Age Determination of Fifteen Old to Intermediate-Age Small Magellanic Cloud Star Clusters

M.C. PARISI; ET AL.; A.E. PIATTI

ASTRONOMICAL JOURNAL

IOP PUBLISHING LTD

Año: 2014 vol. 147 p. 1 - 17

0004-6256

    We present CMDs in the V and I bands for fifteen star clusters in the SmallMagellanic Cloud (SMC) based on data taken with the Very Large Telescope(VLT, Chile). We selected these clusters from our previous work (Parisi et al.2009), wherein we derived cluster velocities and metallicities from Calcium IIinfrared triplet (CaT) spectra also taken with the VLT. PSF photometry wasperformed on FORS2 preimaging exposures which were purposely lengthened inorder to obtain deep photometry to derive accurate ages. In the CMD of eachcluster we measured the δV parameter and derived cluster ages using the cali-bration of Carraro & Chiosi (1994) and Salaris et al. (2004). We found thatthe calibration of Salaris et al. (2004) tends to overestimate ages. The age dif-ference between these two calibrations increases for old clusters. We discoveredthat the ages of six of our clusters have been appreciably underestimated by pre-vious works, which used comparatively small telescopes, graphically illustratingthe need for large apertures to obtain reliable ages of old and intermediate-ageSMC star clusters. In particular, three of these clusters (L 4, L 6 and L 110), turnout to be amongst the oldest SMC clusters known. The determination of theirfundamental properties allow us to better trace the early chemical evolution ofthis galaxy. Using the calibration of Carraro & Chiosi (1994), L 110, L 4 and L 6are 7.61 ± 1.03, 7.93 ± 1.07 and 8.71 ± 1.18 Gyr old, respectively, helping to fillthe ?SMC cluster age gap? (Glatt et al. 2008a). In order to corroborate the an-cient nature of these three clusters, we also calculated their ages from two otherindependent methods. First, we derived a new calibration using clusters studiedby Glatt et al. (2008a,b) using deep HST data, which can be considered the bestconstrained SMC cluster age calibration. Secondly, we fit theoretical isochronesto the CMD, inspired by the procedures of Glatt et al. (2008a,b). Both methodsconfirm that all three clusters are indeed very old. Finally, using ages from thecalibration of Carraro & Chiosi (1994) and metallicities from (Parisi et al. 2009),we analyze the age distribution, age gradient and age metallicity relation (AMR)of a sample of SMC clusters measured homogeneously. There is a suggestion ofbimodality in the age distribution but it does not show a constant formation forthe first 4 Gyr (Piatti et al. 2011b), and we find no evidence for an age gradient.Due to the improved ages of our cluster sample, we find that our AMR is nowbetter represented in the intermediate/old period than that we derived in Parisiet al. 2009, where we simply took ages available in the literature. Additionally,clusters younger than ∼ 4 Gyr now maintain better agreement with the burst-ing model of Pagel & Tautvaiˇien ̇ (1998), but we confirm that this model is                                s enot a good representation of the AMR during the intermediate-age/old period.A more complicated model is needed to explain the SMC chemical evolution inthat period.