Thanks to its unmatched angular resolution, sensitivity, image quality, and excellent frequency coverage, ALMA is revolutionizing our understanding of star formation.

The number of stars with a given mass at birth, so called IMF, is among the very few key parameters transcending astrophysical fields. It is of paramount importance to current theories ranging from cosmology to stellar physics and exo-planets. The IMF is considered to be universal through virtually all galactic environments studied so far. Its origin, arguably the most central question in star formation, remains a major open issue in modern astrophysics.

For the past 20 years, both observational surveys and theories of star formation have claimed that stellar masses are determined by cloud fragmentation, and thus by the gas mass of cores. Until today however, observational constraints from so-called core mass functions (CMFs) were limited to regions in our Solar neighborhood, which only form stars with 0.1 − 5 Mo. These regions are unrepresentative of the Galaxy; they do not capture clouds which form stars > 5 Mo, high-mass cloud environments, or our Galaxy in its vast extent and range of conditions. Our current understanding of the origin of stellar masses is therefore biased if not distorted. Studying massive protoclusters is mandatory to test if the IMF origin can be independent of cloud characteristics.

The ALMA-IMF large program questions the origin of the IMF through the analysis fifteen massive star-forming regions.

It will also provide the community with an unprecedented database with high legacy value for cores, hot cores, stellar clusters, and extragalactic studies. The fine ALMA mosaics resulting from the ALMA-IMF project will broaden our view of high-mass star formation by simultaneously capturing all relevant scales involved in this process, from 1 pc down to 0.01 pc, as well as the concurrent formation of high-mass and low-mass stars.

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