resulting in a “non–detection” of this isotopologue. We thus ignore the mask 3 pixels in further analysis of the emission from Taurus.

Calculation of the Column Density
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We wish to exploit the large linear dynamic range of our map to examine the structure in the column density, and thus wish to determine the column density for as many pixels as possible. This is also important for accurately determining the total molecular mass of the region. In what follows we divide the problem into two parts. The first is determination of the carbon monoxide column density. While subject to its own uncertainties due to excitation, optical depth, and limited signal to noise ratio, we can carry out this step of the analysis based only on data in hand. The second step is conversion of the carbon monoxide column densities to molecular hydrogen column densities, and finally to total cloud mass. This is evidently dependent on the processes which determine the fractional abundance of the various isotopologues observed. Since the additional uncertainties in the second step are large, we present results first in terms of the carbon monoxide distribution and subsequently give results for the molecular hydrogen distribution and the total 