First spectroscopic evidence for carbon stars outside the local group:: Properties of a massive star cluster in NGC 7252

We present near-infrared [1-2.3 mum] spectroscopy of the massive intermediate age star cluster W3 in the merger remnant and proto-elliptical galaxy NGC 7252, obtained with the NTT telescope. This cluster has an age when the integrated near-infrared properties of a stellar population are dominated by the cool and luminous Asymptotic Giant Branch (AGB). We compare the data with instantaneous burst model predictions from new evolutionary synthesis models that include: (i) the computation of the evolution through the thermally pulsing AGB (TP-AGB) for low- and intermediate-massive stars, with the initial mass and metallicity dependent formation of carbon stars; (ii) spectroscopic data from a new stellar library in which differences between static red giants, variable oxygen rich TP-AGB stars and carbon stars are accounted for. The new evolutionary model predicts that the contribution of carbon rich stars to the luminosities in the near-IR passbands is a strong function of metallicity. The comparison of the data to the models clearly shows that carbon stars are present: for the first time, carbon rich star spectral features are thus detected directly outside the Local Group galaxies. Good fits to the available optical/near-IR photometry and the near-IR spectrum of NGC 7252-W3 are found for an age of 300-400 Myr and A(V) similar or equal to 0.6-0.8. The models show that these parameters depend weakly on the model metallicity in the range of Z/Z. = 0:4 1, with higher likelihood for solar metallicity models. At solar metallicity, a mixture of carbon rich and oxygen rich stars is predicted. The strength of the near-IR molecular bands that originated from oxygen rich AGB stars can be used to constrain the absolute T-eff scale of these objects, i.e. a relation between colour and T-eff. We found that, in the framework of our set of evolutionary tracks, the data are more consistent with the temperature scale calibrated on Long Period Variables than on giant stars. At a given colour, variable AGB stars have a lower T-eff than static (or quasi-static) M giants.