Detection of a high-mass prestellar core candidate in W43-MM1
  Aims. To constrain the physical processes that lead to the birth of high-mass
stars it is mandatory to study the very first stages of their formation. We
search for high-mass analogs of low-mass prestellar cores in W43-MM1.
  Methods. We conducted a 1.3 mm ALMA mosaic of the complete W43-MM1 cloud,
which has revealed numerous cores with ~ 2000 au FWHM sizes. We investigated
the nature of cores located at the tip of the main filament, where the
clustering is minimum. We used the continuum emission to measure the core
masses and the $^{13}$CS(5-4) line emission to estimate their turbulence level.
We also investigated the prestellar or protostellar nature of these cores by
searching for outflow signatures traced by CO(2-1) and SiO(5-4) line emission,
and for molecular complexity typical of embedded hot cores.
  Results. Two high-mass cores of ~ 1300 au diameter and ~ $60~M_\odot$ mass
are observed to be turbulent but gravitationally bound. One drives outflows and
is associated with a hot core. The other core, W43-MM1\#6, does not yet reveal
any star formation activity and thus is an excellent high-mass prestellar core
candidate.
