The AGB phase-transition outside the local group: K-band observations of young star clusters in NGC 7252

We have extended the study of the young star clusters observed in the merge r remnant galaxy NGC 7252 by obtaining K band photometry for these clusters . Our K band data significantly complement the optical photometry and spect roscopy in the literature: K band data are fundamental to study the Asympto tic Giant Branch (AGB) population of these clusters, since the AGE phase tr ansition (occuring between the age of similar to 200 Myr and similar to1 Gy r) causes abrupt changes in the near-infrared luminosity of the clusters wh ile producing only small changes in the optical. Therefore, the e.g. V-K co lour is ideal to study this evolutionary phase of stellar populations. For the present analysis we present models for Simple Stellar Populations which include the contribution of the AGE stellar phase, calibrated with the you ng and intermediate age star clusters of the Magellanic Clouds. The compari son with the colour distribution of the NGC 7252 star clusters shows that t hey are indeed intermediate age clusters undergoing the AGE phase transitio n. The AGE phase transition is observed for the first time outside the Loca l Group. Most of the studied clusters span the very narrow age range 300-50 0 Myr, and likely base metallicities 0.5-1 Z(circle dot). A very important exception is the cluster W32, which has already completed its AGE epoch, it s colours being consistent with an age of similar to1-2 Gyr. This impacts o n the duration of the merger-induced starburst. The strengths of the magnes ium and iron lines in the spectrum of the best observed cluster W3, and in the spectrum of the diffuse central light of NGC 7252, do not show an overa bundance in alpha -elements, in contrast to the bulk stellar population of elliptical galaxies.