Astronomy and Astrophysics, volume 469, 529-537 (2007/7-2)
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.
KRAVTSOV V., ALCAINO G., MARCONI G. and ALVARADO F.
Abstract (from CDS):
We carry out and analyze new multi-color photometry of the Galactic globular cluster (GC) M 75 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 M 75, 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 ΔUTOBHB 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, ΔUTORGB, 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 ΔUTORGB on [Fe/H] and have estimated its analytical expression, by assuming a linear relation between the parameter and metallicity. Its slope, ΔUTORGB/Δ[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 M 75 are also discussed.
Galaxy: globular clusters: general - Galaxy: globular clusters: individual: M 75 - stars: population II
VizieR on-line data:
<Available at CDS (J/A+A/469/529): m75phot.dat>
Table 2: Cl* NGC 6864 KAM A (Nos A-D, G). Catalog: Cl* NGC 6864 KAM NNNN (Nos 1-2681).
Status at CDS
All or part of tables of objects will not be ingested in SIMBAD.
View the reference in ADS