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The origins and birthplaces of massive O-type stars (with masses greater
than about 16 Msun) in our Galaxy and in others, including starbursts,
are among the fundamental contentious problems in current astrophysics.
One of the big questions is: do massive stars always form in clusters and
associations (i.e. in groups) or can they be born in isolation (i.e. as
field stars).
The answer to this question is important, as it discriminates between
different formation processes (competitive accretion vs. turbulent core
models).
The nearby magnitude-limited Galactic O-star catalog (version 2, Sota et
al.)
lists 370 entries of which about 100 are classified as field stars.
A recent study (Schilbach and Roeser 2008) has shown that the vast majority
can be traced back to a parent cluster or association (proper motion
analysis).
I will thus review the evidence that massive star formation occurs almost
exclusively in a cluster and association environment, while the field
O-stars
can be explained as runaway stars dynamically ejected from dense clusters
(Bonnell, Larson, Zinnecker 2007, PPV; Fuji \& Portegies Zwart 2011,
Science).
The two formation sites (birthplaces) of massive stars, 1) the dense bound
OB cluster environment (Orion TC, NGC3603, 30 Dor), and 2) the loose,
unbound OB association environment (Ori OB1, Sco OB2, NGC 604), will
be discussed, and explained as two extremes in a common theoretical
framework of hierarchical protoclusters (Zinnecker \& Yorke 2007, ARAA).
Finally, I will present an idea how SOFIA, the Stratospheric Observatory for
Infrared Astronomy, will allow us to pinpoint and study the location and the
dynamics of protocluster formation in molecular clouds via [CII]
observations.
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