Herschel Observations of the W3 GMC (II): Clues to the Formation of
  Clusters of High-Mass Stars
  The W3 GMC is a prime target for investigating the formation of high-mass
stars and clusters. This second study of W3 within the HOBYS Key Program
provides a comparative analysis of subfields within W3 to further constrain the
processes leading to the observed structures and stellar population.
Probability density functions (PDFs) and cumulative mass distributions (CMDs)
were created from dust column density maps, quantified as extinction Av. The
shape of the PDF, typically represented with a lognormal function at low Av
"breaking" to a power-law tail at high Av, is influenced by various processes
including turbulence and self-gravity. The breaks can also be identified, often
more readily, in the CMDs. The PDF break from lognormal (Av(SF)= 6-10 mag)
appears to shift to higher Av by stellar feedback, so that high-mass
star-forming regions tend to have higher PDF breaks. A second break at Av > 50
mag traces structures formed or influenced by a dynamic process. Because such a
process has been suggested to drive high-mass star formation in W3, this second
break might then identify regions with potential for hosting high-mass
stars/clusters. Stellar feedback appears to be a major mechanism driving the
local evolution and state of regions within W3. A high initial star formation
efficiency in a dense medium could result in a self-enhancing process, leading
to more compression and favourable star-formation conditions (e.g., colliding
flows), a richer stellar content, and massive stars. This scenario would be
compatible with the "convergent constructive feedback" model introduced in our
previous Herschel study.
