HCN and HNC mapping of the protostellar core Chamaeleon-MMS1

Aims. 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. Methods. The core was mapped in the J = 1-0 rotational lines of HCN, HNC, and HN(13)C. The column densities of H(13)CN, HN(13)C, H(15)NC and NH(3) were estimated towards the centre of the core. Results. 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 +/- 0.1 K. The HN(13)C/H(13)CN column density ratio is between 3 and 4, i.e. similar to values found in several other cold cores. The HN(13)C/H(15)NC column density ratio is similar to 7. In case no (15)N fractionation occurs in HNC (as suggested by recent modelling results), the HNC/HN(13)C abundance ratio is in the range 30-40, which indicates a high degree of (13)C fractionation in HNC. Assuming no differential (13)C fractionation the HCN and HNC abundances are estimated to be similar to 7 x 10(-10) and similar to 2 x 10(-9), respectively, the former being nearly two orders of magnitude smaller than that of NH(3). 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: chi(NH(3)) > chi(HC(3)N) > chi( HNC) > chi( HCN) > chi(N(2)H(+)). Conclusions. 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 (13)C isotopic substitutes cannot be excluded on the basis of the present and other available data.