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The study of molecules and their chemistry in star-forming regions isfundamental to understand the physical process occurring in such regions. TheHCN and HNC J=1-0 emissions were used to derive their integrated lineintensities (I), to probe a relation recently appeared in the literaturebetween the kinetic temperatures (T$_{K}$) and the isomeric (I) ratio, and toobtain the isomers abundances (X) in 55 high-mass star-forming regions. Theselast ones are classified, according to the evolutive stage, as infrared darkclouds, high-mass protostellar objects, hot molecular cores, and ultracompactHII regions. It is inferred that the T$_{K}$ obtained from the isomericintegrated intensity ratio (I$^{HCN/HNC}$) are underestimated, and hence wesuggest that this relation cannot be employed as an universal thermometer inthe interstellar medium. The isomers abundances show a behavior that can beexplained from the chemistry occurring as the temperature and the UV radiationincrease according to the evolutive stage. We found that the abundance ratio(X$^{HCN/HNC}$) hardly could be used as a chemical clock, and we suggest thatit can be approximated by I$^{HCN/HNC}$. This work is part of an on-going studyof multiple molecules that stand in the sample of analyzed regions whichintends to contribute in the chemical knowledge of high-mass star formation.

An analysis of the isomers HCN and HNC in the evolution of high-mass star-forming regions