Molecular line studies of dense cores have shown that NH3 is an excellent tracer of pre-stellar gas, which is the earliest phase in the formation of stars. Unlike most other molecules (mainly CO), ammonia does not deplete out in dense cores. This apparent non-depletion is still a mystery. To make matters worse, ammonia abundance has been shown to increase at the highest densities. The answer might lie in the choice of ammonia transitions that have been observed to propose non-depletion. These are the para-NH3 23 GHz rotational transitions that have critical densities of virgul 10^4 cm^-3 while NH_3 depletion is expected to occur at densities two orders of magnitude higher! However, the NH3 ground state transition at 572.5 GHz observable with Herschel has a critical density few orders of magnitude higher, close to where we expect to see depletion. We propose to unravel the ..seeming. ammonia non-depletion in dense cores by mapping the densest region of a pre-stellar core (with heavy CO depletion) in ammonia. The molecular depletion is closely linked to molecular deuteration. NH2D is expected to be abundant in cold regions with significant CO depletion. The D-H ratio derived from the ratio of NH3 and NH2D column densities is consistently higher than that derived from other molecules, most importantly N2H+. The discrepancy is worse for higher mass cores where NH2D-NH3 ratios of up to 0.8 have been found! While other astrochemical processes may be at play, one of the prime suspects is again the poor choice of para-NH3 (1,1) transitions as dense gas tracer. We believe that this leads to ammonia column densities being underestimate. Here, we propose to observe NH3 J_K= 1_0-0_0 toward a high mass core where very high deuteration ratio has been found and derive the true NH3 column density. This proposal will address two fundamental issues with our understanding of ammonia chemistry in dense cores, (i) the seeming ammonia ...non-depletion and (ii) very high ammonia deuteration.
Herschel was launched on 14 May 2009! It is the fourth 'cornerstone' mission in the ESA science programme. With a 3.5 m Cassegrain telescope it is the largest space telescope ever launched. It is performing photometry and spectroscopy in approximately the 55-671 µm range, bridging the gap between earlier infrared space missions and groundbased facilities.