region is uniformly 15 K, somewhat higher than well-shielded dense gas, which is plausible in view of increased heating in the peripheral regions surrounding regions of high extinction. [e.g. @li2003b]. We take advantage of the large number of pixels in our map, and bin the data according to the excitation temperature of the determined as described above. In each bin, we have a sufficient number of pixels that the  J = 1$\rightarrow$0 line is detected with good signal to noise ratio. For each $T_{ex}$ bin, we then have the  excitation temperature and the observed / integrated intensity ratio. The data generally have the observed intensity ratio decreasing with increasing $T_{ex}$, from $\simeq$ 22 for $T_{ex}$ = 4.5 K to $\simeq$ 13 for $T_{ex}$ = 2.5 K. The free parameters are the  column density, the 2 density, and the / abundance ratio. The latter cannot be assumed to be a fixed value (e.g. 65), due to the complicating presence of isotopic enhancement due to chemical and/or photo effects [e.g. @watson1976; @bally1982; @chu1983; @vandishoeck1988]. We thus consider $R$ = / between 25 and 65.

[cccccc]{} 4.5 &21.7 &32321 &125 &0.7 &30\
5.5 &21.7 &113923 &200 &1.0 &35\
6.5 &19.6 &202328 &250 &1.4 