The chemical composition of the circumstellar envelopes around yellow hypergiant stars ^*,**

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
³ Observatorio Astronómico Nacional (IGN), Alfonso XII N3, 28014 Madrid, Spain e-mail: j.alcolea@oan.es

Received: 6 February 2007
Accepted: 24 April 2007

Abstract

Context.The yellow hypergiant stars (YHGs) are extremely luminous and massive objects whose general properties are poorly known. Only two of this kind of star show massive circumstellar envelopes, IRC +10420 and AFGL 2343.

Aims.We aim to study the chemistry of the circumstellar envelopes around these two sources, by comparison with well known AGB stars and protoplanetary nebulae. We also estimate the abundances of the observed molecular species.

Methods.We have performed single-dish observations of different transitions for twelve molecular species. We have compared the ratio of the intensities of the molecular transitions and of the estimated abundances in AFGL 2343 and IRC +10420 with those in O-rich and C-rich AGB stars and protoplanetary nebulae.

Results.Both YHGs, AFGL 2343, and IRC +10420, have been found to have an O-rich chemistry similar to that in O-rich AGB stars, though for AFGL 2343 the emission of most molecules compared with ¹³CO lines is relatively weak. Clear differences with the other evolved sources appear when we compare the line intensity corrected for distance and the profile widths which are, respectively, very intense and very wide in YHGs. The abundances obtained for IRC +10420 agree with those found in AGB stars, but in general those found in AFGL 2343, except for ¹³CO, are too low. This apparently low molecular abundance in AFGL 2343 could be due to the fact that these molecules are present only in an inner region of the shell where the mass is relatively low.