and sensitivity, is very impressive.

**\[!htbp\]**

![image](f2.pdf)

COLUMN DENSITY, COLUMN DENSITY DISTRIBUTIONS, AND CLOUD MASS {#coldens}
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Mask Regions
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In order to facilitate analysis of the data to determine column densities, we have broken the Taurus data up into 4 regions, according to the detection or nondetection of  and . The detection thresholds are defined by the requirement that the integrated intensity over the velocity range extending from 0  to 12 be a minimum of 3.5 times larger than the rms noise in an individual pixel over this 12  velocity interval. The median values are $\sigma_{T_{int}}$ = 0.18  for  and $\sigma_{T_{int}}$ = 0.40  for . Since the peak values of the integrated intensity are 6  for  and 18  for , the peak integrated intensities are 30 to 50 $\sigma_{T_{int}}$.

We define mask 0 to be the region in which neither  nor  is detected, mask 1 to be the region in which  is detected but is not, mask 2 to be the region in which both isotopologues are detected, and mask 3 to be the region in which  is detected but  is not. The different regions and the number of pixels in each are given in Table \[maskregiontable\].

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