GEPI

Determining the shape of the stellar initial mass function (IMF), and whether it is constant or varies in a range of environments, is the Holy Grail of modern astrophysics, with profound implications for theories of galaxy formation. On a theoretical ground, it is expected that the extreme conditions for star formation encountered in the most powerful starbursts in the Universe favour the formation of massive stars. Direct methods of IMF determination, however, cannot probe such systems, because of the severe dust obscuration affecting their UV starlight. The next best option is to observe CNO bearing molecules in the interstellar medium at millimetre/submillimetre wavelengths, which, in principle, provides the best indirect evidence for IMF variations. In this talk, I first reassess the roles of massive stars, asymptotic giant branch stars, and novae in the production of the rare isotopes 13C, 15N, 17O, and 18O, along with the more abundant 12C, 14N, and 16O, and calibrate a Galactic chemical evolution code using Milky Way data. The code is then extended to discuss extragalactic data. Though significant uncertainties still hamper our knowledge of the evolution of CNO isotopes in galaxies, some evidence is found for an IMF skewed towards high-mass stars in a sample of submillimetre galaxies observed with ALMA at the peak of the star formation activity of the Universe. This issue deserve further investigations, as outlined at the end of the talk.