IRS spectroscopy of extremely young massive protostars

We have established a sample of massive pre-stellar core candidates using the ISOPHOT Serendipity Survey (ISOSS) and ground based follow-up observations in the (sub)mm continuum and of molecular lines. Thirteen of these sources were successfully observed with IRAC and MIPS during GO Cycle 2. Despite their early evolutionary stage, indicated by low dust and gas kinetic temperatures, all objects have embedded point sources detected with MIPS at 24 um. The origin of this mid-infrared emission is yet unknown but might be due to the potential energy released during the initial gravitational collapse of the core, becoming a massive protostar or protocluster. We have therefore selected a subsample of six objects for follow-up observations with IRS and ask for 5 hours of Spitzer time. The IRS spectra are mandatory to characterize the physical conditions in the discovered mid-infrared sources and unveil their true nature. This will be achieved by means of radiative transfer modeling and warm gas diagnostics.