Circumstellar envelopes ( CSEs ) around asymptotic giant branch ( AGB ) are the main sites of molecular formation .
OH 231.8 + 4.2 is a well studied oxygen-rich CSE around an intermediate-mass evolved star that , in dramatic contrast to most AGB CSEs , displays bipolar molecular outflows accelerated up to \sim 400 km s ^ { -1 } .
OH 231.8 + 4.2 also presents an exceptional molecular richness probably due to shock-induced chemical processes .
We report the first detection in this source of four nitrogen-bearing species , HNCO , HNCS , HC _ { 3 } N , and NO , which have been observed with the IRAM-30 m radiotelescope in a sensitive mm-wavelength survey towards this target .
HNCO and HNCS are also first detections in CSEs .
The observed line profiles show that the emission arises in the massive ( \sim 0.6 M _ { \odot } ) central component of the envelope , expanding with low velocities of V _ { \mathrm { exp } } \sim 15-30 km s ^ { -1 } , and at the base of the fast lobes .
The NO profiles ( with FWHM \sim 40-50 km s ^ { -1 } ) are broader than those of HNCO , HNCS , and HC _ { 3 } N and , most importantly , broader than the line profiles of ^ { 13 } CO , which is a good mass tracer .
This indicates that the NO abundance is enhanced in the fast lobes relative to the slow , central parts .
From LTE and non-LTE excitation analysis , we estimate beam-average rotational temperatures of T _ { \mathrm { rot } } \sim 15-30 K ( and , maybe , up to \sim 55 K for HC _ { 3 } N ) and fractional abundances of X ( HNCO ) \sim [ 0.8-1 ] \times 10 ^ { -7 } , X ( HNCS ) \sim [ 0.9-1 ] \times 10 ^ { -8 } , X ( HC _ { 3 } N ) \sim [ 5-7 ] \times 10 ^ { -9 } , and X ( NO ) \sim [ 1-2 ] \times 10 ^ { -6 } .
NO is , therefore , amongst the most abundant N-bearing species in OH 231.8 + 4.2 .
We performed thermodynamical chemical equilibrium and chemical kinetics models to investigate the formation of these N-bearing species in OH 231.8 + 4.2 .
The model underestimates the observed abundances for HNCO , HNCS , and HC _ { 3 } N by several orders of magnitude , which indicates that these molecules can hardly be products of standard UV-photon and/or cosmic-ray induced chemistry in OH 231.8 + 4.2and that other processes ( e.g .
shocks ) play a major role in their formation .
For NO , the model abundance , \approx 10 ^ { -6 } , is compatible with the observed average value ; however , the model fails to reproduce the NO abundance enhancement in the high-velocity lobes ( relative to the slow core ) inferred from the broad NO profiles .
The new detections presented in this work corroborate the particularly rich chemistry of OH 231.8 + 4.2 , which is likely to be profoundly influenced by shock-induced processes , as proposed in earlier works .