Evolution of young protoclusters embedded in dense massive clumps. A new grid of population synthesis SED models and a new set of L/M evolutionary tracks

A grid of 20 millions 3-1100$mu$m SED models is presented for synthetic young clusters embedded in dense clumps. The models depend on four primary parameters: the clump mass M$_{clump}$ and dust temperature T$_{dust}$, the fraction of mass f$_{core}$ locked in dense cores, and the age of the clump t$_{SF}$. We populate the YSO clusters using the IMF from Kroupa(2001) and the YSOs SED models grid of Robitaille et al. (2006). We conduct extensive testing of SED fitting using a simulated dataset and we find that M$_{clump}$ essentially depends on the submillimeter portion of the SED, while T$_{dust}$ is mostly determined from the shape of the SED in the 70-350$mu$m range. Thanks to the large number of models computed we verify that the combined analysis of L/M, [8-24] and [24-70] colours removes much of the SEDs f$_{core}$-t$_{SF}$ degeneracy. The L/M values are particularly useful to diagnose f$_{core}$. L/M$leq$1 identifies protoclusters with f$_{core}leq$0.1 and t$_{SF} leq 10^5$ years, while L/M$geq$10 excludes f$_{core}leq$0.1. We characterize lower limits of L/M where ZAMS stars are not found in models, and we also find models with L/M $geq$10 and no ZAMS stars, in which [8-24]$geq0.8pm 0.1$ independently from M$_{clump}$, temperature and luminosity. This is the first set of synthesis SED models suited to model for embedded and unresolved clusters of YSOs. A set of new evolutionary tracks in the L/M diagram is also presented.