The earliest phases of massive star formation within entire molecular cloud complexes

We started (sub-)millimeter continuum and line studies of entire molecular cloud complexes located at intermediate distances from the Sun (1-3 kpc). Such an unbiased approach allows to identify and characterize the earliest phases of high-mass stars overlooked by IRAS or MSX. Our complete MAMBO-2 surveys of the Cygnus X and NGC 7538 complexes reveal a large population of ∼ 0.1 pc-size massive young stellar objects (MYSOs) harboring high-mass infrared-quiet protostars. The determination of the nature of all the new millimeter sources is still in progress but we have already collected evidence that the infrared-quiet (or class 0-like) protostellar phase might last as long as the better-known infrared-bright protostellar phase. Besides, our complete census of MYSOs fails to discover the high-mass analogues of pre-stellar dense cores. We propose that the observed lower-density pre-stellar clumps ({>} 1 pc) rapidly concentrate and collapse as also found in the kinematical studies of other prominent clumps. Indeed, CS and HCO^+ mappings in W43 and Cygnus X suggest global supersonic contraction with inward velocities of several km s-1 on parsec scales. Our work and similar studies of entire star-forming complexes will thus definitively contribute to a better knowledge of the earliest phases of high-mass star formation.