A ring/disk/outflow system associated with W51 North: a very massive
  star in the making
  Sensitive and high angular resolution ($\sim$ 0.4\arcsec) SO$_2$[22$_{2,20}$
$\to$ 22$_{1,21}$] and SiO[5$\to$4] line and 1.3 and 7 mm continuum
observations made with the Submillimeter Array (SMA) and the Very Large Array
(VLA) towards the young massive cluster W51 IRS2 are presented. We report the
presence of a large (of about 3000 AU) and massive (40 M$_\odot$) dusty
circumstellar disk and a hot gas molecular ring around a high-mass protostar or
a compact small stellar system associated with W51 North. The simultaneous
observations of the silicon monoxide molecule, an outflow gas tracer, further
revealed a massive (200 M$_\odot$) and collimated ($\sim14^\circ$) outflow
nearly perpendicular to the dusty and molecular structures suggesting thus the
presence of a single very massive protostar with a bolometric luminosity of
more than 10$^5$ L$_\odot$. A molecular hybrid LTE model of a Keplerian and
infalling ring with an inner cavity and a central stellar mass of more than 60
M$_\odot$ agrees well with the SO$_2$[22$_{2,20}$ $\to$ 22$_{1,21}$] line
observations. Finally, these results suggest that mechanisms, such as mergers
of low- and intermediate- mass stars, might be not necessary for forming very
massive stars.
