The AGB stars of the intermediate-age LMC cluster NGC 1846 Variability and age determination

Aims. We investigate variability and we model the pulsational behaviour of AGB variables in the intermediate-age LMC cluster NGC 1846. Methods. Our own photometric monitoring has been combined with data from the MACHO archive to detect 22 variables among the cluster's AGB stars and to derive pulsation periods. According to the global parameters of the cluster we construct pulsation models taking into account the effect of the C/O ratio on the atmospheric structure. In particular, we have used opacities appropriate for both O-rich stars and carbon stars in the pulsation calculations. Results. The observed P-L-diagram of NGC 1846 can be fitted using a mass of the AGB stars of about 1.8 M_⊙. We show that the period of pulsation is increased when an AGB star turns into a carbon star. Using the mass on the AGB defined by the pulsational behaviour of our sample we derive a cluster age of 1.4 × 10⁹ years. This is the first time the age of a cluster has been derived from the variability of its AGB stars. The carbon stars are shown to be a mixture of fundamental and first overtone radial pulsators.