Census and lifetime of high-mass prestellar cores : what are the forces in presence to prevent the gravitational collapse?

The study of the early phases of high-mass star formation is a major topic in astrophysics that is still poorly understood. Several formation scenarios are in competition. To elucidate which precise processes of formation are in action, 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. Here we survey for high-mass prestellar core candidates in the data cubes of massive proto-clusters mapped in the ALMA-IMF program (ALMA Large Program; Motte et al. 2022). For this purpose, an automatized method for systematically detecting outflows from proto-stellar cores is developed. Cores without significant outflows are considered as excellent candidates to be prestellar cores. For this we compare the spectrum of each source (on source) with the spectrum of its surrounding environment (off source). We use both the CO(2-1) and SiO(5-4) spectral lines to identify outflowing gas. In addition to the spectra, we use outflows maps in order to confirm signs for bipolar outflow around each. 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_⊙, and for a total amount of around 51 high-mass proto-stellar cores in the survey. Among those 30 cores, 12 have a mass greater than 16 M_⊙ and will most certainly form a high-mass star in the future. We can then derive statistical lifetimes in several bins of mass with lifetimes below 105 yr for the most massive cores. We can then conclude that the high-mass pre-stellar cores are rare with short statistical lifetimes. But on the other hand, when we compare with the typical free-fall times inside the detected cores, the obtained lifetimes are relatively large with values of the order of 10 x T_ff. Such large values in terms of T_ff strongly suggest 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 turbulence is enough for around 1/3 of the 12 cores to prevent gravitational collapse, but not enough for the two other thirds of the cores. Additional magnetic field with strength between 5 and 50 mG would be needed to justify the long lifetimes of these cores.