Volume 575, March 2015
|Number of page(s)||22|
|Section||Interstellar and circumstellar matter|
|Published online||02 March 2015|
New N-bearing species towards OH 231.8+4.2
Grupo de Astrofísica Molecular. Instituto de Ciencia de Materiales de
CSIC, c/ Sor Juana Inés de la Cruz 3, 28049
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
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.
Key words: astrochemistry / line: identification / molecular processes / stars: AGB and post-AGB / circumstellar matter
Based on observations carried out with the IRAM-30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).
Appendices are available in electronic form at http://www.aanda.org
© ESO, 2015
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