
Instituto de Astronomía, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile e-mail: vkravtsov@ucn.cl
2
Sternberg Astronomical Institute, University Avenue 13, 119899 Moscow, Russia
3
Isaac Newton Institute of Chile, Ministerio de Educación de Chile, Casilla 8-9, Correo 9, Santiago, Chile e-mail: inewton@terra.cl, falvarad@eso.org
4
ESO - European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile e-mail: gmarconi@eso.org
Received:
9
November
2006
Accepted:
22
March
2007
Aims.We carry out and analyze new multi-color photometry of the
Galactic globular cluster (GC) M 75 in 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.
Methods.Along with the presented photometry of M 75, we use our collection
of photometric data on GCs belonging to the metal-poor range,
[Fe/H] dex, obtained from observations with different
equipment, but calibrated by standard stars situated in the observed
cluster fields.
Results.We confirm our earlier finding, and extend it to a larger magnitude range. We demonstrate
that 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 0.4–0.5 mag smaller as
compared to GCs observed with the 100″ telescope and 1.3 m
Warsaw telescope of the Las Campanas Observatory. At the same time,
, 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
on
[Fe/H] and have estimated its analytical expression, by assuming a linear relation between the parameter and metallicity. Its slope,
/Δ[Fe/H] ~ 1.2 mag/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 M 75 are also discussed.
Key words: Galaxy: globular clusters: general / Galaxy: globular clusters: individual: M 75 / stars: population II
© ESO, 2007
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