CCD photometry of the Large Magellanic cloud cluster NGC 1866 observational results (1989)
Keywords :
clusters globular - galaxies: Magellanic Clouds - photometry - stars: evolution
Abstract:The first part of this paper is devoted to a new CCD investigation of stars in the field of the LMC cluster NGC 1866. Two main frames have been analyzed, one covering the cluster's central region and the other covering an area about 10' from the cluster's center, and data has been collected for 2207 objects. On this basis, a C-M diagram has been derived, which shows a well-developed blue sequence reaching a limiting magnitude of about V=21mag together with the already known sequence of the cluster's He-burning giants. A new feature in our data is represented by the evidence for a second group of less luminous giants clumping around V=19mag, which we interpret as field He-burning giants. The second part of the paper is devoted to a comparison of
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The first part of this paper is devoted to a new CCD investigation of
stars in the field of the LMC cluster NGC 1866. Two main frames have
been analyzed, one covering the cluster's central region and the other
covering an area about 10' from the cluster's center, and data has
been collected for 2207 objects. On this basis, a C-M diagram has been
derived, which shows a well-developed blue sequence reaching a
limiting magnitude of about V=21mag together with the already known
sequence of the cluster's He-burning giants. A new feature in our data
is represented by the evidence for a second group of less luminous
giants clumping around V=19mag, which we interpret as field He-burning
giants. The second part of the paper is devoted to a comparison of
these data with theoretical calculations. On the basis of new
evolutionary tracks we find that the composition Y=0.28, Z=0.02
already used by Becker and Mathews (1983ApJ...270..155B) offers an
adequate fitting of the observed range in color spanned by cluster He
giants. Theoretical isochrones for this composition have been produced
and are compared with the observed distribution of cluster stars. A
good agreement with the observed C-M diagram is achieved for a cluster
age of the order of t=100Myr, whereas field stars appear to be
significantly older, with ages of about 500 Myr or more. The
distribution of cluster stars is studied, and both the luminosity
function of the blue sequence (BS) and number of He giants relative to
BS stars are examined. We find that both these cluster characteristics
can be reproduced with a Salpeter slope for the initial mass function,
overcoming previous difficulties raised by suggestions of a serious
disagreement between theory and observations in this cluster. The
Appendix reports data for the evolutionary models and for the
corresponding isochrones, covering both the H- and He-burning phases