Deriving physical parameters of unresolved star clusters

I. Age, mass, and extinction degeneracies^⋆

¹ Vilnius University Observatory, Čiurlionio 29, 03100 Vilnius, Lithuania
e-mail: philippe.demeulenaer@ff.stud.vu.lt
² Center for Physical Sciences and Technology, Savanoriu 231, 02300 Vilnius, Lithuania

Received: 31 October 2012
Accepted: 28 November 2012

Abstract

Context. Stochasticity and physical parameter degeneracy problems complicate the derivation of the parameters (age, mass, and extinction) of unresolved star clusters when using broad-band photometry.

Aims. We develop a method to simulate stochasticity and degeneracies and to investigate their influence on the accuracy of derived physical parameters. Then we apply it to star cluster samples of M 31 and M 33 galaxies.

Methods. Age, mass, and extinction of observed star clusters are derived by comparing their broad-band UBVRI integrated magnitudes to the magnitudes of a large grid of star cluster models with fixed metallicity Z = 0.008. Masses of stars for a cluster model are randomly sampled from the initial mass function. Models of star clusters from the model grid, which have all of their magnitudes located within 3 observational errors from the magnitudes of the observed cluster, are selected for computing their age, mass, and extinction.

Results. In the case of the M 31 galaxy, the extinction range is wide and the age-extinction degeneracy is strong for a fraction of its clusters. Because of a narrower extinction range, the age-extinction degeneracy is weaker for the M 33 clusters. By using artificial cluster sample, we show that age-extinction degeneracy can be reduced significantly if the range of intrinsic extinction within the host galaxy is narrow.