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

TL;DR

This paper introduces an approximate method to estimate ammonia column density in star-forming regions using only the (1,1) inversion transition, simplifying observations and enabling analysis of low signal-to-noise areas.

Contribution

The authors develop a new approach that calculates ammonia column density with a single transition, reducing observational complexity and uncertainties compared to traditional methods.

Findings

The method provides comparable accuracy to full two-transition approaches.

It enables mapping of ammonia in regions with low signal-to-noise ratios.

Application to W3 and Perseus regions demonstrates its effectiveness.

Abstract

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 signal to 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 YSOs, particularly in the outskirts of the protostellar envelope where the (2,2) ammonia line is often undetectable on the short timescales necessary for large area mapping.