Estimating column density from ammonia (1,1) emission in star-forming regions

We present a new, approximate method of calculating the column density of ammonia in mapping observations of the 23 GHz inversion lines. The temperature regime typically found in star-forming regions allows for the assumption of a slowly varying partition function for ammonia. It is therefore possible to determine the column density using only the (J=1, K=1) inversion transition rather than the typical combination of the (1,1) and (2,2) transitions, with additional uncertainties comparable to or less than typical observational error. The proposed method allows column density and mass estimates to be extended into areas of lower signalto-noise ratio. We show examples of column-density maps around a number of cores in the W3 and Perseus star-forming regions made using this approximation, along with a comparison to the corresponding results obtained using the full two-transition approach. We suggest that this method is a useful tool in studying the distribution of mass around young stellar objects, particularly in the outskirts of the protostellar envelope where the (2,2) ammonia line is often undetectable on the short time-scales necessary for large-area mapping. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society