Integrated parameters of star clusters: A comparison of theory and observations

TL;DR

This study compares theoretical models and observations of integrated magnitudes and colours of star clusters, focusing on main-sequence populations and stochastic fluctuations, across the Milky Way, LMC, and SMC.

Contribution

It provides synthetic cluster models that explain the evolution of integrated colours and highlights discrepancies in observed versus predicted colours for LMC and SMC clusters.

Findings

Colour evolution of MS populations is unaffected by stochastic fluctuations.

Fluctuations significantly impact the colour evolution of entire clusters.

Observed colours of LMC and SMC clusters show deviations from model predictions.

Abstract

(Abridged) This paper presents integrated magnitude 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 MS population of galactic open clusters, LMC and SMC star clusters. It is found that the colour evolution of 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 MS population of the clusters in the Milky Way, LMC and SMC, obtained in the present study are well explained by the present synthetic cluster model. The observed integrated $(B-V)$ colours of MS population of LMC star clusters having age $\geq$ 500 Myr seem to be distributed around $Z=$ 0.004 model, whereas $(V-I)$ colours are found to be more bluer than those predicted by the $Z=$ 0.004 model. $(V-I)$ vs $(B-V)$ 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.