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 \mathrm { HCN } , \mathrm { HNC } , and \mathrm { HN ^ { 13 } C } .
The column densities of \mathrm { H ^ { 13 } CN } , \mathrm { HN ^ { 13 } C } , \mathrm { H ^ { 15 } NC } and \mathrm { 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 \mathrm { NH _ { 3 } } ( 1,1 ) and ( 2,2 ) inversion lines , is 12.1 \pm 0.1 K. The \mathrm { HN ^ { 13 } C } / \mathrm { H ^ { 13 } CN } column density ratio is between 3 and 4 , i.e .
similar to values found in several other cold cores .
The \mathrm { HN ^ { 13 } C } / \mathrm { H ^ { 15 } NC } column density ratio is \sim 7 .
In case no ^ { 15 } N fractionation occurs in \mathrm { HNC } ( as suggested by recent modelling results ) , the \mathrm { HNC } / \mathrm { HN ^ { 13 } C } abundance ratio is in the range 30 - 40 , which indicates a high degree of ^ { 13 } C fractionation in \mathrm { HNC } .
Assuming no differential ^ { 13 } C fractionation the \mathrm { HCN } and \mathrm { HNC } abundances are estimated to be \sim 7 10 ^ { -10 } and \sim 2 10 ^ { -9 } , respectively , the former being nearly two orders of magnitude smaller than that of \mathrm { 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 ( \mathrm { NH _ { 3 } } ) > \chi ( \mathrm { HC _ { 3 } N } ) > \chi ( \mathrm { HNC } ) > \chi ( \mathrm { % HCN } ) > \chi ( \mathrm { 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 can not be excluded on the basis of the present and other available data .