HCN/HNC intensity ratio: a new chemical thermometer for the molecular
  ISM
  Context. The gas kinetic temperature (TK) determines the physical and
chemical evolution of the Interestellar Medium (ISM). However, obtaining
reliable TK estimates usually requires expensive observations including the
combination of multi-line analysis and dedicated radiative transfer
calculations. Aims. This work explores the use of HCN and HNC observations, and
particularly its I(HCN)/I(HNC) intensity ratio of their J=1-0 lines, as direct
probe of the gas kinetic temperature in the molecular ISM. Methods. We obtained
a new set of large-scale observations of both HCN and HNC (1-0) lines along the
Integral Shape Filament (ISF) in Orion. In combination with ancillary gas and
dust temperature measurements, we find a systematic temperature dependence of
the observed I(HCN)/I(HNC) intensity ratio across our maps. Additional
comparisons with chemical models demonstrate that these observed I(HCN)/I(HNC)
variations are driven by the effective destruction and isomerization mechanisms
of HNC under low energy barriers. Results. The observed variations of
I(HCN)/I(HNC) with TK can be described with a two-part linear function. This
empirical calibration is then used to create a temperature map of the entire
ISF. Comparisons with similar dust temperature measurements in this cloud, as
well as in other regions and galactic surveys, validate this simple technique
to obtain direct estimates of the gas kinetic temperature in a wide range of
physical conditions and scales with an optimal working range between 15 K and
40 K. Conclusions. Both observations and models demonstrate the strong
sensitivity of the I(HCN)/I(HNC) ratio to the gas kinetic temperature. Since
these lines are easily obtained in observations of local and extragalactic
sources, our results highlight the potential use of this observable as new
chemical thermometer for the ISM.
