2006A&A...456.1037T

The purpose of this study is to investigate the distributions of the isomeric molecules HCN and HNC and estimate their abundance ratio in the protostellar core Cha-MMS1 located in Chamaeleon I. The core was mapped in the J=1-0 rotational lines of HCN, HNC, and HN¹³C. The column densities of H¹³CN, HN¹³C, H¹⁵NC and NH₃ were estimated towards the centre of the core. The core is well delineated in all three maps. The kinetic temperature in the core, derived from the NH₃ (1, 1) and (2, 2) inversion lines, is 12.1 ±0.1K. The HN¹³C/H¹³CN column density ratio is between 3 and 4, i.e. similar to values found in several other cold cores. The HN¹³C/H¹⁵NC column density ratio is ∼7. In case no ¹⁵N fractionation occurs in HNC (as suggested by recent modelling results), the HNC/HN¹³C abundance ratio is in the range 30-40, which indicates a high degree of ¹³C fractionation in HNC. Assuming no differential ¹³C fractionation the HCN and HNC abundances are estimated to be ∼7x10^–10 and ∼2x10^–9, respectively, the former being nearly two orders of magnitude smaller than that of NH₃. Using also previously determined column densities in Cha-MMS1, we can put the most commonly observed nitrogenous molecules in the following order according to their fractional abundances: χ(NH₃)>χ(HC₃N)>χ(HNC)>χ(HCN)>χ(N₂H⁺). The relationships between molecular abundances suggest that Cha-MMS1 represents an evolved chemical stage, experiencing at present the ``late-time'' cyanopolyyne peak. The possibility that the relatively high HNC/HCN ratio derived here is only valid for the ¹³C isotopic substitutes cannot be excluded on the basis of the present and other available data.