New N-bearing species towards OH 231.8+4.2

HNCO, HNCS, HC₃N, and NO^⋆,⋆⋆

¹ Grupo de Astrofísica Molecular. Instituto de Ciencia de Materiales de Madrid, CSIC, c/ Sor Juana Inés de la Cruz 3, 28049 Cantoblanco, Madrid, Spain
e-mail: lvelilla@icmm.csic.es
² Centro de Astrobiología, INTA-CSIC, 28691 Villanueva de la Cañada, Madrid, Spain
³ Centro de Astrobiología, INTA-CSIC, Ctra. de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁴ Université de Bordeaux, LAB, UMR 5804, 33270 Floirac, France
⁵ Observatorio Astronómico Nacional (IGN), Alfonso XII No 3, 28014 Madrid, Spain
⁶ Observatorio Astronómico Nacional (IGN), Ap 112, 28803 Alcalá de Henares, Madrid, Spain
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 7 August 2014
Accepted: 4 December 2014

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 V_exp~ 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 T_rot~ 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.