Astronomical Applications Department, U.S. Naval Observatory thesis Page 104
7 Massive Star Formation and
the Global Gas Distribution
Comparisons of relative H and FUV morphologies established that the output
from massive stars is a viable mechanism for star formation on local scales. On
larger scales, this mechanism can also in uence new star formation activity as high
pressures from supernovae push gas away from aging OB associations. Although
they basically rely on the same process, a distinction is made between the two
triggering mechanisms due to their degree of in uence. Sequential star formation
is de ned as intermediate scale triggering since it typically acts on only one side
of a molecular cloud. Large-scale triggering describes the process by which a
centralized source destroys the remainder of its parental molecular cloud, creating
a void in the gas and a dense ridge of swept up material carried to large distances.
Dynamical instabilities in the dense ridge cause new stars to form, allowing a
means for star formation to propagate on large scales.
The reshaping of the ISM via massive stars on global scales can be seen in
observations of the neutral gas component. Comparisons of the relative FUV
and HI morphologies highlight the connection between massive star formation
and the HI voids, called HI holes or shells. Since FUV observations isolate the
massive stellar component on timescales similar to the kinematical ages of HI
holes, they can provide an observational smoking gun" to the theoretical concept
of pressure driven shells. In addition, FUV observations allow insight into the sites
of secondary star formation in the dense ridges con rming that star formation
triggering via giant HI shells is another viable mechanism for star formation. FUV
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