Multi-directional mass-accretion and collimated outflows in W51
  We observed the W51 high-mass star-forming complex with ALMA's
longest-baseline configurations, achieving an angular resolution of $\sim$20
milliarcseconds, corresponding to a linear resolution of $\sim$100 au at
$D_{\mathrm{W51}}=5.4$ kpc. The observed region contains three high-mass
protostars in which the dust continuum emission at 1.3 mm is optically-thick up
to a radius $\lesssim$1000 au and has brightness temperatures $\gtrsim$200 K.
The high luminosity ($\gtrsim10^4$ L$_{\odot}$) in the absence of free-free
emission suggests the presence of massive stars ($M\gtrsim20$ M$_{\odot}$) at
the earliest stages of their formation. Our continuum images reveal remarkably
complex and filamentary structures arising from compact cores. Molecular
emission shows no clear signs of rotation nor infall on scales from 150 to 2000
au: we do not detect disks. The central sources drive young ($\sim$100 years),
fast ($\sim 100$ km s$^{-1}$), powerful ($\dot{M}>10^{-4}$ M$_{\odot} \
yr^{-1}$), collimated outflows. These outflows provide indirect evidence of
accretion disks on scales $r\lesssim$100--500 au (depending on the object). The
active outflows are connected to fossil flows that have different orientations
on larger spatial scales, implying that the orientations of these small disks
change over time. These results together support a variant of an accretion
model for high-mass star formation in which massive protostars do not form a
large, stable Keplerian disk during their early stages, but instead they
accrete material from multiple massive flows with different angular momentum
vectors. This scenario therefore contrasts with the simplified classic paradigm
of a stable disk+jet system, which is the standard model for low-mass star
formation, and provides an experimental confirmation of a multi-directional and
unsteady accretion model for massive star formation.
