Astronomical Applications Department, U.S. Naval Observatory thesis Page 115
101
stars is su cient to create the HI holes that encircle them. The largest hole in
HoII actually contains two star forming regions and these are added together to
estimate the energy input. All the star forming regions in HoII interior to HI
holes are in the oldest age group group 4, 6.3 Myr. The derived ages in Table
7.1 for these regions are upper limits in the sense that these regions are likely
much older. The technique to derive the ages depends upon the presence of H
emission and is not reliable for absolute ages 6.3 Myr. For these regions, E
SN
is
a lower limit and would increase linearly with the true cluster age. Region 13 in
HoII has signi cantly more energy than the creation energy of its associated HI
hole. It is possible that this region is also responsible for an adjacent hole 22,
which would increase E
o
by 35
10
50
ergs. The best observational evidence for a
hole cluster correlation, region 9 in IC2574, also provides an excellent quantitative
test of the correlation since a reliable age estimate of this region is known. Given
the young average age of the region, it is likely that a sizable percentage of the
energy driving the shell has provided by stellar winds alone. These comparisons
con rm that massive stars do provide su cient mechanical energy to account for
the observed properties of associated HI holes.
Galaxy Reg Hole Age
L
FUV
Mass
E
o
E
SN
Myr 10
36
ergs s
,
1
10
3
M 10
50
ergs 10
50
ergs
HoII
28
48 8.5
7.4
7.57
470.3
608.8
13
23 8.5
9.3
9.53
41.1
766.4
36,37 21 10.0
23.6
32.10
2025
3036.9
IC2574
9
35 3.7
332.5
59.40
420
693.0
Table 7.1. Derived Properties of Clusters in HI holes
