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Valeriy Kravtsov |
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"U-based CMDs of the Metal-Poor
([Fe/H < -1.0) Galactic Globular Clusters" |
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The U bandpass is typically rarely used in studies of globular clusters (GCs), although it is a source of potentially valuable observational information on stellar populations of the clusters. The present talk is mainly devoted to an overall morphology of U-based CMDs of the metal-poor Galactic GCs, with [Fe/H] < 1.0 dex. In particular, we focus on the level both of the horizontal (HB) and red giant branches (RGB) relative to the main-sequence turnoff (TO) in U magnitude. In our analysis, we rely on a collection of photometric data gathered with different Chilean telescopes and equipment, but calibrated by standard stars situated in the observed cluster fields. We demonstrate that Delta U (TO BHB) expressing the difference in U magnitude between the TO point and the level of the blue HB, near its red boundary, of metal-poor GCs is bimodal. Specifically, if U band response curve is close to the standard one then Delta U (TO - BHB) is about 0.4 - 0.5 mag larger as compared to its value obtained from photometry in U bandpass with response curve blue-shifted of the Balmer jump. At the same time, Delta U (TO - RGB), the difference in U magnitude between the TO and RGB inflection (brightest) points, does not show such an apparent dependence on this kind of variation in U band response curve, but it notably depends on cluster metallicity. The dependence of the parameter Delta U (TO - RGB) on [Fe/H] is approximately a factor of two steeper than, for example, the dependence on metallicity of the RGB bump position in V magnitude. However, some factors are able to affect both Delta U (TO - RGB) and Delta U (TO - RGB), particularly an essential red leak of U filters. In addition, we briefly mention a possibility to estimate age of metal-poor GCs in a distance-independent way, using main-sequence photometry in two-color diagrams with the U - B index. |