New N-bearing species towards OH 231.8+4.2

Abstract

Circumstellar envelopes (CSEs) around asymptotic giant branch (AGB) stars 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 ~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, HC3N, 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 (~0.6 M⊙) central component of the envelope, expanding with low velocities of Vexp~ 15–30 km s-1, and at the base of the fast lobes. The NO profiles (with FWHM~ 40–50 km s-1) are broader than those of HNCO, HNCS, and HC3N and, most importantly, broader than the line profiles of 13CO, 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 Trot~ 15–30 K (and, maybe, up to ~55 K for HC3N) and fractional abundances relative to H2 of X(HNCO) ~ [0.8–1] × 10-7, X(HNCS) ~ [0.9–1] × 10-8, X(HC3N) ~ [5–7] × 10-9, and X(NO) ~ [1–2] × 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 HC3N 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+4.2 and that other processes (e.g. shocks) play a major role in their formation. For NO, the model abundance, ≈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.