Molecular analysis of a high-mass prestellar core candidate in W43-MM1
  High-mass analogues of low-mass prestellar cores are searched for to
constrain the models of high-mass star formation. Several high-mass cores, at
various evolutionary stages, have been recently identified towards the massive
star-forming region W43-MM1 and amongst them a high-mass prestellar core
candidate. We aim to characterise the chemistry in this high-mass prestellar
core candidate, referred to as W43-MM1 core #6, and its environment. Using ALMA
high-spatial resolution data of W43-MM1, we have studied the molecular content
of core #6 and a neighbouring high-mass protostellar core, referred to as #3,
which is similar in size and mass to core #6. We first subtracted the continuum
emission using a method based on the density distribution of the intensities on
each pixel. Then, from the distribution of detected molecules, we identified
the molecules centred on the prestellar core candidate (core #6) and those
associated to shocks related to outflows and filament formation. Then we
constrained the column densities and temperatures of the molecules detected
towards the two cores. While core #3 appears to contain a hot core with a
temperature of about 190 K, core #6 seems to have a lower temperature in the
range from 20 K to 90 K from a rotational diagram analysis. We have considered
different source sizes for core #6 and the comparison of the abundances of the
detected molecules towards the core with various interstellar sources shows
that it is compatible with a core of size 1000 au with $T = 20-90$ K or a core
of size 500 au with $T \sim 80$ K. Core #6 of W43-MM1 remains one of the best
high-mass prestellar core candidates even if we cannot exclude that it is at
the very beginning of the protostellar phase of high-mass star formation.
