Context. The evolution of intermediate and low-mass stars on the asymptotic giant branch is dominated by their strong dust-driven winds. More massive stars evolve into red supergiants with a similar envelope structure and strong wind. These stellar winds are a prime source for the chemical enrichment of the interstellar medium. Aims. We aim to (1) set up simple and general analytical expressions to estimate mass-loss rates of evolved stars, and (2) from those calculate estimates for the mass-loss rates of the asymptotic giant branch, red supergiant, and yellow hypergiant stars in our galactic sample. Methods. The rotationally excited lines of carbon monoxide (CO) are a classic and very robust diagnostic in the study of circumstellar envelopes. When sampling different layers of the circumstellar envelope, observations of these molecular lines lead to detailed profiles of kinetic temperature, expansion velocity, and density. A state-of-the-art, nonlocal thermal equilibrium, and co-moving frame radiative transfer code that predicts CO line intensities in the circumstellar envelopes of late-type stars is used in deriving relations between stellar and molecular-line parameters, on the one hand, and mass-loss rate, on the other. These expressions are applied to our extensive CO data set to estimate the mass-loss rates of 47 sample stars. Results. We present analytical expressions for estimating the mass-loss rates of evolved stellar objects for 8 rotational transitions of the CO molecule and thencompare our results to those of previous studies. Our expressions account for line saturation and resolving of the envelope, thereby allowing accurate determination of very high mass-loss rates. We argue that, for estimates based on a single rotational line, the CO(2-1) transition provides the most reliable mass-loss rate. The mass-loss rates calculated for the asympotic giant branch stars range from 4 × 10 −8 M yr −1 up to 8 × 10 −5 M yr −1 . For red supergiants they reach values between 2 × 10 −7 M yr −1 and 3 × 10 −4 M yr −1 . The estimates for the set of CO transitions allow time variability to be identified in the mass-loss rate. Possible mass-loss-rate variability is traced for 7 of the sample stars. We find a clear relation between the pulsation periods of the asympotic giant branch stars and their derived mass-loss rates, with a levelling off at ∼3 × 10 −5 M yr −1 for periods exceeding 850 days. Conclusions.

Context. The interstellar medium is enriched primarily by matter ejected from evolved low and intermediate mass stars. The outflow from these stars creates a circumstellar envelope in which a rich gas-phase chemistry takes place. Complex shock-induced nonequilibrium chemistry takes place in the inner wind envelope, dust-gas reactions and ion-molecule reactions alter the abundances in the intermediate wind zone, and the penetration of cosmic rays and ultraviolet photons dissociates the molecules in the outer wind region. Aims. Little observational information exists on the circumstellar molecular abundance stratifications of many molecules. Furthermore, our knowledge of oxygen-rich envelopes is not as profound as for the carbon-rich counterparts. The aim of this paper is therefore to study the circumstellar chemical abundance pattern of 11 molecules and isotopologs ( 12 CO, 13 CO, SiS, 28 SiO, 29 SiO, 30 SiO, HCN, CN, CS, SO, SO 2 ) in the oxygen-rich evolved star IK Tau. Methods. We have performed an in-depth analysis of a large number of molecular emission lines excited in the circumstellar envelope around IK Tau. The analysis is done based on a non-local thermodynamic equilibrium (non-LTE) radiative transfer analysis, which calculates the temperature and velocity structure in a self-consistent way. The chemical abundance pattern is coupled to theoretical outer wind model predictions including photodestruction and cosmic ray ionization. Not only the integrated line intensities, but also the line shapes are used as diagnostic tool to study the envelope structure. Results. The deduced wind acceleration is much slower than predicted from classical theories. SiO and SiS are depleted in the envelope, possibly due to the adsorption onto dust grains. For HCN and CS a clear difference with respect to inner wind non-equilibrium predictions is found, either indicating uncertainties in the inner wind theoretical modeling or the possibility that HCN and CS (or the radical CN) participate in the dust formation. The low signal-to-noise profiles of SO and CN prohibit an accurate abundance determination; the modeling of high-excitation SO 2 lines is cumbersome, possibly related to line misidentifications or problems with the collisional rates.

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.

The detection of circumstellar water vapour around the ageing carbon star IRC +10216 challenged the current understanding of chemistry in old stars, because water was predicted to be almost absent in carbon-rich stars. Several explanations for the water were postulated, including the vaporization of icy bodies (comets or dwarf planets) in orbit around the star, grain surface reactions, and photochemistry in the outer circumstellar envelope. With a single water line detected so far from this one carbon-rich evolved star, it is difficult to discriminate between the different mechanisms proposed. Here we report the detection of dozens of water vapour lines in the far-infrared and sub-millimetre spectrum of IRC +10216 using the Herschel satellite. This includes some high-excitation lines with energies corresponding to approximately 1,000 K, which can be explained only if water is present in the warm inner sooty region of the envelope. A plausible explanation for the warm water appears to be the penetration of ultraviolet photons deep into a clumpy circumstellar envelope. This mechanism also triggers the formation of other molecules, such as ammonia, whose observed abundances are much higher than hitherto predicted.

Context. The carbon-rich asymptotic giant branch star IRC +10 216 undergoes strong mass loss, and quasi-periodic enhancements of the density of the circumstellar matter have previously been reported. The star's circumstellar environment is a well-studied and complex astrochemical laboratory, in which many molecular species have been proved to be present. CO is ubiquitous in the circumstellar envelope, while emission from the ethynyl (C 2 H) radical is detected in a spatially confined shell around IRC +10 216. We recently detected unexpectedly strong emission from the N = 4−3, 6−5, 7−6, 8−7, and 9−8 transitions of C 2 H with the IRAM 30 m telescope and with Herschel/HIFI, which challenges the available chemical and physical models. Aims. We aim to constrain the physical properties of the circumstellar envelope of IRC +10 216, including the effect of episodic mass loss on the observed emission lines. In particular, we aim to determine the excitation region and conditions of C 2 H to explain the recent detections and to reconcile them with interferometric maps of the N = 1−0 transition of C 2 H. Methods. Using radiative-transfer modelling, we provide a physical description of the circumstellar envelope of IRC +10 216, constrained by the spectral-energy distribution and a sample of 20 high-resolution and 29 low-resolution CO lines -to date, the largest modelled range of CO lines towards an evolved star. We furthermore present the most detailed radiative-transfer analysis of C 2 H that has been done so far. Results. Assuming a distance of 150 pc to IRC +10 216, the spectral-energy distribution was modelled with a stellar luminosity of 11300 L and a dust-mass-loss rate of 4.0 × 10 −8 M yr −1 . Based on the analysis of the 20 high-frequency-resolution CO observations, an average gas-mass-loss rate for the last 1000 years of 1.5 × 10 −5 M yr −1 was derived. This results in a gas-to-dust-mass ratio of 375, typical for this type of star. The kinetic temperature throughout the circumstellar envelope is characterised by three power laws: T kin (r) ∝ r −0.58 for radii r ≤ 9 stellar radii, T kin (r) ∝ r −0.40 for radii 9 ≤ r ≤ 65 stellar radii, and T kin (r) ∝ r −1.20 for radii r ≥ 65 stellar radii. This model successfully describes all 49 observed CO lines. We also show the effect of density enhancements in the wind of IRC +10 216 on the C 2 H-abundance profile, and the close agreement we find of the model predictions with interferometric maps of the C 2 H N = 1−0 transition and with the rotational lines observed with the IRAM 30 m telescope and Herschel/HIFI. We report on the importance of radiative pumping to the vibrationally excited levels of C 2 H and the significant effect this pumping mechanism has on the excitation of all levels of the C 2 H-molecule.