EDP Sciences
Free access
Volume 390, Number 1, July IV 2002
Page(s) 121 - 132
Section Stellar clusters and associations
DOI http://dx.doi.org/10.1051/0004-6361:20020692

A&A 390, 121-132 (2002)
DOI: 10.1051/0004-6361:20020692

Analysis of colour-magnitude diagrams of rich LMC clusters: NGC 1831

L. O. Kerber1, B. X. Santiago1, R. Castro1 and D. Valls-Gabaud2

1  Universidade Federal do Rio Grande do Sul, IF, CP 15051, Porto Alegre 91501-970 RS, Brazil
2  UMR CNRS 5572, Observatoire Midi-Pyrénées, 14 avenue Édouard Belin, 31400 Toulouse, France

(Received 8 April 2002 / Accepted 6 May 2002 )

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 (~ V) and F814W (~ I) filters, reaching $m_{555} \sim 25$. 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, $8.75 \leq \log\,(\tau/{\rm yr}) \leq 8.80$, $18.54 \leq (m-M)_{0} \leq 18.68$ and $0.00 \leq E(B-V) \leq 0.03$. For the position dependent PDMF slope ( $ \alpha = -{\rm d}\log\Phi(M)/{\rm d}\log M$), we clearly observe the effect of mass segregation in the system: for projected distances $R \leq 30$ arcsec, $\alpha \simeq 1.7$, whereas $2.2 \leq \alpha \leq 2.5$ in the outer regions of NGC 1831.

Key words: galaxies: star clusters -- Magellanic Cloud -- globular clusters: individual: NGC 1831 -- stars: statistics

Offprint request: L. O. Kerber, kerber@if.ufrgs.br

SIMBAD Objects

© ESO 2002