Fig. 5
a) Map of the abundance ratio 
/XC18O. b)
Locations of the regions summarized in Table 4. Colors to indicate the individual regions are the same as those of the
plots in Fig. 6. In panel a), the
contours show the value of /XC18O = 10. The crosses
show the positions of θ1 Ori C, NU Ori, and 42 Ori,
which are the exciting stars of the HII regions, M 42, M 43, and NGC 1977.
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