We calculate the broad-band photometric evolution of unresolved star clusters , including the preferential loss of low-mass stars due to mass segregation , in a simplified way .
The stellar mass function of a cluster evolves due to three effects : ( a ) the evolution of the massive stars reduces their number ; ( b ) tidal effects before cluster-wide mass segregation reduce the mass function homogeneously , i.e .
independently of the stellar mass ; ( c ) after mass segregation has completed , tidal effects preferentially remove the lowest-mass stars from the cluster .
These effects result in a narrowing of the stellar mass range .
These three effects are described quantitatively , following the results of N -body simulations , and taken into account in the calculation of the photometric history , based on the galev cluster evolution models for solar metallicity and a Salpeter mass function .
We find the following results : ( 1 ) During the first \sim 40 % of the lifetime of a cluster its colour evolution is adequately described by the standard galev models ( without mass segregation ) but the cluster gets fainter due to the loss of stars by tidal effects .
During this phase the colour evolution is the same for clusters with and without initial mass segregation . ( 2 ) Between \sim 40 and \sim 80 % of its lifetime ( independent of the total lifetime ) the cluster gets bluer due to the loss of low-mass stars .
This will result in an underestimate of the age of clusters if standard cluster evolution models are used .
The correction increases from 0.15 dex for a cluster with a total lifetime of 3 Gyr to 0.5 dex for clusters with a total lifetime of 30 Gyr . ( 3 ) After \sim 80 % of the total lifetime of a cluster it will rapidly get redder .
This is because stars at the low-mass end of the main sequence , which are preferentially lost , are bluer than the AGB stars that dominate the light at long wavelengths .
This will result in an overestimate of the age of clusters if standard cluster evolution models are used . ( 4 ) Clusters with mass segregation and the preferential loss of low-mass stars evolve along almost the same tracks in colour-colour diagrams as clusters without mass segregation .
Therefore it will be difficult to distinguish this effect from that due to the cluster age for unresolved clusters .
Only if the total lifetime of clusters can be estimated then the colours can be used to give reliable age estimates . ( 5 ) The changes in the colour evolution of unresolved clusters due to the preferential loss of low-mass stars will affect the determination of the star formation histories of galaxies if they are derived from clusters that have a total lifetime of less than about 30 Gyr . ( 6 ) The preferential loss of low-mass stars might explain the presence of old ( \sim 13 Gyr ) clusters in NGC 4365 which are photometrically disguised as intermediate-age clusters ( 2 – 5 Gyr ) , if the expected total lifetime of these clusters is between 16 and 32 Gyr .
It may also explain the concentration of these clusters towards the center of NGC 4365 .