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Context. S-type AGB stars have a C/O ratio which suggests that they are transition objects between oxygen-rich M-type stars and carbon-rich C-type stars. As such, their circumstellar compositions of gas and dust are thought to be sensitive to their precise C/O ratio, and it is therefore of particular interest to examine their circumstellar properties. Aims. We present new Herschel HIFI and PACS sub-millimetre and far-infrared line observations of several molecular species towards the S-type AGB star W Aql. We use these observations, which probe a wide range of gas temperatures, to constrain the circumstellar properties of W Aql, including mass-loss rate and molecular abundances. Methods. We used radiative transfer codes to model the circumstellar dust and molecular line emission to determine circumstellar properties and molecular abundances. We assumed a spherically symmetric envelope formed by a constant mass-loss rate driven by an accelerating wind. Our model includes fully integrated H 2 O line cooling as part of the solution of the energy balance. Results. We detect circumstellar molecular lines from CO, H 2 O, SiO, HCN, and, for the first time in an S-type AGB star, NH 3 . The radiative transfer calculations result in an estimated mass-loss rate for W Aql of 4.0 × 10 −6 M yr −1 based on the 12 CO lines. The estimated 12 CO/ 13 CO ratio is 29, which is in line with ratios previously derived for S-type AGB stars. We find an H 2 O abundance of 1.5 × 10 −5 , which is intermediate to the abundances expected for M and C stars, and an ortho/para ratio for H 2 O that is consistent with formation at warm temperatures. We find an HCN abundance of 3 × 10 −6 , and, although no CN lines are detected using HIFI, we are able to put some constraints on the abundance, 6 × 10 −6 , and distribution of CN in W Aql's circumstellar envelope using ground-based data. We find an SiO abundance of 3 × 10 −6 , and an NH 3 abundance of 1.7 × 10 −5 , confined to a small envelope. If we include uncertainties in the adopted circumstellar model -in the adopted abundance distributions, etc. -the errors in the abundances are of the order of factors of a few. The data also suggest that, in terms of HCN, S-type and M-type AGB stars are similar, and in terms of H 2 O, S-type AGB stars are more like C-type than M-type AGB stars. We detect excess blue-shifted emission in several molecular lines, possibly due to an asymmetric outflow. Conclusions. The estimated abundances of circumstellar HCN, SiO and H 2 O place W Aql in between M-and C-type AGB stars, i.e., the abundances are consistent with an S-type classification.

Aims. In March 2008, the APEX facility instrument was installed on the telescope at the site of Lliano Chajnantor in northern Chile. The main objective of the paper is to introduce the new instrument to the radio astronomical community. It describes the hardware configuration and presents some initial results from the on-sky commissioning. Methods. The heterodyne instrument covers frequencies between 211 GHz and 1390 GHz divided into four bands. The first three bands are sideband-separating mixers operating in a single sideband mode and based on superconductor-insulator-superconductor (SIS) tunnel junctions. The fourth band is a hot-electron bolometer, waveguide balanced mixer. All bands are integrated in a closedcycle temperature-stabilized cryostat and are cooled to 4 K. Results. We present results from noise temperature, sideband separation ratios, beam, and stability measurements performed on the telescope as a part of the receiver technical commissioning. Examples of broad extragalactic lines are also included.

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