Context. Low and intermediate mass stars are known to power strong stellar winds when evolving through the asymptotic giant branch (AGB) phase. Initial mass, luminosity, temperature, and composition determine the pulsation characteristics of the star and the dust species formed in the pulsating photospheric layers. Radiation pressure on these grains triggers the onset of a stellar wind. However, as of today, we still cannot predict the wind mass-loss rates and wind velocities from first principles neither do we know which species are the first to condense in the upper atmospheric regions. Aims. We aim to characterise the dominant physical, dynamical, and chemical processes in the inner wind region of two archetypical oxygen-rich (C/O < 1) AGB stars, that is, the low mass-loss rate AGB star R Dor (Ṁ ~ 1 × 10−7 M⊙ yr−1) and the high mass-loss rate AGB star IK Tau (Ṁ ~ 5 × 10−6 M⊙ yr−1). The purpose of this study is to observe the key molecular species contributing to the formation of dust grains and to cross-link the observed line brightnesses of several species to the global and local properties of the star and its wind. Methods. A spectral line and imaging survey of IK Tau and R Dor was made with ALMA between 335 and 362 GHz (band 7) at a spatial resolution of ~150 mas, which corresponds to the locus of the main dust formation region of both targets. Results. Some two hundred spectral features from 15 molecules (and their isotopologues) were observed, including rotational lines in both the ground and vibrationally excited states (up to v = 5 for SiO). Detected species include the gaseous precursors of dust grains such as SiO, AlO, AlOH, TiO, and TiO2. We present a spectral atlas for both stars and the parameters of all detected spectral features. A clear dichotomy for the sulphur chemistry is seen: while CS, SiS, SO, and SO2 are abundantly present in IK Tau, only SO and SO2 are detected in R Dor. Also other species such as NaCl, NS, AlO, and AlOH display a completely different behaviour. From some selected species, the minor isotopologues can be used to assess the isotopic ratios. The channel maps of many species prove that both large and small-scale inhomogeneities persist in the inner wind of both stars in the form of blobs, arcs, and/or a disk. The high sensitivity of ALMA allows us to spot the impact of these correlated density structures in the spectral line profiles. The spectral lines often display a half width at zero intensity much larger than expected from the terminal velocity, v∞, previously derived for both objects (36 km s−1 versus v∞~ 17.7 km s−1 for IK Tau and 23 km s−1 versus v∞~ 5.5 km s−1 for R Dor). Both a more complex 3D morphology and a more forceful wind acceleration of the (underlying) isotropic wind can explain this trend. The formation of fractal grains in the region beyond ~400 mas can potentially account for the latter scenario. From the continuum map, we deduce a dust mass of ~3.7 × 10−7 M⊙ and ~2 × 10−8 M⊙ for IK Tau and R Dor, respectively. Conclusions. The observations presented here provide important constraints on the properties of these two oxygen-dominated AGB stellar winds. In particular, the ALMA data prove that both the dynamical and chemical properties are vastly different for this high mass-loss rate (IK Tau) and low mass-loss rate (R Dor) star.

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. The sulphur compounds SO and SO 2 have not been widely studied in the circumstellar envelopes of asymptotic giant branch (AGB) stars. By presenting and modelling a large number of SO and SO 2 lines in the low mass-loss rate M-type AGB star R Dor, and modelling the available lines of those molecules in a further four M-type AGB stars, we aim to determine their circumstellar abundances and distributions. Methods. We use a detailed radiative transfer analysis based on the accelerated lambda iteration method to model circumstellar SO and SO 2 line emission. We use molecular data files for both SO and SO 2 that are more extensive than those previously available. Results. Using 17 SO lines and 98 SO 2 lines to constrain our models for R Dor, we find an SO abundance of (6.7 ± 0.9) × 10 −6 and an SO 2 abundance of 5 × 10 −6 with both species having high abundances close to the star. We also modelled 34 SO and found an abundance of (3.1 ± 0.8) × 10 −7 , giving an 32 SO/ 34 SO ratio of 21.6 ± 8.5. We derive similar results for the circumstellar SO and SO 2 abundances and their distributions for the low mass-loss rate object W Hya. For the higher mass-loss rate stars, we find shell-like SO distributions with peak abundances that decrease and peak abundance radii that increase with increasing mass-loss rate. The positions of the peak SO abundance agree very well with the photodissociation radii of H 2 O. We also modelled SO 2 in two higher mass-loss rate stars but our models for these were less conclusive. Conclusions. We conclude that for the low mass-loss rate stars, the circumstellar SO and SO 2 abundances are much higher than predicted by chemical models of the extended stellar atmosphere. These two species may also account for all the available sulphur. For the higher mass-loss rate stars we find evidence that SO is most efficiently formed in the circumstellar envelope, most likely through the photodissociation of H 2 O and the subsequent reaction between S and OH. The S-bearing parent molecule does not appear to be H 2 S. The SO 2 models for the higher mass-loss rate stars are less conclusive, but suggest an origin close to the star for this species. This is not consistent with current chemical models. The combined circumstellar SO and SO 2 abundances are significantly lower than that of sulphur for these higher mass-loss rate objects.

Context. Asymptotic giant branch (AGB) stars are in one of the latest evolutionary stages of low to intermediate-mass stars. Their vigorous mass loss has a significant effect on the stellar evolution, and is a significant source of heavy elements and dust grains for the interstellar medium. The mass-loss rate can be well traced by carbon monoxide (CO) line emission. Aims. We present new Herschel/HIFI and IRAM 30 m telescope CO line data for a sample of 53 galactic AGB stars. The lines cover a fairly large range of excitation energy from the J = 1 → 0 line to the J = 9 → 8 line, and even the J = 14 → 13 line in a few cases. We perform radiative transfer modelling for 38 of these sources to estimate their mass-loss rates. Methods. We used a radiative transfer code based on the Monte Carlo method to model the CO line emission. We assume spherically symmetric circumstellar envelopes that are formed by a constant mass-loss rate through a smoothly accelerating wind. Results. We find models that are consistent across a broad range of CO lines for most of the stars in our sample, i.e., a large number of the circumstellar envelopes can be described with a constant mass-loss rate. We also find that an accelerating wind is required to fit, in particular, the higher-J lines and that a velocity law will have a significant effect on the model line intensities. The results cover a wide range of mass-loss rates (∼10 −8 to 2 × 10 −5 M yr −1 ) and gas expansion velocities (2 to 21.5 km s −1 ) , and include M-, S-, and C-type AGB stars. Our results generally agree with those of earlier studies, although we tend to find slightly lower mass-loss rates by about 40%, on average. We also present "bonus" lines detected during our CO observations.

Aims. We aim to constrain the temperature and velocity structures, and H 2 O abundances in the winds of a sample of M-type asymptotic giant branch (AGB) stars. We further aim to determine the effect of H 2 O line cooling on the energy balance in the inner circumstellar envelope. Methods. We use two radiative-transfer codes to model molecular emission lines of CO and H 2 O towards four M-type AGB stars. We focus on spectrally resolved observations of CO and H 2 O from HIFI aboard the Herschel Space Observatory. The observations are complemented by ground-based CO observations, and spectrally unresolved CO and H 2 O observations with PACS aboard Herschel. The observed line profiles constrain the velocity structure throughout the circumstellar envelopes (CSEs), while the CO intensities constrain the temperature structure in the CSEs. The H 2 O observations constrain the o-H 2 O and p-H 2 O abundances relative to H 2 . Finally, the radiative-transfer modelling allows to solve the energy balance in the CSE, in principle including also H 2 O line cooling. Results. The fits to the line profiles only set moderate constraints on the velocity profile, indicating shallower acceleration profiles in the winds of M-type AGB stars than predicted by dynamical models, while the CO observations effectively constrain the temperature structure. Including H 2 O line cooling in the energy balance was only possible for the low-mass-loss-rate objects in the sample, and required an ad hoc adjustment of the dust velocity profile in order to counteract extreme cooling in the inner CSE. H 2 O line cooling was therefore excluded from the models. The constraints set on the temperature profile by the CO lines nevertheless allowed us to derive H 2 O abundances. The derived H 2 O abundances confirm previous estimates and are consistent with chemical models. However, the uncertainties in the derived abundances are relatively large, in particular for p-H 2 O, and consequently the derived o/p-H 2 O ratios are not well constrained.