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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We present observational evidence that the OH/IR star OH 12.8-0.9 is the fourth in a class of objects previously dubbed "water-fountain" sources. Using the Very Long Baseline Array, we produced the first images of the water maser emission associated with OH 12.8-0.9. We find that the masers are located in two compact regions with an angular separation of ~109 mas on the sky. The axis of separation between the two maser regions is at a position angle of 1.5 deg. East of North with the blue-shifted (-80.5 to -85.5 km/s) masers located to the North and the red-shifted (-32.0 to -35.5 km/s) masers to the South. In addition, we find that the blue- and red-shifted masers are distributed along arc-like structures ~10-12 mas across oriented roughly perpendicular to the separation axis. The morphology exhibited by the water masers is suggestive of an axisymmetric wind with the masers tracing bow shocks formed as the wind impacts the ambient medium. This bipolar jet-like structure is typical of the three other confirmed water-fountain sources. When combined with the previously observed spectral characteristics of OH 12.8-0.9, the observed spatio-kinematic structure of the water masers provides strong evidence that OH 12.8-0.9 is indeed a member of the water-fountain class.Comment: 12 pages, 2 figures (1 color), accepted for publication in the Ap J Letter

-We present results of VLBI observations of the water masers associated with IRAS 4A and IRAS 4B in the NGC 1333 star-forming region taken in four epochs over a two month period. Both objects have been classified as extremely young sources and each source is known to be a multiple system. Using the Very Long Baseline Array, we detected 35 masers in Epoch I, 40 masers in Epoch II, 35 in Epoch III, and 24 in Epoch IV. Only one identified source in each system associates with these masers. These data are used to calculate proper motions for the masers and trace the jet outflows within 100 AU of IRAS 4A2 and IRAS 4BW. In IRAS 4A2 there are two groups of masers, one near the systemic cloud velocity and one red-shifted. They expand linearly away from each other at velocities of 53 km s −1 . In IRAS 4BW, masers are observed in two groups that are blue-shifted and red-shifted relative to the cloud velocity. They form complex linear structures with a thickness of 3 mas (1 AU at a distance of 320 pc) that expand linearly away from each other at velocities of 78 km s −1 . Neither of the jet outflows traced by the maser groups align with the larger scale outflows. We suggest the presence of unresolved companions to both IRAS 4A2 and 4BW.

I present results from a sensitive imaging search for CH 3 OH and C 2 H in two oxygen-rich stars (IK Taurus and TX Camelopardalis) carried out with the Owens Valley Millimeter Array. Neither CH 3 OH nor C 2 H are detected in these stars, in agreement with single-dish observations by Charnley and Latter. The observations place upper limits on the fractional abundance of C 2 H of 9:7 ; 10 À9 in IK Tau and 3:2 ; 10 À8 in TX Cam and also place upper limits on the fractional abundance of CH 3 OH of 4:5 ; 10 À9 in IK Tau and 1:1 ; 10 À8 in TX Cam. A chemical model developed by Charnley and coworkers explaining the presence of HCN in these objects is clearly ruled out by these observations. The HCN distribution in these oxygen-rich stars has been mapped with a resolution of %3 00 . The HCN emission is marginally resolved, centrally concentrated, and does not appear to be well fitted by a thin-shell model. Continuum fluxes for the sources have been measured. At 96.7 GHz, the observed continuum fluxes were 6:1 AE 0:4 mJy for IK Tau and 3:7 AE 0:4 mJy for TX Cam. The continuum emission is unresolved at the %3 00 level, as expected from a model for the emission mechanism proposed by Reid and Menten.

We report observations of water masers around the semiregular variable RT Virginis, which have been made with the Very Long Baseline Array of the National Radio Astronomy Observatory at five epochs, each separated by 3 weeks of time. We detected about 60 maser features at each epoch. Overall, 61 features, detected at least twice, were tracked by their radial velocities and proper motions. The three-dimensional maser kinematics exhibited a circumstellar envelope that is expanding roughly spherically with a velocity of '8 km s À1 . Asymmetries in both the spatial and velocity distributions of the maser features were found in the envelope, but they were less significant than those found in other semiregular variables. Systematic radial velocity drifts of individual maser features were found with amplitudes of 2 km s À1 yr À1 . For one maser feature, we found a quadratic position shift with time along a straight line on the sky. This apparent motion indicates an acceleration with an amplitude of 33 km s À1 yr À1 , implying the passage of a shock wave driven by the stellar pulsation of RT Vir. The acceleration motion is likely seen only on the sky plane because of a large velocity gradient formed in the accelerating maser region. We estimated the distance to RT Vir to be about 220 pc on the basis of both the statistical parallax and model-fitting methods for the maser kinematics.