The structure and chemistry of the massive shell around AFGL 2343: SiO and HCN as tracers of high-excitation regions^*

¹ Observatorio Astronómico Nacional (IGN), Apdo. 112, 28803 Alcalá de Henares, Spain e-mail: [g.quintana;v.bujarrabal]@oan.es
² Institut de RadioAstronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France e-mail: ccarrizo@iram.fr

Received: 12 February 2008
Accepted: 7 April 2008

Abstract

Context. The yellow hypergiant stars (YHGs) are very massive objects that are expected to pass through periods of intense mass loss during their evolution. Despite this, massive circumstellar envelopes have been found in only two of them, IRC +10420 and AFGL 2343.

Aims. The envelopes around these objects and the processes that form them are poorly known. We aim to study the structure, dynamics, and chemistry of the envelope around AFGL 2343.

Methods. We obtained interferometric maps of the rotational lines ²⁹SiO –1, HCN –0 and SO –1₁ towards AFGL 2343. We used an LVG excitation model to analyze the new observations and some previously published line profiles of AFGL 2343.

Results. The analysis of the observational data and the fitting results show the presence of a thin, hot, and dense component within the previously identified CO shell. This component can be associated with recently shocked gas, but it could also be caused by a phase of extremely copious mass loss. We suggest that this shell is responsible for the whole ²⁹SiO emission and that it significantly contributes to the HCN emission. The presence of such a dense shell rich in SiO can be related with the one previously found for IRC +10420, which was also suggested to result from a shock. This may be a common feature in the evolution of these stars, as a consequence of the episodic mass loss periods they go through during their evolution. We present new results for the mass loss pattern, the total mass of the circumstellar envelope, and the molecular abundances of some species in AFGL 2343.