Hot ammonia around young O-type stars. III. High-mass star formation and
  hot core activity in W51~Main
  This paper is the third in a series of ammonia multilevel imaging studies in
well-known high-mass star forming regions. Using the JVLA, we have mapped the
hot and dense molecular gas in W51 Main, with about 0.2 arcsec angular
resolution, in five highly-excited metastable inversion transitions of ammonia
(ammonia): (J,K)=(6,6), (7,7), (9,9), (10,10), and (13,13). We have identified
and characterised two main centers of high-mass star formation in W51-Main: the
W51e2 complex and the W51e8 core (6 arcsec southward of W51e2). The former
breaks down into three further sub-cores: W51e2-W, which surrounds the well
known HC HII region, where hot ammonia is observed in absorption, and two
additional dusty cores, W51e2-E (~0.8" to the East) and W51e2-NW (~1" to the
North), where hot ammonia is observed in emission. The velocity maps towards
the HC HII region show a clear velocity gradient that may indicate rotation,
though we do not directly observe a Keplerian velocity profile. The absence of
outflow and/or maser activity and the low amount of molecular gas available for
accretion (~5 solar masses) with respect to the mass of the central YSO (>20
solar masses), both indicate that the central YSO has already accreted most of
its final mass. On the other hand, in the nearby W51e2-E object, the relatively
large amount of hot molecular gas available for accretion (~20 solar masses,
within about half an arcsecond or 2500 AU), along with strong outflow and maser
activity, indicates that the main accretion center in the W51e2 complex is
W51e2-E rather than W51e2-W. Finally, W51e2-NW and W51e8, although less dense,
are also hot cores and contain a significant amount of molecular gas (~30 and
70 solar masses, respectively). We speculate that they may host high-mass YSOs
either at a previous evolutionary stage or with lower mass than W51e2-E and
W51e2-W.
