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We analyse continuum and molecular emission, observed with Atacama Large Millimetre/submillimetre Array, from the dust-enshrouded intermediate-mass asymptotic giant branch (AGB) star OH 30.1−0.7. We find a secondary peak in the continuum maps, 'feature B', separated by 4.6 arcsec from the AGB star, which corresponds to a projected separation of |$1.8\times 10^{4}$| au, placing a lower limit on the physical separation. This feature is most likely composed of cold dust and is likely to be ejecta associated with the AGB star, though we cannot rule out that it is a background object. The molecular emission we detect includes lines of CO, SiS, CS, |$\mathrm{SO}_2$|⁠ , NS, NaCl, and KCl. We find that the NS emission is off centre and arranged along an axis perpendicular to the direction of feature B, indicative of a UV-emitting binary companion (e.g. a G-type main sequence star or hotter), perhaps on an eccentric orbit, contributing to its formation. However, the NaCl and KCl emission constrain the nature of that companion to not be hotter than a late B-type main-sequence star. We find relatively warm emission arising from the inner wind and detect several vibrationally excited lines of SiS (⁠|$\upsilon =1$|⁠), NaCl (up to |$\upsilon =4$|⁠), and KCl (up to |$\upsilon =2$|⁠), and emission from low-energy levels in the mid to outer envelope, as traced by |$\mathrm{SO}_2$|⁠. The CO emission is abruptly truncated around 3.5 arcsec or 14 000 au from the continuum peak, suggesting that mass loss at a high rate may have commenced as little as 2800 yr ago. [ABSTRACT FROM AUTHOR]

Context. The chemical evolution of asymptotic giant branch (AGB) stars is driven by repeated thermal pulses (TPs). The duration of a TP is only a few hundred years, whereas an inter-pulse period lasts 104 − 105 yr. Direct observations of TPs are hence unlikely. However, the detached shells seen in CO line emission that are formed as a result of a TP provide indirect constraints on the changes experienced by the star during the pulse. Aims. We aim to resolve the spatial and kinematic sub-structures in five detached-shell sources to provide detailed constraints for hydrodynamic models that describe the formation and evolution of the shells. Methods. We used observations of the 12CO (1 − 0) emission towards five carbon-AGB stars with ALMA (Atacama Large Millimeter/submillimeter Array), including previously published observations of the carbon AGB star U Ant. The data have angular resolutions of 0″​​.3 to 1″ and a velocity resolution of 0.3 km s−1. This enabled us to quantify spatial and kinematic structures in the shells. Combining the ALMA data with single-dish observations of the 12CO (1 − 0) to 12CO (4 − 3) emission towards the sources, we used radiative transfer models to compare the observed structures with previous estimates of the shell masses and temperatures. Results. The observed emission is separated into two distinct components: a more coherent, bright outer shell and a more filamentary, fainter inner shell. The kinematic information shows that the inner sub-shells move at a higher velocity relative to the outer sub-shells. The observed sub-structures reveal a negative velocity gradient outwards across the detached shells, confirming the predictions from hydrodynamical models. However, the models do not predict a double-shell structure, and the CO emission likely only traces the inner and outer edges of the shell, implying a lack of CO in the middle layers of the detached shell. Previous estimates of the masses and temperatures are consistent with originating mainly from the brighter subshell, but the total shell masses are likely lower limits. Also, additional structures in the form of partial shells outside the detached shell around V644 Sco, arcs within the shell of R Scl, and a partially filled shell for DR Ser indicate a more complicated evolution of the shells and mass-loss process throughout the TP cycle than previously assumed. Conclusions. The observed spatial and kinematical splittings of the shells appear consistent with results from the hydrodynamical models, provided the CO emission does not trace the H2 density distribution in the shell but rather traces the edges of the shells. The hydrodynamical models predict very different density profiles depending on the evolution of the shells and the different physical processes involved in the wind-wind interaction (e.g. heating and cooling processes). It is therefore not possible to constrain the total shell mass based on the CO observations alone. Additional features outside and inside the shells complicate the interpretation of the data. Complementary observations of, for example, CI as a dissociation product of CO would be necessary to understand the distribution of CO compared to H2, in addition to new detailed hydrodynamical models of the pre-pulse, pulse, and post-pulse wind. Only a comprehensive combination of observations and models will allow us to constrain the evolution of the shells and the changes in the star during the thermal-pulse cycle. [ABSTRACT FROM AUTHOR]

Context. The mass loss experienced on the asymptotic giant branch (AGB) at the end of the lives of low- and intermediate-mass stars is widely accepted to rely on radiation pressure acting on newly formed dust grains. Dust formation happens in the extended atmospheres of these stars, where the density, velocity, and temperature distributions are strongly affected by convection, stellar pulsation, and heating and cooling processes. The interaction between these processes and how that affects dust formation and growth is complex. Hence, characterising the extended atmospheres empirically is paramount to advance our understanding of the dust formation and wind-driving processes. Aims. We aim to determine the density, temperature, and velocity distributions of the gas in the extended atmosphere of the AGB star R Dor. Methods. We acquired observations using ALMA towards R Dor to study the gas through molecular line absorption and emission. We modelled the observed 12CO v = 0,  J = 2 − 1, v = 1,  J = 2 − 1, and 3 − 2 and 13CO v = 0,  J = 3 − 2 lines using the 3D radiative transfer code LIME to determine the density, temperature, and velocity distributions up to a distance of four times the radius of the star at sub-millimetre wavelengths. Results. The high angular resolution of the sub-millimetre maps allows for even the stellar photosphere to be spatially resolved. By analysing the absorption against the star, we infer that the innermost layer in the near-side hemisphere is mostly falling towards the star, while gas in the layer above that seems to be mostly outflowing. Interestingly, the high angular resolution of the ALMA Band 7 observations reveal that the velocity field of the gas seen against the star is not homogenous across the stellar disc. The gas temperature and density distributions have to be very steep close to the star to fit the observed emission and absorption, but they become shallower for radii larger than ∼1.6 times the stellar sub-millimetre radius. While the gas mass in the innermost region is hundreds of times larger than the mass lost on average by R Dor per pulsation cycle, the gas densities just above this region are consistent with those expected based on the mass-loss rate and expansion velocity of the large-scale outflow. Our fits to the line profiles require the velocity distribution on the far side of the envelope to be mirrored, on average, with respect to that on the near side. Using a sharp absorption feature seen in the CO v = 0,  J = 2 − 1 line, we constrained the standard deviation of the stochastic velocity distribution in the large-scale outflow to be ≲0.4 km s−1. We characterised two blobs detected in the CO v = 0,  J = 2 − 1 line and found densities substantially larger than those of the surrounding gas. The two blobs also display expansion velocities that are high relative to that of the large-scale outflow. Monitoring the evolution of these blobs will lead to a better understanding of the role of these structures in the mass-loss process of R Dor. [ABSTRACT FROM AUTHOR]

Context. The asymptotic giant branch (AGB) marks the final evolutionary stage of stars with initial masses between ~0.8 and 8 M®. During this phase, stars undergo copious mass loss, which contributes significantly to the enrichment of the interstellar medium. The well-accepted mass-loss mechanism requires radiation pressure acting on dust grains that form in the density-enhanced and extended AGB stellar atmospheres. The details of the mass-loss process are not yet well understood, however. For oxygen-rich AGB stars, which are the focus of this study, the dust grains that drive the wind are expected to scatter visible light very efficiently because their sizes are relative large. Aims. We study the distribution of dust in the inner wind of oxygen-rich AGB stars to advance our understanding of the wind-driving process. Methods. We observed light scattered off dust grains that form around three oxygen-rich AGB stars (W Hya, SWVir, and RCrt) with mass-loss rates between 10-7 and 10-6 M® yr-1 using the extreme-adaptive-optics imager and polarimeter SPHERE/ZIMPOL with three filters centred at 0.65, 0.75 and 0.82 μm. We compared the observed morphologies and the spectral dependence of the scattered light between the three sources and determined the radial profile, per image octant, of the dust density distribution around the closest target, WHya. Results. We find the distribution of dust to be asymmetric for the three targets. A biconical morphology is seen for R Crt, with a position angle that is very similar to those inferred from interferometric observations of maser emission and of mid-infrared continuum emission. The cause of the biconical outflow cannot be inferred from the ZIMPOL data, but we speculate that it might be the consequence of a circumstellar disc or of the action of strong magnetic fields. The dust grains polarise light more efficiently at 0.65 μm for R Crt and SWVir and at 0.82 μm for WHya. This indicates that at the time of the observations, the grains around SWVir and R Crt had sizes <0.1 μm, while those aroundWHya were larger, with sizes &0.1 μm. The asymmetric distribution of dust around R Crt makes the interpretation more uncertain for this star, however. We find that polarised light is produced already from within the visible photosphere of W Hya, which we reproduce using models with an inner dust shell that is optically thick to scattering. We fit radiative transfer models to the radial profile of the polarised light observed around WHya and find a steep dust density profile, with steepness varying considerably with direction. We find the wind-acceleration region of WHya to extend to at least ~7 R. This is in agreement with theoretical predictions of wind acceleration up to ~12 R, and highlights that ZIMPOL observations probe the crucial region around AGB stars where dust forms and is accelerated. [ABSTRACT FROM AUTHOR]

Context. The initial conditions for mass loss during the asymptotic giant branch (AGB) phase are set in their extended atmospheres, where, among others, convection and pulsation driven shocks determine the physical conditions. Aims. High resolution observations of AGB stars at (sub)millimetre wavelengths can now directly determine the morphology, activity, density, and temperature close to the stellar photosphere. Methods. We used Atacama Large Millimeter/submillimeter Array (ALMA) high angular resolution observations to resolve the extended atmospheres of four of the nearest AGB stars: W Hya, Mira A, R Dor, and R Leo. We interpreted the observations using a parameterised atmosphere model. Results. We resolve all four AGB stars and determine the brightness temperature structure between 1 and 2 stellar radii. For W Hya and R Dor we confirm the existence of hotspots with brightness temperatures > 3000 to 10 000 K. All four stars show deviations from spherical symmetry. We find variations on a timescale of days to weeks, and for R Leo we directly measure an outward motion of the millimetre wavelength surface with a velocity of at least 10.6 ± 1.4 km s−1. For all objects but W Hya we find that the temperature-radius and size-frequency relations require the existence of a (likely inhomogeneous) layer of enhanced opacity. Conclusions. The ALMA observations provide a unique probe of the structure of the extended AGB atmosphere. We find highly variable structures of hotspots and likely convective cells. In the future, these observations can be directly compared to multi-dimensional chromosphere and atmosphere models that determine the temperature, density, velocity, and ionisation structure between the stellar photosphere and the dust formation region. However, our results show that for the best interpretation, both very accurate flux calibration and near-simultaneous observations are essential. [ABSTRACT FROM AUTHOR]

Aims. We characterise the gas in the extended atmospheres of the oxygen-rich asymptotic giant branch (AGB) stars W Hya and R Dor using high angular resolution ALMA observations. Methods. We report the detection and investigate the properties of high-excitation Λ-doubling line emission of hydroxyl (OH). Results. The OH lines are produced very close to the central stars and seem optically thin and with no maser effect. We analyse the molecular excitation using a population diagram and find rotational temperatures of ∼2500 K and column densities of ∼1019 cm−2 for both sources. For W Hya, we observe emission from vibrationally excited H2O arising from the same region as the OH emission. Moreover, CO v = 1, J = 3 − 2 emission also shows a brightness peak in the same region. Considering optically thin emission and the rotational temperature derived for OH, we find a CO column density ∼15 times higher than that of OH, within an area of (92 × 84) mas2 centred on the OH emission peak. These results should be considered tentative because of the simple methods employed. The observed OH line frequencies differ significantly from the predicted transition frequencies in the literature, and provide the possibility of using OH lines observed in AGB stars to improve the accuracy of the Hamiltonian used for the OH molecule. We predict stronger OH Λ-doubling lines at millimetre wavelengths than those we detected. These lines will be a good probe of shocked gas in the extended atmosphere and are possibly even suitable as probes of the magnetic field in the atmospheres of close-by AGB stars through the Zeeman effect. [ABSTRACT FROM AUTHOR]

Context. On the asymptotic giant branch (AGB), Sun-like stars lose a large portion of their mass in an intensive wind and enrich the surrounding interstellar medium with nuclear processed stellar material in the form of molecular gas and dust. For a number of carbon-rich AGB stars, thin detached shells of gas and dust have been observed. These shells are formed during brief periods of increased mass loss and expansion velocity during a thermal pulse, and open up the possibility to study the mass-loss history of thermally pulsing AGB stars. Aims. We study the properties of dust grains in the detached shell around the carbon AGB star R Scl and aim to quantify the influence of the dust grain properties on the shape of the spectral energy distribution (SED) and the derived dust shell mass. Methods. We modelled the SED of the circumstellar dust emission and compared the models to observations, including new observations of Herschel/PACS and SPIRE (infrared) and APEX/LABOCA (sub-millimeter). We derived present-day mass-loss rates and detached shell masses for a variation of dust grain properties (opacities, chemical composition, grain size, and grain geometry) to quantify the influence of changing dust properties to the derived shell mass. Results. The best-fitting mass-loss parameters are a present-day dust mass-loss rate of 2 × 10−10M⊙ yr−1 and a detached shell dust mass of (2.9 ± 0.3) × 10−5M⊙. Compared to similar studies, the uncertainty on the dust mass is reduced by a factor of 4. We find that the size of the grains dominates the shape of the SED, while the estimated dust shell mass is most strongly affected by the geometry of the dust grains. Additionally, we find a significant sub-millimeter excess that cannot be reproduced by any of the models, but is most likely not of thermal origin. [ABSTRACT FROM AUTHOR]

Context. The stellar outflows of low- to intermediate-mass stars are characterised by a rich chemistry. Condensation of molecular gas species into dust grains is a key component in a chain of physical processes that leads to the onset of a stellar wind. In order to improve our understanding of the coupling between the micro-scale chemistry and macro-scale dynamics, we need to retrieve the abundance of molecules throughout the outflow. Aims. Our aim is to determine the radial abundance profile of SiO and HCN throughout the stellar outflow of R Dor, an oxygen-rich AGB star with a low mass-loss rate. SiO is thought to play an essential role in the dust-formation process of oxygen-rich AGB stars. The presence of HCN in an oxygen-rich environment is thought to be due to non-equilibrium chemistry in the inner wind. Methods. We analysed molecular transitions of CO, SiO, and HCN measured with the APEX telescope and all three instruments on the Herschel Space Observatory, together with data available in the literature. Photometric data and the infrared spectrum measured by ISO-SWS were used to constrain the dust component of the outflow. Using both continuum and line radiative transfer methods, a physical envelope model of both gas and dust was established. We performed an analysis of the SiO and HCN molecular transitions in order to calculate their abundances. Results. We have obtained an envelope model that describes the dust and the gas in the outflow, and determined the abundance of SiO and HCN throughout the region of the stellar outflow probed by our molecular data. For SiO, we find that the initial abundance lies between 5.5 × 10-5 and 6.0 × 10-5 with respect to H2. The abundance profile is constant up to 60 ± 10 R*, after which it declines following a Gaussian profile with an e-folding radius of 3.5 ± 0.5 × 1013 cm or 1.4 ± 0.2 R*. For HCN, we find an initial abundance of 5.0 × 10-7 with respect to H2. The Gaussian profile that describes the decline starts at the stellar surface and has an e-folding radius re of 1.85 ± 0.05 × 1015 cm or 74 ± 2 R*. Conclusions. We cannot unambiguously identify the mechanism by which SiO is destroyed at 60 ± 10 R*. The initial abundances found are higher than previously determined (except for one previous study on SiO), which might be due to the inclusion of higher-J transitions. The difference in abundance for SiO and HCN compared to high mass-loss rate Mira star IK Tau might be due to different pulsation characteristics of the central star and/or a difference in dust condensation physics. [ABSTRACT FROM AUTHOR]

Context. Low- and intermediate-mass stars go through a period of intense mass-loss at the end of their lives, during the asymptotic giant branch (AGB) phase. While on the AGB a significant part, or even most, of their initial mass is expelled in a stellar wind. This process controls the final stages of the evolution of these stars and contributes to the chemical evolution of galaxies. However, the wind-driving mechanism of AGB stars is not yet well understood, especially so for oxygen-rich sources. Characterizing both the present-day mass-loss rate and wind structure and the evolution of the mass-loss rate of such stars is paramount to advancing our understanding of this processes. Aims. We study the dusty wind of the oxygen-rich AGB star WHya to understand its composition and structure and shed light on the mass-loss mechanism. Methods. We modelled the dust envelope of WHya using an advanced radiative transfer code. We analysed our dust model in the light of a previously calculated gas-phase wind model and compared it with measurements available in the literature, such as infrared spectra, infrared images, and optical scattered light fractions. Results. We find that the dust spectrum of WHya can partly be explained by a gravitationally bound dust shell that probably is responsible for most of the amorphous Al2O3 emission. The composition of the large (∼0.3 μm) grains needed to explain the scattered light cannot be constrained, but probably is dominated by silicates. Silicate emission in the thermal infrared was found to originate from beyond 40 AU from the star. In our model, the silicates need to have substantial near-infrared opacities to be visible at such large distances. The increase in near-infrared opacity of the dust at these distances roughly coincides with a sudden increase in expansion velocity as deduced from the gas-phase CO lines. The dust envelope of WHya probably contains an important amount of calcium but we were not able to obtain a dust model that reproduces the observed emission while respecting the limit set by the gas mass-loss rate. Finally, the recent mass loss of WHya is confirmed to be highly variable and we identify a strong peak in the mass-loss rate that occurred about 3500 years ago and lasted for a few hundred years. [ABSTRACT FROM AUTHOR]

Context. The evolution of low- and intermediate-mass stars on the asymptotic giant branch (AGB) is mainly controlled by the rate at which these stars lose mass in a stellar wind. Understanding the driving mechanism and strength of the stellar winds of AGB stars and the processes enriching their surfaces with products of nucleosynthesis are paramount to constraining AGB evolution and predicting the chemical evolution of galaxies. Aims. In a previous paper we have constrained the structure of the outflowing envelope of W Hya using spectral lines of the 12CO molecule. Here we broaden this study by including an extensive set of H2O and 28SiO lines. It is the first time such a comprehensive study is performed for this source. The oxygen isotopic ratios and the 28SiO abundance profile can be connected to the initial stellar mass and to crucial aspects of dust formation at the base of the stellar wind, respectively. Methods. We model the molecular emission observed by the three instruments on board Herschel Space Observatory using a stateof- the-art molecular excitation and radiative transfer code. We also account for the dust radiation field in our calculations. Results. We find an H2O ortho-to-para ratio of 2.5 -1.0+2.5, consistent with what is expected for an AGB wind. The O16/O17 ratio indicates that W Hya has an initial mass of about 1.5 M☉. Although the ortho- and para-H2O lines observed by HIFI appear to trace gas of slightly different physical properties, we find that a turbulence velocity of 0.7 ± 0.1 kms-1 fits the HIFI lines of both spin isomers and those of 28SiO well. Conclusions. The modelling of H2O and 28SiO confirms the properties of the envelope model of W Hya, as derived from 12CO lines, and allows us to constrain the turbulence velocity. The ortho- and para-H216 O and 28SiO abundances relative to H2 are (6-2+3) × 10-4, (3-1+2) × 10-4, and (3.3 ± 0.8) × 10-5, respectively, in agreement with expectations for oxygen-rich AGB outflows. Assuming a solar silicon-to-carbon ratio, the 28SiO line emission model is consistent with about one-third of the silicon atoms being locked up in dust particles. [ABSTRACT FROM AUTHOR]

Aims. Spectra, taken with the heterodyne instrument, HIFI, aboard the Herschel Space Observatory, of O-rich asymptotic giant branch (AGB) stars which form part of the guaranteed time key program HIFISTARS are presented. The aim of this program is to study the dynamical structure, mass-loss driving mechanism, and chemistry of the outflows from AGB stars as a function of chemical composition and initial mass. Methods. We used the HIFI instrument to observe nine AGB stars, mainly in the H2O and high rotational CO lines. We investigate the correlation between line luminosity, line ratio and mass-loss rate, line width and excitation energy. Results. A total of nine different molecules, along with some of their isotopologues have been identified, covering a wide range of excitation temperature. Maser emission is detected in both the ortho- and para-H2O molecules. The line luminosities of ground state lines of ortho- and para-H2O, the high-J CO and NH3 lines show a clear correlation with mass-loss rate. The line ratios of H2O and NH3 relative to CO J = 6-5 correlate with the mass-loss rate while ratios of higher CO lines to the 6-5 is independent of it. In most cases, the expansion velocity derived from the observed line width of highly excited transitions formed relatively close to the stellar photosphere is lower than that of lower excitation transitions, formed farther out, pointing to an accelerated outflow. In some objects, the vibrationally excited H2O and SiO which probe the acceleration zone suggests the wind reaches its terminal velocity already in the innermost part of the envelope, i.e., the acceleration is rapid. Interestingly, for R Dor we find indications of a deceleration of the outflow in the region where the material has already escaped from the star. [ABSTRACT FROM AUTHOR]