Modelling the abundance structure of isocyanic acid (HNCO) toward the
  low-mass solar type protostar IRAS 16293-2422
  Isocyanic acid (HNCO), the most stable of the simplest molecules containing
the four main elements essential for organic chemistry, has been observed in
several astrophysical environments such as molecular clouds, star-forming
regions, external galaxies and comets. In this work, we model HNCO spectral
line profiles toward the low-mass solar type protostar IRAS 16293$-2$422
observed with the ALMA interferometer, the IRAM, JCMT and APEX single-dish
radio telescopes, and the HIFI instrument on board the Herschel Space
Observatory. In star-forming environments, the HNCO emission is not always in
Local Thermodynamical Equilibrium (LTE). A non-LTE radiative transfer approach
is necessary to properly interpret the line profiles, and accurate collisional
rate coefficients are needed. Here, we used the RADEX package with a completely
new set of collisional quenching rates between HNCO and both ortho-H$_2$ and
para-H$_2$ obtained from quantum chemical calculations yielding a novel
potential energy surface in the rigid rotor approximation. We find that the
lines profiles toward IRAS 16293$-$2422 are very well reproduced if we assume
that the HNCO emission arises from a compact, dense and hot physical component
associated with the hot corino, a warm component associated with the internal
part of the protostellar envelope, and a cold and more extended component
associated with the outer envelope. The derived HNCO abundances from our model
agree well with those computed with the Nautilus chemical code.
