coefficient for H$_2$$^{13}$CO is 1.32 $\times$ 10$^{13}$. The column densities in the 1$_{11}$ level divided by the a priori unknown quantity $T_{ex}$ for our sources are listed in Columns 6 and 7 of Table \[Table2\]. Used velocity ranges are given in Columns 4 and 5.

We derived the H$_2$$^{12}$CO/H$_2$$^{13}$CO isotope ratios in two different ways: (1) from Gaussian least square fits and (2) using planimetry to derive the ratio of their column densities (Col.(8) of Table \[Table2\]; see details in Wilson et al. ). Since the ratios obtained by these two methods differ by only 10% within the permissible margin of error, we average these two ratios to get the final H$_2$$^{12}$CO/H$_2$$^{13}$CO ratios. The average ratios corrected for telescope gain (see details in Section \[sec:obser\]) for our sample are listed in Col.(4) of Table \[Table3\].

[c|cc|cc|cc|cc]{}

\[Table2\] W3IRS5 & 0.088 & 0.0018 & -44 & -34 & 0.353 $\pm$ 0.002 & 0.464 $\pm$ 0.082 & 76 $\pm$ 14 & 10.3\
Mol12 & 0.464 & 0.0221 & -1 & 8 & 1.401 $\pm$ 0.008 & 4.698 $\pm$ 0.839 & 30 $\pm$ 5 & 9.71\
NGC2024 & 0.205 & 0.0024 & 7 & 15 & 0.465 $\pm$ 0.001 & 0.778 $\pm$ 0.100 