Aims. A multi-transition survey of HCN (sub-) millimeter line emission from a large sample of asymptotic giant branch (AGB) stars of different chemical type is presented. The data are analysed and circumstellar HCN abundances are estimated. The sample stars span a large range of properties such as mass-loss rate and photospheric C/O-ratio. The analysis of the new data allows for more accurate estimates of the circumstellar HCN abundances and puts new constraints on chemical models. Methods. In order to constrain the circumstellar HCN abundance distribution a detailed non-local thermodynamic equilibrium (LTE) excitation analysis, based on the Monte Carlo method, is performed. Effects of line overlaps and radiative excitation from dust grains are included. Results. The median values for the derived abundances of HCN (with respect to H 2 ) are 3 × 10 −5 , 7 × 10 −7 and 10 −7 for carbon stars (25 stars), S-type AGB stars (19 stars) and M-type AGB stars (25 stars), respectively. The estimated sizes of the HCN envelopes are similar to those obtained in the case of SiO for the same sample of sources and agree well with previous results from interferometric observations, when these are available. Conclusions. We find that there is a clear dependence of the derived circumstellar HCN abundance on the C/O-ratio of the star, in that carbon stars have about two orders of magnitude higher abundances than M-type AGB stars, on average. The derived HCN abundances of the S-type AGB stars have a larger spread and typically fall in between those of the two other types, however, slightly closer to the values for the M-type AGB stars. For the M-type stars, the estimated abundances are much higher than what would be expected if HCN is formed in thermal equilibrium. However, the results are also in contrast to predictions from recent non-LTE chemical models, where very little difference is expected in the HCN abundances between the various types of AGB stars.

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Abstract. We present sub-milliarcsecond observations of SiO masers in the late-type stars IRC +10011 and χ Cyg. We have used the NRAO Very Long Baseline Array (VLBA) to map the 43 GHz (v = 1, 2 J = 1-0) and the 86 GHz (v = 1, 2 J = 2−1) SiO masers. All the transitions have been imaged except the v = 2 J = 2-1 in IRC +10011. We report the first VLBI map of the v = 1 J = 2-1 28 SiO maser in IRC +10011 as well as the first VLBA images of SiO masers in an S-type Mira variable, χ Cyg. In this paper we have focused on the study of the relative spatial distribution of the different observed lines. We have found that in some cases the observational results are not reproduced by the current theoretical pumping models, either radiative or collisional. In particular, for IRC +10011, the v = 1 J = 1-0 and J = 2-1 28 SiO lines have different spatial distributions and emitting region sizes, the J = 2-1 emission being located in an outer region of the envelope. For χ Cyg, the distributions also differ, but the sizes of the masing regions are comparable. We suggest that the line overlaps between rovibrational transitions of two abundant molecular species, H 2 O and 28 SiO, is a possible explanation for the discrepancies found between the observations and the theoretical predictions. We have introduced this overlapping process in the calculations of the excitation of the SiO molecule. We conclude that the line overlaps can strongly affect the excitation of SiO and may reproduce the unexpected observational results for the two sources studied.

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