High-mass prestellar cores: what are the forces in presence to prevent the gravitationnal collapse ?

To better understand the formation of high-mass stars, it is necessary to identify the youngest precursors of massive stars (M > 8 M_⊙), even before collapse (so-called prestellar cores), within high-mass star-forming regions. Using a new automatized method for systematically detecting outflows from proto-stellar cores, we search high-mass prestellar cores in the ALMA-IMF survey, i.e. cores without significant outflowing emission. For this, we rely on the CO(2-1) and SiO(5-4) lines. We show that 30 cores with a mass greater than 8 M_⊙ are such good prestellar candidates in the ALMA-IMF survey (Valeille-Manet et al. subm). They cover a mass range from 8 to 54 M_⊙, with 12 cores more massive than 16 M_⊙ that will most certainly form a high-mass star in the future. We can then derive statistical lifetimes in several bins of mass with short lifetimes below 10^5 yr for the most massive cores. But on the other hand, the obtained lifetimes are relatively large with values of the order of 10 x T_ff, suggesting the need of a support against gravity to prevent the cores to collapse on a free-fall time as expected. Using the DCN(3-2) and 13CS(5-4) lines, we studied the turbulence in the 12 most massive prestellar core candidates and found supersonic turbulence in all the cores (Valeille-Manet et al in prep). A Virial analysis reveals that additional magnetic field strengths between 5 and 50 mG would be needed for around 2/3 of the 12 cores to justify their long lifetimes.