2019MNRAS.486.4508M

A grid of 20 millions 3-1100 µm spectral energy distribution (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 young stellar object (YSO) clusters using the Kroupa initial mass function and the YSOs SED models grid of Robitaille et al. We conduct extensive testing of SED fitting using a simulated data set and we find that M_clump essentially depends on the submillimetre portion of the SED, while T_dust is mostly determined from the shape of the SED in the 70-350 µm 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 <= 1 identifies protoclusters with f_core ≤ 0.1 and t_SF ≲ 10^5 yr, while L/M ≳ 10 excludes f_core ≤ 0.1. We characterize lower limits of L/M where zero-age main sequence (ZAMS) stars are not found in models, and we also find models with L/M >=10 and no ZAMS stars, in which [8-24] ≳ 0.8±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.