HCN/HNC chemistry in shocks: a study of L1157-B1 with ASAI
  HCN and its isomer HNC play an important role in molecular cloud chemistry
and the formation of more complex molecules. We investigate here the impact of
protostellar shocks on the HCN and HNC abundances from high-sensitivity IRAM
30m observations of the prototypical shock region L1157-B1 and the envelope of
the associated Class 0 protostar, as a proxy for the pre-shock gas. The
isotopologues H$^{12}$CN, HN$^{12}$C, H$^{13}$CN, HN$^{13}$C, HC$^{15}$N,
H$^{15}$NC, DCN and DNC were all detected towards both regions. Abundances and
excitation conditions were obtained from radiative transfer analysis of
molecular line emission under the assumption of Local Thermodynamical
Equilibrium. In the pre-shock gas, the abundances of the HCN and HNC
isotopologues are similar to those encountered in dark clouds, with a HCN/HNC
abundance ratio $\approx 1$ for all isotopologues. A strong D-enrichment
(D/H$\approx 0.06$) is measured in the pre-shock gas. There is no evidence of
$^{15}$N fractionation neither in the quiescent nor in the shocked gas. At the
passage of the shock, the HCN and HNC abundances increase in the gas phase in
different manners so that the HCN/HNC relative abundance ratio increases by a
factor 20. The gas-grain chemical and shock model UCLCHEM allows us to
reproduce the observed trends for a C-type shock with pre-shock density $n$(H)=
$10^5$cm$^{-3}$ and shock velocity $V_s= 40$km/s. We conclude that the HCN/HNC
variations across the shock are mainly caused by the sputtering of the grain
mantle material in relation with the history of the grain ices.
