The origin of the highest mass stars is still an enigma in modern astrophysics. Only massive clumps, at the
onset of star formation, can reveal the initial conditions and shed light on the necessary physical processes
leading to their formation. The 870 micron ATLASGAL survey of the inner Galaxy is ideally suited to search for
massive clumps still at their early evolutionary stage. Based on ATLASGAL, we identified the complete sample of
infrared quiet massive clumps located at a distance closer than 5 kpc. I will present results from an ALMA
follow-up study to search for embedded protostars in this sample of young massive clumps. ALMA reveals that
their fragmentation properties are not consistent with pure thermal processes and that at the early stage of
massive clumps fragmentation is limited to a few number of very massive cores showing a significant
concentration of mass at small scales. Finally, I will present a detailed study of a high-mass protostellar
envelope reaching 400 au physical scales.
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