Abstract:
This paper presents integrated magnitudes and colours for synthetic clusters. The integrated parameters
have been obtained for the whole cluster population as well as for the main-sequence
(MS) population of star clusters. We have also estimated observed integrated magnitudes and
colours of the MS population of galactic open clusters, Large Magellanic Cloud (LMC) and
Small Magellanic Cloud (SMC) star clusters. It is found that the colour evolution of the MS
population of star clusters is not affected by the stochastic fluctuations, however, these fluctuations
significantly affect the colour evolution of the whole cluster population. The fluctuations
are maximum in (V − I ) colour in the age range 6.7 < log(age) < 7.5. Evolution of integrated
colours of the MS population of clusters in the Milky Way, LMC and SMC, obtained in the
present study is well explained by the present synthetic cluster model. The observed integrated
(B − V ) colours of the MS population of LMC star clusters having age ≥500 Myr seem to be
distributed around the Z = 0.004 model, whereas (V − I ) colours are found to be bluer than
those predicted by the Z = 0.004 model. The (V − I) versus (B − V ) two-colour diagram
for the MS population of the Milky Way star clusters shows a fair agreement between the
observations and present model, however, the diagrams for LMC and SMC clusters indicate
that observed (V − I ) colours are relatively bluer. Possible reasons for this anomaly have been
discussed. Comparison of the synthetic (U − B) versus (B − V ) relation with the observed
integrated parameters of the whole cluster population of the Milky Way, LMC and SMC star
clusters indicates that the majority of the bluest clusters [(B − V )₀ < 0.0] follow the MS
population relation. The colour evolution of young MilkyWay, LMC and SMC clusters [6.5 ≤ log(age) ≤8.0] also indicates that a large number of young clusters follow the MS population
relation. Therefore, in the absence of a careful modelling of stochastic effects, age determination
of young star clusters by comparing their integrated colours with whole cluster synthetic
colours may yield erroneous results.
