We present the results of our investigation of the intermediate-age star cluster BS 90 , located in the vicinity of the H ii region N 66 in the SMC , observed with HST/ACS .
The high-resolution data provide a unique opportunity for a very detailed photometric study performed on one of the rare intermediate-age rich SMC clusters .
The complete set of observations is centered on the association NGC 346 and contains almost 100,000 stars down to V \simeq 28 mag .
In this study we focus on the northern part of the region , which covers almost the whole stellar content of BS 90 .
We construct its stellar surface density profile and derive structural parameters .
Isochrone fits on the CMD of the cluster results in an age of about 4.5 Gyr .
The luminosity function is constructed and the present-day mass function of BS 90 has been obtained using the mass-luminosity relation , derived from the isochrone models .
We found a slope between -1.30 and -0.95 , comparable or somewhat shallower than a typical Salpeter IMF .
Examination of the radial dependence of the mass function shows a steeper slope at larger radial distances , indicating mass segregation in the cluster .
The derived half-mass relaxation time of 0.95 Gyr suggests that the cluster is mass segregated due to its dynamical evolution .
From the isochrone model fits we derive a metallicity for BS 90 of [ Fe / H ] = -0.72 , which adds an important point to the age-metallicity relation of the SMC .
We discuss our findings on this relation in comparison to other SMC clusters .