Cluster formation versus star formation rates around six regions in the Large Magellanic Cloud

Indian Institute of Astrophysics, II Block, Koramangala, Bangalore 560034, India e-mail: purni@iiap.ernet.in

Received: 5 January 2004
Accepted: 15 June 2004

Abstract

The stellar population and star clusters around six regions in the Large Magellanic Cloud (LMC) are studied to understand the correlation between star formation and cluster formation rates. We used the stellar data base of the OGLE II LMC survey and the star cluster catalogues. The observed distributions of stellar density in the colour-magnitude diagrams (CMDs) were compared with synthetic ones generated from stellar evolutionary models. By minimising the reduced  values, the star formation history of the regions were obtained in terms of star formation rates (SFR). All the regions were found to show large SFRs between the ages 500-2 Gyr with lower values for younger and older ages. A correlated peak in the cluster and SFRs is found for ages ~1 Gyr, and for ages less than 100 Myr. Five of the six regions show significant cluster formation in the age range of 100-300 Myr, when the SFRs were found to be very low. This indicates anti-correlation between star and cluster formation rates for the 100-300 Myr age range. A possible reason may be that the stars are predominantly formed in clusters, whether bound or unbound, as a result of star formation during the above age range. The enhanced cluster formation rate in the 100-300 Myr age range could be correlated with the encounter of the LMC with the Small Magellanic Cloud, while the enhanced star and cluster formation at ~1 Gyr does not correspond to any interaction. This could indicate that the star formation induced by interactions is biased towards group or cluster formation of stars.