Analysis of colour-magnitude diagrams of rich LMC clusters: NGC1831

We present the analysis of a deep colour-magnitude diagram (CMD) of NGC 1831, a rich star cluster in the LMC. The data were obtained with HST/WFPC2 in the F555W (similar toV) and F814W (similar toI) filters, reaching m(555) similar to 25. We discuss and apply a method of correcting the CMD for sampling incompleteness and field star contamination. Efficient use of the CMD data was made by means of direct comparisons of the observed to model CMDs. The model CMDs are built by an algorithm that generates artificial stars from a single stellar population, characterized by an age, a metallicity, a distance, a reddening value, a present day mass function and a fraction of unresolved binaries. Photometric uncertainties are empirically determined from the data and incorporated into the models as well. Statistical techniques are presented and applied as an objective method to assess the compatibility between the model and data CMDs. By modelling the CMD of the central region in NGC 1831 we infer a metallicity Z = 0.012, 8.75 less than or equal to log (tau/yr) less than or equal to 8.80, 18.54 less than or equal to (m - M)(0) less than or equal to 18.68 and 0.00 less than or equal to E(B - V) less than or equal to 0.03. For the position dependent PDMF slope (alpha = -d logPhi(M)/d log M), we clearly observe the effect of mass segregation in the system. for projected distances R less than or equal to 30 arcsec, alpha similar or equal to 1.7, whereas 2.2 less than or equal to alpha less than or equal to 2.5 in the outer regions of NGC 1831.