The asymptotic giant branch star R Sculptoris is surrounded by a detached shell of dust and gas 1,2 . The shell originates from a thermal pulse during which the star undergoes a brief period of increased mass loss 3,4 . It has hitherto been impossible to constrain observationally the timescales and mass-loss properties during and after a thermal pulse − parameters that determine the lifetime on the asymptotic giant branch and the amount of elements returned by the star. Here we report observations of CO emission from the circumstellar envelope and shell around R Sculptoris with an angular resolution of 1.3". What was hitherto thought to be only a thin, spherical shell with a clumpy structure, is revealed to contain a spiral structure. Spiral structures associated with circumstellar envelopes have been seen previously, from which it was concluded that the systems must be binaries 5,6,7,8 . Using the data, combined with hydrodynamic simulations, we conclude that R Sculptoris is a binary system that underwent a thermal pulse ≈1800 years ago, lasting ≈200 years. About 3×10 !3 M ! of mass was ejected at a velocity of 14.3 km s −1 and at a rate ≈30 times higher than the prepulse mass-loss rate. This shows that ≈3 times more mass is returned to the interstellar medium during and immediately after a pulse than previously thought. The detached shell around R Sculptoris was observed in CO(J = 3 − 2) emission at 345 GHz using the Atacama Large Millimeter/submillimeter Array (ALMA) during Cycle 0 operations (Fig.1, and supplementary information). The data clearly show the well-centered detached shell with a radius of 18.5", and reveal a spiral structure extending from the central star outwards to the shell. Previous observations of R Sculptoris show structure in the form of clumps. However, this was interpreted as clumpy material within the shell itself, and not as a structure interior to the shell 2 . Until now no clear signs of binary companions have been observed in the detached shell sources (with a possible exception for the detached shell around TT Cyg 9 ). The observed structure around R Sculptoris, however, indicates the presence of a companion, shaping the stellar wind into a spiral shell structure 8 . Smoothed particle hydrodynamics (SPH) models show that a wide binary companion can have a significant effect in the shaping of the wind, leading to elliptical and spiral structures (e.g. as observed in the case of the envelope of AFGL 3068) 5,6 . The observed shapes of the circumstellar envelopes (CSEs) around binary AGB stars depend on the physical parameters of the binary system (e.g., separation and mass ratio 10 ), the density contrasts imprinted on the wind, the temperatures in the CSE, the viewing angle, and, in the case of the gas, the chemistry and excitation 11 . The temporal variations of the mass-loss-rate and the expansion velocity further affect the structure of the CSE. Hence, the observed spiral structure and detached shell allow us to measure these important properties, and to directly link them to th...

Context. In the recent literature there has been some doubt as to the reliability of CO multi-transitional line observations as a massloss-rate estimator for AGB stars. Aims. Using new well-calibrated CO radio line observations, the main aim of the work presented here is to carefully evaluate the reliability of CO mass-loss-rate estimates for intermediate-to high-mass-loss-rate AGB stars with different photospheric chemistries. Methods. Mass-loss rates for 10 intermediate-to high-mass-loss-rate AGB stars are derived using a detailed non-LTE, non-local radiative transfer code based on the Monte-Carlo method to model the CO radio line intensities. The circumstellar envelopes are assumed to be spherically symmetric and formed by constant mass-loss rates. The energy balance is solved self-consistently and the effects of dust on the radiation field and thermal balance included. An independent estimate of the mass-loss rate is also obtained from the combination of dust radiative transfer modelling with a dynamical model of the gas and dust particles. Results. We find that the CO radio line intensities and shapes are successfully reproduced for the majority of our objects when assuming a constant mass-loss rate. Moreover, the CO line intensities are only weakly dependent on the adopted micro-turbulent velocity, in contrast to recent claims in the literature. The two methods used in the present work to derive mass-loss rates are consistent within a factor of ∼3 for intermediate-to high-mass-loss-rate objects, indicating that this is a lower limit to the uncertainty in present mass-loss-rate estimates. We find a tentative trend with chemistry. Mass-loss rates from the dust/dynamical model are systematically higher than those from the CO model for the carbon stars and vice versa for the M-type stars. This could be ascribed to a discrepancy in the adopted CO/H 2 -abundance ratio, but we caution that the sample is small and systematic errors cannot be excluded.

Aims. The aim of this paper is to investigate the evolution of the 12 C/ 13 C ratio along the AGB through the circumstellar 12 CO/ 13 CO abundance ratio. This is the first time a sample including a significant number of M-and S-type stars is analysed together with a carbon-star sample of equal size, making it possible to investigate trends among the different types and establish evolutionary effects. Methods. The circumstellar 12 CO/ 13 CO abundance ratios are estimated through a detailed radiative transfer analysis of single-dish radio line emission observations. Several different transitions have been observed for each source to ensure that a large extent of the circumstellar envelope is probed and the radiative transfer model is well constrained. The radiative transfer model is based on the Monte Carlo method and has been benchmarked against a set of similar codes. It assumes that the radiation field is non-local and solves the statistical equilibrium equations in full non-local thermodynamic equilibrium. The energy balance equation, determining the gas temperature distribution, is solved self-consistently, and the effects of thermal dust radiation (as estimated from the spectral energy distribution) are taken into account. First, the 12 CO radiative transfer is solved, assuming an abundance (dependent on the chemical type of the star), to give the physical parameters of the gas, i.e. mass-loss rate,Ṁ, gas expansion velocity, υ e , and gas temperature distribution. Then, the 13 CO radiative transfer is solved using the results of the 12 CO model giving the 13 CO abundance. Finally, the 12 CO/ 13 CO abundance ratio is calculated. Results. The circumstellar 12 CO/ 13 CO abundance ratio differs between the three spectral types. This is consistent with what is expected from stellar evolutionary models assuming that the spectral types constitute an evolutionary sequence; however, this is the first time this has been shown observationally for a relatively large sample covering all three spectral types. The median value of the 13 CO abundance in the inner circumstellar envelope is 1.6 × 10 −5 , 2.3 × 10 −5 , and 3.0 × 10 −5 for the M-type, S-type, and carbon stars of the sample, respectively, corresponding to 12 CO/ 13 CO abundance ratios of 13, 26, and 34, respectively. The spread in the 13 CO abundance, quantified by the ratio between the 90th and 10th percentile, is 4, 3, and 15 for the M-type, S-type, and carbon stars, respectively. Interestingly, the abundance ratio spread of the carbon stars is much larger than for the M-and S-type stars, even when excluding J-type carbon stars, in line with what could be expected from evolution on the AGB. We find no correlation between the isotopologue ratio and the mass-loss rate, as would be expected if both increase as the star evolves.

Aims. The main aim is to derive reliable mass-loss rates and circumstellar SiO abundances for a sample of 40 S-type AGB stars based on new multi-transitional CO and SiO radio line observations. In addition, the results are compared to previous results for M-type AGB stars and carbon stars to look for trends with chemical type. Methods. The circumstellar envelopes are assumed to be spherically symmetric and formed by a constant mass-loss rate. The massloss rates are estimated from fitting the CO observations using a non-local, non-LTE radiative transfer code based on the Monte Carlo method. In the excitation analysis, the energy balance equation is solved self-consistently simultaneously as the radiative transfer and the temperature structure of the gas is derived. Effects of dust grains are also included in the molecular excitation analysis. Once the physical properties of the circumstellar envelopes are determined, the same radiative transfer code is used to model the observed SiO lines in order to derive circumstellar abundances and the sizes of the SiO line-emitting regions. Results. We have estimated mass-loss rates of 40 S-type AGB stars and find that the derived mass-loss rates have a distribution that resembles those previously derived for similar samples of M-type AGB stars and carbon stars. The estimated mass-loss rates also correlate well with the corresponding expansion velocity of the envelope, in accordance with results for M-type AGB stars and carbon stars. In all, this indicates that the mass loss is driven by the same mechanism in all three chemical types of AGB stars. In addition, we have estimated the circumstellar fractional abundance of SiO relative to H 2 in 26 of the sample S-type AGB stars. The derived SiO abundances are, on average, about an order of magnitude higher than predicted by stellar atmosphere thermal equilibrium chemistry, indicating that non-equilibrium chemical processes determines the abundance of SiO in the circumstellar envelope. Moreover, a comparison with the results for M-type AGB stars and carbon stars show that for a certain mass-loss rate, the circumstellar SiO abundance seems independent (although with a large scatter) of the C/O-ratio. Conclusions. In our comparison of S-type AGB stars with carbon stars and M-type AGB stars, we find no large differences in circumstellar physical properties or SiO abundances depending on the chemical type of the star.

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.