We identified star clusters in archived HST/WFPC2 images of the merger and ultra-luminous infrared galaxy NGC 6240 , with the aim of investigating whether star cluster properties ( luminosity , age and mass ) in such an extreme environment differ from those of clusters in less luminous starburst galaxies .
We found 54 star clusters in all of the F450W , F547M and F814W exposures , of which 41 are located in the main body of NGC 6240 and 13 in the galactic tails .
Given that only two colours are available to derive two independent variables ( cluster reddening and age ) , we adopted an ad hoc procedure to statistically derive cluster parameters under the assumption that the cluster metallicity is LMC-like .
The colours of each cluster are fitted to STARBURST99 models of fixed mass and variable ages and reddenings .
All cluster reddening and age solutions with \chi ^ { 2 } < 1 
are considered to be consistent with the data .
Masses are derived by scaling the luminosity of the models with best-fit \chi ^ { 2 } < 1 by the observed V luminosity , after correction for reddening and distance .
Therefore , each cluster is described by a range of reddening values , ages and masses ; for each of these parameters we derive probability functions .
We thus infer that the most probable age of the observed clusters is between 5 and 13 Myr and their most probable mass is about ( 1 - 2 ) \times 10 ^ { 5 } M _ { \odot } .
A low probability exists for clusters as massive as 10 ^ { 8 } M _ { \odot } , as well as for the trend that the mean cluster mass increases towards the double nuclei of NGC 6240 .
Comparison with star clusters in starburst galaxies seems to point to more massive clusters being formed in more massive galaxies and gas-rich mergers , while the overall cluster mass distribution might be relatively independent of the details of the associated starburst where dense , massive clusters preferentially form .