We have measured the ortho-to-para ratio of ammonia in the blueshifted gas of the L1157 outflow by observing the six metastable inversion lines from ( J,K ) = ( 1 , 1 ) to ( 6 , 6 ) .
The highly excited ( 5 , 5 ) and ( 6 , 6 ) lines were first detected in the low- mass star forming regions .
The rotational temperature derived from the ratio of four transition lines from ( 3 , 3 ) to ( 6 , 6 ) is 130–140 K , suggesting that the blueshifted gas is heated by a factor of \sim 10 as compared to the quiescent gas .
The ortho-to-para ratio of the NH _ { 3 } molecules in the blueshifted gas is estimated to be 1.3–1.7 , which is higher than the statistical equilibrium value .
This ratio provides us with evidence that the NH _ { 3 } molecules have been evaporated from dust grains with the formation temperature between 18 and 25 K. It is most likely that the NH _ { 3 } molecules on dust grains have been released into the gas phase through the passage of strong shock waves produced by the outflow .
Such a scenario is supported by the fact that the ammonia abundance in the blueshifted gas is enhanced by a factor of \sim 5 with respect to the dense quiescent gas .