92
The distribution of these masses depends upon the IMF, which scales as dN
dlog
M
M
,
where M
and N
represent the number and mass of stars in a dis-
tribution with a given IMF slope,
,
. For example, a distribution with = 1
:
0
produces a cluster with roughly 5 times the number of stars with masses M =
7
,
30M than with masses M
30M . McCray & Kafatos 1987 relate the
energy input during the two phases with the radius of the expanding structure.
For the rst few million years, the mechanical energy driving the expansion of the
shell is provided by the stars in the range M
30M via stellar winds. During
its main-sequence lifetime and Wolf-Rayet phase, each star M
30M imparts
a net mechanical energy of
10
51
ergs via stellar winds. The ionizing radiation
and stellar wind power decrease sharply after
5 Myr, as the O star population
terminates as supernovae. At this point, the radius of the shell is typically
100
pc. Each supernova explosion provides a net mechanical energy of
10
51
ergs
driving the expansion until
50 Myr, when the least massive stars M = 7M
explode. Expansion of the structure continues until radiative cooling of the hot
interior becomes a factor.
The above process produces an expanding hot bubble surrounded by a thin,
cold shell. As it expands, the dense ridge accumulates and mixes gas clouds in
its path. Described by Elmegreen 1992 as the collect and collapse" method,
dynamical instability of the accumulated gas collapses under self gravity to form
new stars along the periphery of the shell. New star formation is regulated by the
relationship between the density in the shell and the critical density for collapse.
The ridge surrounding the shell may fragment leaving chains or arcs of new star
formation.
