The NH3/N2H+ abundance ratio in dense cores

Abstract

We have observed the dense cores Barnard 217 (B217) and LDN 1262 (L1262) in the (J, K) = (1, 1) and (2, 2) inversion lines of ammonia and in the J = 1-0 rotational line of diazenylium. The two cores are morphologically similar, in the sense that both contain a Class-I young stellar object (YSO) 1-2′ away from the core centre. The NH3 and N2H+ column densities show the same pattern in both cores: the average NH3/N2H+ abundance ratios are 140-190 in the starless main bodies of the cores, while they drop to about 60-90 in the regions around the YSOs. Comparison with the dust continuum emission of B217 suggests that this pattern is due to an enhanced fractional ammonia abundance in the quiescent part of the core, where we find N(NH3)/N(H2) = 5 × 10-8. On the outskirts of the core the fractional ammonia abundance is about 3 × 10-8, in accordance with our previous results. We discuss these findings in the light of recent chemical models including molecular depletion.