Washington photometry of 14 intermediate-age to old star clusters in the Small Magellanic Cloud

Abstract

We present CCD photometry in the Washington system C, T1 and T2 passbands down to T1~ 23 in the fields of L3, L28, HW66, L100, HW79, IC1708, L106, L108, L109, NGC643, L112, HW84, HW85 and HW86, 14 Small Magellanic Cloud (SMC) clusters, most of them poorly studied objects. We measured T1 magnitudes and C-T1 and T1-T2 colours for a total of 213516 stars spread throughout cluster areas of 14.7 × 14.7arcmin2 each. We carried out an in-depth analysis of the field star contamination of the colour-magnitude diagrams (CMDs) and statistically cleaned the cluster CMDs. Based on the best fits of isochrones computed by the Padova group to the (T1, C-T1) CMDs, as well as from the δ(T1) index and the standard giant branch procedure, we derived ages and metallicities for the cluster sample. With the exception of IC1708, a relatively metal-poor Hyades-age cluster, the remaining 13 objects are between intermediate and old age (from 1.0 to 6.3Gyr), their [Fe/H] values ranging from -1.4 to -0.7dex. By combining these results with others available in the literature, we compiled a sample of 43 well-known SMC clusters older than 1Gyr, with which we produced a revised age distribution. We found that the present clusters' age distribution reveals two primary excesses of clusters at t~ 2 and 5Gyr, which engraves the SMC with clear signs of enhanced formation episodes at both ages. In addition, we found that from the birth of the SMC cluster system until approximately the first 4Gyr of its lifetime, the cluster formation resembles that of a constant formation rate scenario.