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We analyse continuum and molecular emission, observed with ALMA, from the dust-enshrouded intermediate-mass AGB star OH 30.1 -0.7. We find a secondary peak in the continuum maps, "feature B", separated by 4.6" 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, 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 (3 = 1), NaCl (up to 3 = 4) and KCl (up to 3 = 2), and emission from low energy levels in the mid to outer envelope, as traced by SO$_2$. The CO emission is abruptly truncated around 3.5" or 14,000 au from the continuum peak, suggesting that mass loss at a high rate may have commenced as little as 2800 years ago., Comment: Accepted for publication in MNRAS

With a temperature of $\sim 285$ K WISE0855 is the coldest brown dwarf observed so far. Using the James Webb Space Telescope (JWST) we obtained observations that allow us to characterize WISE0855s atmosphere focusing on vertical variation in the water steam abundance, measuring trace gas abundances and receiving bulk parameters for this cold object. We observed the ultra cool dwarf WISE0855 using the Mid-Infrared Instrument Medium Resolution Spectrometer (MIRI/MRS) onboard JWST at a spectral resolution of up to 3750. We combined the observation with published data from the Near Infrared Spectrograph (NIRSpec) G395M and PRISM modes yielding a spectrum ranging from 0.8 to 22 um. We apply atmospheric retrievals using petitRADTRANS to measure atmospheric abundances, the pressure-temperature structure, radius and gravity of the brown dwarf. We also employ publicly available clear and cloudy self-consistent grid models to estimate bulk properties of the atmosphere such as the effective temperature, radius, gravity and metallicity. Atmospheric retrievals constrain a variable water abundance profile in the atmosphere, as predicted by equilibrium chemistry. We detect the 15NH3 isotopologue and infer a ratio of mass fraction of 14NH3/15NH3 = 332+63-43 for the clear retrieval. We measure the bolometric luminosity by integrating the presented spectrum and obtain a value of log(L/L$_{\odot}$) = -7.291+/-0.008. The detected water depletion indicates that water condenses out in the upper atmosphere due to the very low effective temperature of WISE0855. The height in the atmosphere where this occurs is covered by the MIRI/MRS data, and thus demonstrates the potential of MIRI to characterize cold gas giants atmospheres. Comparing the data to retrievals and self-consistent grid models, we do not detect signs for water ice clouds, although their spectral features have been predicted in previous studies., Comment: Submitted to A&A, 29 pages, 21 figures

Binary interactions have been proposed to explain a variety of circumstellar structures seen around evolved stars, including asymptotic giant branch (AGB) stars and planetary nebulae. Studies resolving the circumstellar envelopes of AGB stars have revealed spirals, discs and bipolar outflows, with shaping attributed to interactions with a companion. For the first time, we have used a combined chemical and dynamical analysis to reveal a highly eccentric and long-period orbit for W Aquilae, a binary system containing an AGB star and a main sequence companion. Our results are based on anisotropic SiN emission, the first detections of NS and SiC towards an S-type star, and density structures observed in the CO emission. These features are all interpreted as having formed during periastron interactions. Our astrochemistry-based method can yield stringent constraints on the orbital parameters of long-period binaries containing AGB stars, and will be applicable to other systems., Comment: This version of the article has been accepted for publication, after peer review (when applicable) and is subject to Springer Nature's AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at: https://doi.org/10.1038/s41550-023-02154-y

Observing exoplanets through transmission spectroscopy supplies detailed information on their atmospheric composition, physics, and chemistry. Prior to JWST, these observations were limited to a narrow wavelength range across the near-ultraviolet to near-infrared, alongside broadband photometry at longer wavelengths. To understand more complex properties of exoplanet atmospheres, improved wavelength coverage and resolution are necessary to robustly quantify the influence of a broader range of absorbing molecular species. Here we present a combined analysis of JWST transmission spectroscopy across four different instrumental modes spanning 0.5-5.2 micron using Early Release Science observations of the Saturn-mass exoplanet WASP-39b. Our uniform analysis constrains the orbital and stellar parameters within sub-percent precision, including matching the precision obtained by the most precise asteroseismology measurements of stellar density to-date, and further confirms the presence of Na, K, H$_2$O, CO, CO$_2$, and SO$_2$ atmospheric absorbers. Through this process, we also improve the agreement between the transmission spectra of all modes, except for the NIRSpec PRISM, which is affected by partial saturation of the detector. This work provides strong evidence that uniform light curve analysis is an important aspect to ensuring reliability when comparing the high-precision transmission spectra provided by JWST., Comment: 34 pages, 11 figures, 3 tables. Nat Astron (2024)

As the observed occurrence for planets or stellar companions orbiting low and intermediate-mass evolved stars is increasing, so does the importance of understanding and evaluating the strength of their interactions. One of the fundamental mechanisms to understand this interaction is the tidal dissipation in these stars, as it is one of the engines of orbital/rotational evolution of star-planet/star-star systems. This article builds on previous works studying the evolution of the tidal dissipation along the pre-MS and the MS, which have shown the strong link between the structural and rotational evolution of stars and tidal dissipation. This article provides for the first time a complete picture of tidal dissipation along the entire evolution of low and intermediate-mass stars, including the advanced phases of evolution. Using stellar evolutionary models, the internal structure of the star is computed from the pre-MS all the way up to the white dwarf phase, for stars with initial mass between 1 and 4 Msun. Tidal dissipation is separated into two components: the dissipation of the equilibrium (non-wavelike) tide and the dissipation of the dynamical (wavelike) tide. For evolved stars the dynamical tide is constituted by progressive internal gravity waves. The significance of both the equilibrium and dynamical tide dissipation becomes apparent within distinct domains of the parameter space. The dissipation of the equilibrium tide is dominant when the star is large in size or the companion is far away from the star. Conversely the dissipation of the dynamical tide is important when the star is small in size or the companion is close to the star. Both the equilibrium and the dynamical tides are important in evolved stars, and therefore both need to be taken into account when studying the tidal dissipation in evolved stars and the evolution of planetary or/and stellar companions orbiting them.

We constrain wind parameters of a sample of 18 O-type stars in the LMC, through analysis with stellar atmosphere and wind models including the effects of optically thick clumping. This allows us to determine the most accurate spectroscopic mass-loss and wind structure properties of massive stars at sub-solar metallicity to date and gain insight into the impact of metallicity on massive stellar winds. Combining high signal to noise (S/N) ratio spectroscopy in the UV and optical gives us access to diagnostics of multiple different physical processes in the stellar wind. We produce synthetic spectra using the stellar atmosphere modelling code FASTWIND, and reproduce the observed spectra using a genetic algorithm based fitting technique. We empirically constrain 15 physical parameters associated with the stellar and wind properties, including temperature, surface gravity, surface abundances, rotation, macroturbulence and wind parameters. We find, on average, mass-loss rates a factor of 4-5 lower than those predicted by Vink et al. 2001, in good agreement with predictions from Bjorklund et al. 2021, and the best agreement with those from Krticka et al. 2018. In the 'weak-wind' regime we find mass-loss rates orders of magnitude below any theoretical predictions. We find a positive correlation of clumping factors (fcl) with effective temperature with an average fcl = 14 +- 8 for the full sample. Above 38 kK an average 46 +- 24% of the wind velocity span is covered by clumps and the interclump density is 10-30% of the mean wind. Below an effective temperature of roughly 38 kK there must be additional light leakage for supergiants. For dwarf stars at low temperatures there is a statistical preference for very low clump velocity spans, however it is unclear if this can be physically motivated as there are no clearly observable wind signatures in UV diagnostics., Comment: 12 pages with 6 figures, plus 16 pages and 18 figures in appendix. Accepted for publication in A&A

The chemistry of an astrophysical environment is closely coupled to its dynamics, the latter often found to be complex. Hence, to properly model these environments a 3D context is necessary. However, solving chemical kinetics within a 3D hydro simulation is computationally infeasible for a even a modest parameter study. In order to develop a feasible 3D hydro-chemical simulation, the classical chemical approach needs to be replaced by a faster alternative. We present mace, a Machine learning Approach to Chemistry Emulation, as a proof-of-concept work on emulating chemistry in a dynamical environment. Using the context of AGB outflows, we have developed an architecture that combines the use of an autoencoder (to reduce the dimensionality of the chemical network) and a set of latent ordinary differential equations (that are solved to perform the temporal evolution of the reduced features). Training this architecture with an integrated scheme makes it possible to successfully reproduce a full chemical pathway in a dynamical environment. mace outperforms its classical analogue on average by a factor 26. Furthermore, its efficient implementation in PyTorch results in a sub-linear scaling with respect to the number of hydrodynamical simulation particles.

Context. Two protoplanets have recently been discovered within the PDS 70 protoplanetary disk. JWST/NIRCam offers a unique opportunity to characterize them and their birth environment at wavelengths difficult to access from the ground. Aims. We aim to image the circumstellar environment of PDS 70 at 1.87 $\mu$m and 4.83 $\mu$m, assess the presence of Pa-$\alpha$ emission due to accretion onto the protoplanets, and probe any IR excess indicative of heated circumplanetary material. Methods. We obtain non-coronagraphic JWST/NIRCam images of PDS 70 within the MINDS (MIRI mid-INfrared Disk Survey) program. We leverage the Vortex Image Processing (VIP) package for data reduction, and develop dedicated routines for optimal stellar PSF subtraction, unbiased imaging of the disk, and protoplanet flux measurement in this type of dataset. A radiative transfer model of the disk is used to disentangle the contributions from the disk and the protoplanets. Results. We re-detect both protoplanets and identify extended emission after subtracting a disk model, including a large-scale spiral-like feature. We interpret its signal in the direct vicinity of planet c as tracing the accretion stream feeding its circumplanetary disk, while the outer part of the feature may rather reflect asymmetric illumination of the outer disk. We also report a bright signal consistent with a previously proposed protoplanet candidate enshrouded in dust, near the 1:2:4 mean-motion resonance with planets b and c. The 1.87 $\mu$m flux of planet b is consistent with atmospheric model predictions, but not that of planet c. We discuss potential origins for this discrepancy, including significant Pa-$\alpha$ line emission. The 4.83 $\mu$m fluxes of planets b and c suggest enshrouding dust or heated CO emission from their circumplanetary environment., Comment: 6+11 pages, 3+10 figures (text+appendix). Accepted for publication in A&A

Using the medium resolution spectrograph X-shooter, spectra of 235 OB and Wolf-Rayet (WR) stars in sub-solar metallicity environments have been secured. [...]This second paper focuses on the optical observations of 232 Magellanic Clouds targets. It describes the uniform reduction of the UVB (300 - 560 nm) and VIS (550 - 1020 nm) XShootU data as well as the preparation of advanced data products [...] . The data reduction of the raw data is based on the ESO CPL X-shooter pipeline. We paid particular attention to the determination of the response curves [...] We implemented slit-loss correction, absolute flux calibration, (semi-)automatic rectification to the continuum, and a correction for telluric lines. The spectra of individual epochs were corrected for the barycentric motion, re-sampled and co-added, and the spectra from the two arms were merged into a single flux calibrated spectrum covering the entire optical range with maximum signal-to-noise ratio. [...] We provide three types of data products: (i) two-dimensional spectra for each UVB and VIS exposure; (ii) one-dimensional UVB and VIS spectra before and after response-correction, as well as after applying various processing, including absolute flux calibration, telluric removal, normalisation and barycentric correction; and (iii) co-added flux-calibrated and rectified spectra over the full optical range, for which all available XShootU exposures were combined. For many of the targets, the final signal-to-noise ratio per resolution element is above 200 in both the UVB and the VIS co-added spectra. The reduced data and advanced scientific data products will be made available to the community upon publication of this paper. [...], Comment: 22 pages, 19 figures; accepted for publication in Astronomy & Astrophysics. Links to online tables and databases will be included upon publication

The red hypergiant VY CMa is famous for its very visible record of high mass loss events. Recent CO observations with ALMA revealed three previously unknown large scale outflows (Paper I). In this paper we use the CO maps to investigate the motions of a cluster of four clumps close to the star, not visible in the optical or infrared images. We present their proper motions measured from two epochs of ALMA images and determine the line of sight velocities of the gas in emission at the clumps. We estimate their masses and ages, or time since ejection, and conclude that all four were ejected during VY CMa's active period in the early 20th century. Together with two additional knots observed with HST, VY CMa experienced at least six massive outflows during a 30 year period with a total mass lost greater than 0.07 Msun. The position-velocity map of the $^{12}$CO emission reveals previously unnoticed attributes of the older outer ejecta. In a very narrow range of Doppler velocities, $^{12}$CO absorption and emission causes some of this outer material to be quite opaque. At those frequencies the inner structure is hidden and we see only emission from an extended outer region. This fact produces a conspicuous but illusory dark spot if one attempts to subtract the continuum in a normal way., Comment: To appear in the Astronomical Journal

The dusty winds of cool evolved stars are a major contributor of the newly synthesised material enriching the Galaxy and future generations of stars. However, the details of the physics and chemistry behind dust formation and wind launching have yet to be pinpointed. Recent spatially resolved observations show the importance of gaining a more comprehensive view of the circumstellar chemistry, but a comparative study of the intricate interplay between chemistry and physics is still difficult because observational details such as frequencies and angular resolutions are rarely comparable. Aiming to overcome these deficiencies, ATOMIUM is an ALMA Large Programme to study the physics and chemistry of the circumstellar envelopes of a diverse set of oxygen-rich evolved stars under homogeneous observing conditions at three angular resolutions between ~0.02"-1.4". Here we summarize the molecular inventory of these sources, and the correlations between stellar parameters and molecular content. Seventeen oxygen-rich or S-type asymptotic giant branch (AGB) and red supergiant (RSG) stars have been observed in several tunings with ALMA Band 6, targeting a range of molecules to probe the circumstellar envelope and especially the chemistry of dust formation close to the star. We systematically assigned the molecular carriers of the spectral lines and measured their spectroscopic parameters and the angular extent of the emission of each line from integrated intensity maps. Across the ATOMIUM sample, we detect 291 transitions of 24 different molecules and their isotopologues. This includes several first detections in oxygen-rich AGB/RSG stars: PO v=1, SO2 v1=1 and v2=2, and several high energy H2O transitions. We also find several first detections in S-type AGB stars: vibrationally excited HCN v2=2,3 and SiS v=4,5,6, as well as first detections of the molecules SiC, AlCl, and AlF in W Aql..., Comment: 19 pages plus appendices, forthcoming publication in Astronomy & Astrophysics

Context. Clouds seem unavoidable in cool and dense environments, and hence, are necessary to explain observations of exoplanet atmospheres, most recently of WASP 96b with JWST. Understanding the formation of cloud condensation nuclei in non-terrestrial environments is therefore crucial to develop accurate models to interpret present and future observations. Aims. The goal of the paper is to support observations with infrared spectra for (TiO2)N clusters in order to study cloud formation in exoplanet atmospheres. Methods. Vibrational frequencies are derived from quantum-chemical calculations for 123 (TiO2)-clusters and their isomers, and line-broadening mechanisms are evaluated. Cluster spectra are calculated for several atmospheric levels for two example exoplanet atmospheres (WASP 121b-like and WASP 96b-like) to identify possible spectral fingerprints for cloud formation. Results. Rotational motion of and transitions in the clusters cause significant line broadening, so that individual vibrational lines are broadened beyond the spectral resolution of the medium resolution mode of the JWST mid-infrared instrument MIRI at R = 3000. However, each individual cluster isomer exhibits a "fingerprint" IR spectrum. In particular, larger (TiO2)-clusters have distinctly different spectra from smaller clusters. Morning and evening terminator for the same planet can exhibit different total absorbances due to different cluster sizes being more abundant. Conclusions. The largest (TiO2)-clusters are not necessarily the most abundant (TiO2)-clusters in the high-altitude regions of ultra-hot Jupiters, and the different cluster isomers will contribute to the local absorbance. Planets with a considerable day-night asymmetry will be most suitable to search for (TiO2)-cluster isomers in order to improve cloud formation modelling., Comment: 8 pages, 8 figures, 1 table, accepted for publication in A&A

Water and the hydroxyl radical are major constituents of the envelope of O-rich late-type stars. Transitions involving energy levels that are highly excited have been observed in both H$_2$O and OH. These and more recently discovered transitions can now be observed at a high sensitivity and angular resolution with the ALMA Array. Spectra and maps of H$_2$O and OH observed with an angular resolution of 20 to $\sim$200 mas were obtained at two epochs with the ALMA array. Observations with the Compact Array were also used to check for time variability of water transitions. Radiative transfer models of water were revisited to characterize masing conditions and up-to-date chemical models were used for comparison with our observations. Ten rotational transitions of H$_2$O with energies up to 9000 K were observed in various vibrational states. All but one are new detections in space, and from these we have derived accurate rest frequencies. Hyperfine split $\Lambda$-doubling transitions in v = 0, J = 27/2 and 29/2 levels of the $^2\Pi_{3/2}$ state and, $J = 33/2$ and 35/2 of the $^2\Pi_{1/2}$ state of OH with excitation energies up to 8900 K were also observed. Four of these transitions are new detections in space. Combining our measurements with earlier observations of OH, the v = 0 and v = 1 $\Lambda$-doubling frequencies have been improved. Our H$_2$O maps show compact emission and extensions up to twelve stellar radii or more. The 268.149 GHz emission line of water in the v$_2$ = 2 state is time variable, tends to be masing with dominant radiative pumping, and is widely excited. The widespread but weaker 262.898 GHz water line in v$_2$ = 1 also shows signs of maser emission. Emission and absorption of both H$_2$O and OH reveal an infall of matter and complex kinematics influenced by binarity. From our observed column densities, we derived OH/H$_2$O abundance ratios in a few stars., Comment: 41 pages with references and 25 figures in main text. 4 Tables in Appendix A. 10 figures in Appendix B. 2 figures in Appendix C. 14 figures in Appendix D. 5 figures in Appendix E. 2 figures in Appendix F

Asymptotic Giant Branch (AGB) stars shed a significant amount of their mass in the form of a stellar wind, creating a vast circumstellar envelope (CSE). Owing to the ideal combination of relatively high densities and cool temperatures, CSEs serve as rich astrochemical laboratories. {While the chemical structure of AGB outflows has been modelled and analysed in detail for specific physical setups, there is a lack of understanding regarding the impact of changes in the physical environment on chemical abundances. A systematic sensitivity study is necessary to comprehend the nuances in the physical parameter space, given the complexity of the chemistry. This is crucial for estimating uncertainties associated with simulations and observations. In this work, we present the first sensitivity study of the impact of varying outflow densities and temperature profiles on the chemistry. With the use of a chemical kinetics model, we report on the uncertainty in abundances, given a specific uncertainty on the physical parameters. }Additionally, we analyse the molecular envelope extent of parent species and compare our findings to observational studies. Mapping the impact of differences in physical parameters throughout the CSE on the chemistry is a strong aid to observational studies., Comment: 21 pages, 25 figures

The winds of massive stars have an impact on stellar evolution and on the surrounding medium. The maximum speed reached by these outflows, the terminal wind speed, is a global wind parameter and an essential input for models of stellar atmospheres and feedback. With the arrival of the ULLYSES programme, a legacy UV spectroscopic survey with HST, we have the opportunity to quantify the wind speeds of massive stars at sub-solar metallicity (in the Large and Small Magellanic Clouds, 0.5Z and 0.2Z) at an unprecedented scale. We empirically quantify the wind speeds of a large sample of OB stars, including supergiants, giants, and dwarfs at sub-solar metallicity. Using these measurements, we investigate trends of terminal wind speed with a number of fundamental stellar parameters, namely effective temperature, metallicity, and surface escape velocity. We empirically determined the terminal wind speed for a sample of 149 OB stars in the Magellanic Clouds either by directly measuring the maximum velocity shift of the absorption component of the Civ 1548-1550 line profile, or by fitting synthetic spectra produced using the Sobolev with exact integration method. Stellar parameters were either collected from the literature, obtained using spectral-type calibrations, or predicted from evolutionary models. We find strong trends of terminal wind speed with effective temperature and surface escape speed when the wind is strong enough to cause a saturated P Cygni profile in Civ 1548-1550. We find evidence for a metallicity dependence on the terminal wind speed proportional to Z^0.22+-0.03 when we compared our results to previous Galactic studies. Our results suggest that effective temperature rather than surface escape speed should be used as a straightforward empirical prediction of terminal wind speed and that the observed metallicity dependence is steeper than suggested by earlier works., Comment: 21 pages, 16 figures, 8 tables. Accepted in A&A

The 'Great Dimming' of the prototypical red supergiant Betelgeuse, which occurred between December 2019 and April 2020, gives us unprecedented insight into the processes occurring on the stellar surface and in the inner wind of this type of star. In particular it may bring further understanding of their dust nucleation and mass loss processes. Here, we present and analyse VLTI/MATISSE observations in the N-band (8 - 13 $\mu$m) taken near the brightness minimum in order to assess the status of the dusty circumstellar environment. We explore the compatibility of a dust clump obscuring the star with our mid-infrared interferometric observations using continuum 3D radiative transfer modelling, and probe the effect of adding multiple clumps close to the star on the observables. We also test the viability of a large cool spot on the stellar surface without dust present in the ambient medium. Using the visibility data, we derive a uniform disk diameter of 59.02 $\pm$ 0.64 mas in the spectral range 8 to 8.75 $\mu$m. We find that both the dust clump and the cool spot models are compatible with the data. Further to this, we note that the extinction and emission of our localised dust clump in the line of sight of the star, directly compensate each other making the clump undetectable in the spectral energy distribution and visibilities. The lack of infrared brightening during the 'Great Dimming' therefore does not exclude extinction due to a dust clump as one of the possible mechanisms. The visibilities can be reproduced by a spherical wind with dust condensing at 13 stellar radii and a dust mass-loss rate of (2.1 - 4.9) $\times$ 10$^{-10}$ $\mathit{M}_{\odot} {\rm yr}^{-1}$, however, in order to reproduce the complexity of the observed closure phases, additional surface features or dust clumps would be needed., Comment: 13 Pages, accepted for publication in A&A

NaCl is a diatomic molecule with a large dipole moment, which allows for its detection even at relatively small abundances. It has been detected towards several evolved stars, among which is the AGB star IK Tau, around which it is distributed in several clumps that lie off-center from the star. We aim to study the three-dimensional distribution of NaCl around the AGB star IK Tau, and to obtain the abundance of NaCl relative to H$_2$ for each of the clumps. First, a new value for the maximum expansion velocity is determined. The observed ALMA channel maps are then deprojected to create a three-dimensional model of the distribution of NaCl. This model is then used as input for the radiative transfer modelling code magritte, which is used to obtain the NaCl abundances of each of the clumps by comparing the observations with the results of the magritte simulations. Additionally, the rotational temperature of the clumps is determined using population diagrams. We derive an updated value for the maximum expansion velocity of IK Tau $\upsilon_\mathrm{exp}$ = 28.4 km/s. A spiral-like shape can be discerned in our three-dimensional distribution model of the NaCl. This spiral lies more or less in the plane of the sky. The distribution is also flatter in the line-of-sight direction than in the plane of the sky. We find clump abundances between $9 \times 10^{-9}$ and $5 \times 10^{-8}$ relative to H$_2$, where the relative abundance is typically lower for clumps closer to the star. For the first time, we used deprojection to understand the three-dimensional environment of an AGB star and calculated the fractional abundance of NaCl in clumps surrounding the star., Comment: Accepted for publication in A&A

Aims. Through the ATOMIUM project, based on an ALMA large program, we aim to present a consistent view of a sample of 17 nearby cool evolved stars (Aymptotic Giant Branch and red supergiant stars). Methods. Here we present VLT/SPHERE-ZIMPOL polarimetric maps obtained in the visible of 14 out of the 17 ATOMIUM sources. They were obtained contemporaneously with the ALMA high spatial resolution data. To help interpret the polarized signal, we produced synthetic maps of light scattering by dust, through 3D radiative transfer simulations with the RADMC3D code. Results. The degree of linear polarization (DoLP) observed by ZIMPOL spreads across several optical filters. We infer that it primarily probes dust located just outside of the point spread function, and in or near the plane of the sky, with a total optical depth close to unity in the line of sight, representing only a fraction of the total circumstellar dust. The maximum DoLP ranges from 0.03-0.38 depending on the source, fractions that can be reproduced by our 3D pilot models for grains composed of common dust species. The spatial structure of the DoLP shows a diverse set of shapes. Only for three sources do we note a correlation between the ALMA CO and SiO lines, which trace the gas density, and the DoLP, which traces the dust. Conclusion. The clumpiness of the DoLP and the lack of a consistent correlation between the gas and the dust location show that, in the inner circumstellar environment (CSE), dust formation occurs at very specific sites. This has potential consequences for the derived mass-loss rates and dust-to-gas ratio in the inner region of the CSE. Except for $\pi^1$~Gru and perhaps GY Aql, we do not detect interactions between the circumstellar wind and the hypothesized companions that shape the wind at larger scales. This suggests that the orbits of any other companions are tilted out of the plane of the sky., Comment: Accepted for publication in Astronomy & Astrophysics. 22 pages, 15 figures, 5 tables

Transmission spectroscopy of exoplanets has revealed signatures of water vapor, aerosols, and alkali metals in a few dozen exoplanet atmospheres. However, these previous inferences with the Hubble and Spitzer Space Telescopes were hindered by the observations' relatively narrow wavelength range and spectral resolving power, which precluded the unambiguous identification of other chemical species$-$in particular the primary carbon-bearing molecules. Here we report a broad-wavelength 0.5-5.5 $\mu$m atmospheric transmission spectrum of WASP-39 b, a 1200 K, roughly Saturn-mass, Jupiter-radius exoplanet, measured with JWST NIRSpec's PRISM mode as part of the JWST Transiting Exoplanet Community Early Release Science Team program. We robustly detect multiple chemical species at high significance, including Na (19$\sigma$), H$_2$O (33$\sigma$), CO$_2$ (28$\sigma$), and CO (7$\sigma$). The non-detection of CH$_4$, combined with a strong CO$_2$ feature, favours atmospheric models with a super-solar atmospheric metallicity. An unanticipated absorption feature at 4$\mu$m is best explained by SO$_2$ (2.7$\sigma$), which could be a tracer of atmospheric photochemistry. These observations demonstrate JWST's sensitivity to a rich diversity of exoplanet compositions and chemical processes., Comment: 41 pages, 4 main figures, 10 extended data figures, 4 tables. Under review in Nature

Red supergiant stars (RSGs, Minit = 10-40Msun) are known to eject large amounts of material, as much as half of their initial mass during this evolutionary phase. However, the processes powering the mass ejection in low- and intermediate-mass stars do not work for RSGs and the mechanism that drives the ejection remains unknown. Different mechanisms have been proposed as responsible for this mass ejection but so far little is known about the actual processes taking place in these objects. Here we present high angular resolution interferometric ALMA maps of VY CMa continuum and molecular emission, which resolve the structure of the ejecta with unprecedented detail. The study of the molecular emission from the ejecta around evolved stars has been shown to be an essential tool in determining the characteristics of the mass loss ejections. Our aim is thus to use the information provided by these observations to understand the ejections undergone by VY CMa and to determine their possible origins. We inspected the kinematics of molecular emission observed. We obtained position-velocity diagrams and reconstructed the 3D structure of the gas traced by the different species. It allowed us to study the morphology and kinematics of the gas traced by the different species surrounding VY CMa. Two types of ejecta are clearly observed: extended, irregular, and vast ejecta surrounding the star that are carved by localized fast outflows. The structure of the outflows is found to be particularly flat. We present a 3D reconstruction of these outflows and proof of the carving. This indicates that two different mass loss processes take place in this massive star. We tentatively propose the physical cause for the formation of both types of structures. These results provide essential information on the mass loss processes of RSGs and thus of their further evolution., Comment: 13 pages, 10 figures

The formation of inorganic cloud particles takes place in several atmospheric environments including those of warm, hot, rocky and gaseous exoplanets, brown dwarfs, and AGB stars. The cloud particle formation needs to be triggered by the in-situ formation of condensation seeds since it can not be assumed that such condensation seeds preexist in these chemically complex gas-phase environments. We aim to develop a methodology to calculate the thermochemical properties of clusters as key inputs to model the formation of condensation nuclei in gases of changing chemical composition. TiO$_2$ is used as benchmark species for cluster sizes N = 1 - 15. We create 90000 candidate geometries, for cluster sizes N = 3 - 15. We employ a hierarchical optimisation approach, consisting of a force field description, density functional based tight binding (DFTB) and all-electron density functional theory (DFT) to obtain accurate energies and thermochemical properties for the clusters. We find B3LYP/cc-pVTZ including Grimmes empirical dispersion to perform most accurately with respect to experimentally derived thermochemical properties of the TiO$_2$ molecule. We present a hitherto unreported global minimum candidate for size N = 13. The DFT derived thermochemical cluster data are used to evaluate the nucleation rates for a given temperature-pressure profile of a model hot Jupiter atmosphere. We find that with the updated and refined cluster data, nucleation becomes unfeasible at slightly lower temperatures, raising the lower boundary for seed formation in the atmosphere. The approach presented in this paper allows to find stable isomers for small (TiO$_2$)$_N$ clusters. The choice of functional and basis set for the all-electron DFT calculations have a measurable impact on the resulting surface tension and nucleation rate and the updated thermochemical data is recommended for future considerations., Comment: 20 pages, 14 figures. Accepted for publication in A&A

In accelerating and supersonic media, the interaction of photons with spectral lines can be of ultimate importance. However, fully accounting for such line forces currently can only be done by specialised codes in 1-D steady-state flows. More general cases and higher dimensions require alternative approaches. We presented a comprehensive and fast method for computing the radiation line-force using tables of spectral line-strength distribution parameters, which can be applied in arbitrary (multi-D, time-dependent) simulations, including those accounting for the line-deshadowing instability, to compute the appropriate opacities. We assumed local thermodynamic equilibrium to compute a flux-weighted line opacity from $>4$ million spectral lines. We derived the spectral line strength and tabulated the corresponding line-distribution parameters for a range of input densities $\rho\in[10^{-20},10^{-10}]gcm^{-3}$ and temperatures $T\in[10^4,10^{4.7}]K$. We found that the variation of the line distribution parameters plays an essential role in setting the wind dynamics in our models. In our benchmark study, we also found a good overall agreement between the O-star mass-loss rates of our models and those derived from steady-state studies using more detailed radiative transfer. Our models reinforce that self-consistent variation of the line-distribution parameters is important for the dynamics of line-driven flows. Within a well-calibrated O-star regime, our results support the proposed methodology. In practice, utilising the provided tables, yielded a factor $>100$ speed-up in computational time compared to specialised 1-D model-atmosphere codes of line-driven winds, which constitutes an important step towards efficient multi-D simulations. We conclude that our method and tables are ready to be exploited in various radiation-hydrodynamic simulations where the line force is important.

Red supergiants are the most common final evolutionary stage of stars that have initial masses between 8 and 35 times that of the Sun. During this stage, which lasts roughly 100,000 years1, red supergiants experience substantial mass loss. However, the mechanism for this mass loss is unknown. Mass loss may affect the evolutionary path, collapse and future supernova light curve of a red supergiant, and its ultimate fate as either a neutron star or a black hole. From November 2019 to March 2020, Betelgeuse - the second-closest red supergiant to Earth (roughly 220 parsecs, or 724 light years, away) - experienced a historic dimming of its visible brightness. Usually having an apparent magnitude between 0.1 and 1.0, its visual brightness decreased to 1.614 +/- 0.008 magnitudes around 7-13 February 2020 - an event referred to as Betelgeuse's Great Dimming. Here we report high-angular-resolution observations showing that the southern hemisphere of Betelgeuse was ten times darker than usual in the visible spectrum during its Great Dimming. Observations and modelling support a scenario in which a dust clump formed recently in the vicinity of the star, owing to a local temperature decrease in a cool patch that appeared on the photosphere. The directly imaged brightness variations of Betelgeuse evolved on a timescale of weeks. Our findings suggest that a component of mass loss from red supergiants is inhomogeneous, linked to a very contrasted and rapidly changing photosphere

This overview paper presents ATOMIUM, a Large Programme in Cycle 6 with the Atacama Large Millimeter-submillimeter Array (ALMA). The goal of ATOMIUM is to understand the dynamics and the gas phase and dust formation chemistry in the winds of evolved asymptotic giant branch (AGB) and red supergiant (RSG) stars. A more general aim is to identify chemical processes applicable to other astrophysical environments. 17 oxygen-rich AGB and RSG stars spanning a range in (circum)stellar parameters and evolutionary phases were observed in a homogeneous observing strategy allowing for an unambiguous comparison. Data were obtained between 213.83 and 269.71 GHz at high (0.025-0.050 arcsec), medium (0.13-0.24 arcsec), and low (about 1 arcsec) angular resolution. The sensitivity per 1.3 km/s channel was 1.5-5 mJy/beam. 13 molecules were designated as primary molecules in the survey: CO, SiO, AlO, AlOH, TiO, TiO2, HCN, SO, SO2, SiS, CS, H2O, and NaCl. The scientific motivation, survey design, sample properties, data reduction, and an overview of the data products are described; and we highlight one scientific result - the wind kinematics of the ATOMIUM sources. The ATOMIUM sources often have a slow wind acceleration, and a fraction of the gas reaches a velocity which can be up to a factor of two times larger than previously reported terminal velocities assuming isotropic expansion, and the wind kinematic profiles establish that the radial velocity described by the momentum equation for a spherical wind structure cannot capture the complexity of the velocity field. In 15 sources, some molecular transitions other than 12CO v=0 J=2-1 reach a higher outflow velocity, with a spatial emission zone that is often greater than 30 stellar radii, but much less than the extent of CO. Binary interaction with a (sub)stellar companion might (partly) explain the non-monotonic behaviour of the projected velocity field.

S-type asymptotic giant branch (AGB) stars are thought to be intermediates in the evolution of oxygen- to carbon-rich AGB stars. The chemical compositions of their circumstellar envelopes are also intermediate, but have not been studied in as much detail as their carbon- and oxygen-rich counterparts. We aim to determine the abundances of AlCl and AlF from rotational lines, which have been observed for the first time towards an S-type AGB star, W Aql. In combination with models based on PACS observations, we aim to update our chemical kinetics network based on these results. We analyse ALMA observations towards W Aql of AlCl in the ground and first two vibrationally excited states and AlF in the ground vibrational state. Using radiative transfer models, we determine the abundances and spatial abundance distributions of Al$^{35}$Cl, Al$^{37}$Cl, and AlF. We also model HCl and HF emission and compare these models to PACS spectra to constrain the abundances of these species. AlCl is found in clumps very close to the star, with emission confined within 0.1$^{\prime\prime}$ of the star. AlF emission is more extended, with faint emission extending 0.2$^{\prime\prime}$ to 0.6$^{\prime\prime}$ from the continuum peak. We find peak abundances, relative to H$_2$, of $1.7\times 10^{-7}$ for Al$^{35}$Cl, $7\times 10^{-8}$ for Al$^{37}$Cl and $1\times 10^{-7}$ for AlF. From the PACS spectra, we find abundances of $9.7\times 10^{-8}$ and $\leq 10^{-8}$, relative to H$_2$, for HCl and HF, respectively. The AlF abundance exceeds the solar F abundance, indicating that fluorine synthesised in the AGB star has already been dredged up to the surface of the star and ejected into the circumstellar envelope. From our analysis of chemical reactions in the wind, we conclude that AlF may participate in the dust formation process, but we cannot fully explain the rapid depletion of AlCl seen in the wind., Comment: Accepted for publication in Astronomy & Astrophysics

The late evolutionary stages of low- and intermediate-mass stars are characterised by mass loss through a dust-driven stellar wind. Recent observations reveal complex structures within these winds, that are believed to be formed primarily via interaction with a companion. How these complexities arise, and which structures are formed in which type of systems, is still poorly understood. Particularly, there is a lack of studies investigating the structure formation in eccentric systems. We aim to improve our understanding of the wind morphology of eccentric AGB binary systems by investigating the mechanism responsible for the different small-scale structures and global morphologies that arise in a polytropic wind with different velocities. Using the smoothed particle hydrodynamics (SPH) code Phantom, we generate nine different high-resolution, 3D simulations of an AGB star with a solar-mass companion with various wind velocity and eccentricity combinations. The models assume a polytropic gas, with no additional cooling. We conclude that for models with a high wind velocity, the short interaction with the companion results in a regular spiral morphology, that is flattened. In the case of a lower wind velocity, the stronger interaction results in the formation of a high-energy region and bow-shock structure that can shape the wind into an irregular morphology if instabilities arise. High-eccentricity models show a complex, phase-dependent interaction leading to wind structures that are irregular in three dimensions. However, the significant interaction with the companion compresses matter into an equatorial density enhancement, irrespective of eccentricity., Comment: 23 pages, 22 figures

Asymptotic giant branch (AGB) stars are known to lose a significant amount of mass by a stellar wind, which controls the remainder of their stellar lifetime. High angular-resolution observations show that the winds of these cool stars typically exhibit mid- to small-scale density perturbations such as spirals and arcs, believed to be caused by the gravitational interaction with a (sub-)stellar companion. We aim to explore the effects of the wind-companion interaction on the 3D density and velocity distribution of the wind, as a function of three key parameters: wind velocity, binary separation and companion mass. For the first time, we compare the impact on the outflow of a planetary companion to that of a stellar companion. We intend to devise a morphology classification scheme based on a singular parameter. With our grid of models we cover the prominent morphology changes in a companion-perturbed AGB outflow: slow winds with a close, massive binary companion show a more complex morphology. Additionally, we prove that massive planets are able to significantly impact the density structure of an AGB wind. We find that the interaction with a companion affects the terminal velocity of the wind, which can be explained by the gravitational slingshot mechanism. We distinguish between two types of wind focussing to the orbital plane resulting from distinct mechanisms: global flattening of the outflow as a result of the AGB star's orbital motion and the formation of an EDE as a consequence of the companion's gravitational pull. We investigate different morphology classification schemes and uncover that the ratio of the gravitational potential energy density of the companion to the kinetic energy density of the AGB outflow yields a robust classification parameter for the models presented in this paper., Comment: 19 pages, 11 figures

Context. Red supergiants are observed to undergo vigorous mass-loss. However, to date, no theoretical model has succeeded in explaining the origins of these objects' winds. This strongly limits our understanding of red supergiant evolution and Type II-P and II-L supernova progenitor properties. Aims. We examine the role that vigorous atmospheric turbulence may play in initiating and determining the mass-loss rates of red supergiant stars. Methods. We analytically and numerically solve the equations of conservation of mass and momentum, which we later couple to an atmospheric temperature structure, to obtain theoretically motivated mass-loss rates. We then compare these to state-of-the-art empirical mass-loss rate scaling formulae as well as observationally inferred mass-loss rates of red supergiants. Results. We find that the pressure due to the characteristic turbulent velocities inferred for red supergiants is sufficient to explain the mass-loss rates of these objects in the absence of the normally employed opacity from circumstellar dust. Motivated by this initial success, we provide a first theoretical and fully analytic mass-loss rate prescription for red supergiants. We conclude by highlighting some intriguing possible implications of these rates for future studies of stellar evolution, especially in light of the lack of a direct dependence on metallicity., Comment: 14 pages, 9 figures, 2 tables

The processes by which red supergiants lose mass are not fully understood thus-far and their mass-loss rates lack theoretical constraints. The ambient surroundings of the nearby M0.5 Iab star Antares offers an ideal environment to obtain detailed empirical information on the outflow properties at its onset, and hence indirectly, on the mode(s) of mass loss. We present and analyse optical VLT/SPHERE/ZIMPOL polarimetric imaging with angular resolution down to 23 milli-arcsec, sufficient to spatially resolve both the stellar disk and its direct surroundings. We detect a conspicuous feature in polarised intensity that we identify as a clump containing dust, which we characterise through 3D radiative transfer modelling. The clump is positioned behind the plane of the sky, therefore has been released from the backside of the star, and its inner edge is only 0.3 stellar radii above the surface. The current dust mass in the clump is $1.3^{+0.2}_{-1.0} \times 10^{-8}$ M$_{\odot}$, though its proximity to the star implies that dust nucleation is probably still ongoing. The ejection of clumps of gas and dust makes a non-negligible contribution to the total mass lost from the star which could possibly be linked to localised surface activity such as convective motions or non-radial pulsations., Comment: 15 pages, accepted for publication in MNRAS

Vigorous mass loss in the classical Wolf-Rayet (WR) phase is important for the late evolution and final fate of massive stars. We develop spherically symmetric time-dependent and steady-state hydrodynamical models of the radiation-driven wind outflows and associated mass loss from classical WR stars. The simulations are based on combining the opacities typically used in static stellar structure and evolution models with a simple parametrised form for the enhanced line-opacity expected within a supersonic outflow. Our simulations reveal high mass-loss rates initiated in deep and hot optically thick layers around T\approx 200kK. The resulting velocity structure is non-monotonic and can be separated into three phases: i) an initial acceleration to supersonic speeds ii) stagnation and even deceleration, and iii) an outer region of rapid re-acceleration. The characteristic structures seen in converged steady-state simulations agree well with the outflow properties of our time-dependent models. By directly comparing our dynamic simulations to corresponding hydrostatic models, we demonstrate explicitly that the need to invoke extra energy transport in convectively inefficient regions of stellar structure and evolution models is merely an artefact of enforcing a hydrostatic outer boundary. Moreover, the "dynamically inflated" inner regions of our simulations provide a natural explanation for the often-found mismatch between predicted hydrostatic WR radii and those inferred from spectroscopy. Finally, we contrast our simulations with alternative recent WR wind models based on co-moving frame radiative transfer for computing the radiation force. Since CMF transfer currently cannot handle non-monotonic velocity fields, the characteristic deceleration regions found here are avoided in such simulations by invoking an ad-hoc very high degree of clumping., Comment: 15 pages, 9 figures

Rotational transitions in vibrationally excited AlO and TiO -- two possible precursors of dust -- were observed in the 300 GHz range (1 mm wavelength) towards the oxygen rich AGB stars R Dor and IK Tau with ALMA, and vibrationally excited AlO was observed towards the red supergiant VY CMa with the SMA. The $J=11 \to 10$ transition of TiO in the $v=1~{\rm{and}}~2$ levels, and the $N = 9 \to 8$ transition in the $v=2$ level of AlO were identified towards R Dor; the $J=11 \to 10$ line of TiO was identified in the $v=1$ level towards IK Tau; and two transitions in the $v=1~{\rm{and}}~2$ levels of AlO were identified towards VY CMa. The newly-derived high vibrational temperature of TiO and AlO in R Dor of $1800 \pm 200$ K, and prior measurements of the angular extent confirm that the majority of the emission is from a region within $\lesssim2R_{\star}$ of the central star. A full radiative transfer analysis of AlO in R Dor yielded a fractional abundance of $\sim$3% of the solar abundance of Al. From a similar analysis of TiO a fractional abundance of $\sim78$% of the solar abundance of Ti was found. The observations provide indirect evidence that TiO is present in a rotating disk close to the star. Further observations in the ground and excited vibrational levels are needed to determine whether AlO, TiO, and TiO$_2$ are seeds of the Al$_2$O$_3$ dust in R Dor, and perhaps in the gravitationally bound dust shells in other AGB stars with low mass loss rates., Comment: Accepted for publication in ApJ

Binary interactions dominate the evolution of massive stars, but their role is less clear for low- and intermediate-mass stars. The evolution of a spherical wind from an asymptotic giant branch (AGB) star into a nonspherical planetary nebula (PN) could be due to binary interactions. We observed a sample of AGB stars with the Atacama Large Millimeter/submillimeter Array (ALMA) and found that their winds exhibit distinct nonspherical geometries with morphological similarities to planetary nebulae (PNe). We infer that the same physics shapes both AGB winds and PNe; additionally, the morphology and AGB mass-loss rate are correlated. These characteristics can be explained by binary interaction. We propose an evolutionary scenario for AGB morphologies that is consistent with observed phenomena in AGB stars and PNe., Comment: 19 pages main journal, 97 pages Supplementary Information

High-angular-resolution observations of asymptotic giant branch (AGB) stars often reveal non-spherical morphologies for the gas and dust envelopes. We aim to make a pilot study to quantify the impact of different geometries (spherically symmetric, spiral-shaped, and disc-shaped) of the dust component of AGB envelopes on spectral energy distributions (SEDs), mass estimates, and subsequent mass-loss rate (MLR) estimates. We also estimate the error made on the MLR if the SED is fitted by an inappropriate geometrical model. We use the 3D Monte-Carlo-based radiative-transfer code RADMC-3D to simulate emission from dusty envelopes with different geometries (but fixed spatial extension). We compare these predictions with each other, and with the SED of the AGB star EP Aqr that we use as a benchmark since its envelope is disc-like and known to harbour spiral arms, as seen in CO. The SEDs involving the most massive envelopes are those for which the different geometries have the largest impact, primarily on the silicate features at 10 and 18 um. These different shapes originate from large differences in optical depths. Massive spirals and discs appear akin to black bodies. Optically thick edge-on spirals and discs (with dust masses of 1e-4 and 1e-5 Msun) exhibit black-body SEDs that appear cooler than those from face-on structures and spheres of the same mass, while optically thick face-on distributions appear as warmer emission. We find that our more realistic models, combined spherical and spiral distributions, are 0.1 to 0.5 times less massive than spheres with similar SEDs. More extreme, less realistic scenarios give that spirals and discs are 0.01 to 0.05 times less massive than corresponding spheres. This means that adopting the wrong geometry for an AGB circumstellar envelope may result in a MLR that is incorrect by as much as 1 to 2 orders of magnitude when derived from SED fitting., Comment: 15 pages, 9 figures, and 8 tables, accepted for publication in A&A in August 2020

Stars with initial masses between $\sim0.8$ and 8~$M_\odot$ present copious mass loss during the asymptotic giant branch (AGB) at the end of their lives. The accepted mass-loss mechanism requires radiation pressure acting on dust grains that form in the extended AGB stellar atmospheres. The details of this process are not yet well understood, however. Using the extreme-adaptive-optics imager and polarimeter SPHERE/ZIMPOL, we observed light polarised by grains around W\,Hya, SW\,Vir, and R\,Crt, which have mass-loss rates between 10$^{-7}$ and 10$^{-6}~M_\odot~{\rm yr^{-1}}$. We find the distribution of dust to be asymmetric around 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 directly inferred from the ZIMPOL data. The dust grains polarise light more efficiently at 0.65~$\mu$m for R\,Crt and SW\,Vir and at 0.82~$\mu$m for W\,Hya. This indicates that at the time of the observations, the grains around SW\,Vir and R\,Crt had sizes $< 0.1~\mu$m, while those around W\,Hya were larger, with sizes $\gtrsim 0.1~\mu$m. 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. The radial profile of the polarised light observed around W\,Hya reveal a steep dust density profile. We find the wind-acceleration region of W\,Hya to extend to at least $\sim 7~R_\star$, in agreement with theoretical predictions of acceleration up to $\sim 12~R_\star$., Comment: 16 pages, 9 figures, 3 tables

We present and analyse SO and SO$_2$, recently observed with high angular resolution and sensitivity in a spectral line survey with ALMA, for two oxygen-rich AGB stars: the low mass-loss rate R Dor and high mass-loss rate IK Tau. We analyse 8 lines of SO detected towards both stars, 78 lines of SO$_2$detected towards R Dor and 52 lines of SO$_2$ detected towards IK Tau. We detect several lines of $^{34}$SO, $^{33}$SO and $^{34}$SO$_2$ towards both stars, and tentatively S$^{18}$O towards R Dor, and hence derive isotopic ratios for these species. The spatially resolved observations show us that the two sulphur oxides are co-located towards R Dor and trace out the same wind structures in the circumstellar envelope (CSE). Much of the emission is well reproduced with a Gaussian abundance distribution spatially centred on the star. Emission from the higher energy levels of SO and SO$_2$ towards R Dor provide evidence in support of a rotating inner region of gas identified in earlier work. The new observations allow us to refine the abundance distribution of SO in IK Tau derived from prior observations with single antennas, and confirm the distribution is shell-like with the peak in the fractional abundance not centred on the star. The confirmation of different types of SO abundance distributions will help fine-tune chemical models and allows for an additional method to discriminate between low and high mass-loss rates for oxygen-rich AGB stars., Comment: Accepted for publication in MNRAS

The late stellar evolutionary phases of low and intermediate-mass stars are strongly constrained by their mass-loss rates. The wind surrounding cool evolved stars frequently shows non-spherical features, thought to be due to an unseen companion orbiting the donor star. We study the morphology of the circumbinary envelope, in particular around oxygen-rich asymptotic giant branch (AGB) stars. We run a grid of 70 3D hydrodynamics simulations of a progressively accelerating wind propagating in the Roche potential formed by a mass-loosing evolved star in orbit with a main sequence companion. We resolve the flow structure both in the immediate vicinity of the secondary, where bow shocks, outflows and wind-captured disks form, and up to 40 orbital separations, where spiral arms, arcs and equatorial density enhancements develop. When the companion is deeply engulfed in the wind, the lower terminal wind speeds and more progressive wind acceleration around oxygen-rich AGB stars make them more prone than carbon-rich AGB stars to display more disturbed outflows, a disk-like structure around the companion and a wind concentrated in the orbital plane. In these configurations, a large fraction of the wind is captured by the companion which leads to a significant shrinking of the orbit over the mass-loss timescale, if the donor star is at least a few times more massive than its companion. Provided the companion has a mass of at least a tenth of the mass of the donor star, it can compress the wind in the orbital plane up to large distances. Our grid of models covers a wide scope of configurations function of the dust chemical content, the terminal wind speed relative to the orbital speed, the extension of the dust condensation region around the cool evolved star and the mass ratio. It provides a frame of reference to interpret high-resolution maps of the outflows surrounding cool evolved stars.

Asteroseismology offers the possibility of probing stellar interiors and testing evolutionary and seismic models. Precise photometry and spectroscopy obtained during multi-site campaigns on young open clusters allows discovering rich samples of pulsating stars and using them in a simultaneous seismic modelling called ensemble asteroseismology. The aim of this study is to obtain the age of the open cluster NGC 6910 by means of ensemble asteroseismology of the early-type pulsating members, to derive their stellar parameters, and to classify the excited modes. We used time-series analysis, performed photometric and spectroscopic mode identification, and calculated grids of evolutionary and seismic models to apply the procedure of ensemble asteroseismology for nine pulsating members of NGC 6910. With two iterations of the procedure of ensemble asteroseismology applied to nine pulsating stars we derived an age of 10.6$^{+0.9}_{-0.8}$ Myr for NGC 6910. Of the nine pulsating stars examined in the paper, eight are $\beta$ Cep stars, including three that are hybrid $\beta$ Cep and slowly pulsating B-type (SPB) pulsators, and one is an SPB star. Interestingly, the least massive $\beta$ Cep star, NGC 6910-38, has a mass of about 5.6 M$_\odot$. The present theory does not predict unstable $p$ modes in B-type stars with such a low mass. The $g$ modes with relatively high frequencies ($>3.5$ d$^{-1}$), observed in three members of the cluster, are also stable according to seismic modelling. Both findings pose a challenge for theoretical calculations and prompt a revision of the opacities. The procedure of ensemble asteroseismology was found to be successful for NGC 6910 and $\chi$ Per on the basis of pulsating B-type stars and can therefore be applied to other young open clusters that are rich in such stars., Comment: 22 pages, 24 figures. Caution: arXiv abstract is shorter than in the article

Chemical modelling of AGB outflows is typically focused on either non-thermodynamic equilibrium chemistry in the inner region or photon-driven chemistry in the outer region. We include, for the first time, a comprehensive dust-gas chemistry in our AGB outflow chemical kinetics model, including both dust-gas interactions and grain-surface chemistry. The dust is assumed to have formed in the inner region, and follows an interstellar-like dust-size distribution. Using radiative transfer modelling, we obtain dust temperature profiles for different dust types in an O-rich and a C-rich outflow. We calculate a grid of models, sampling different outflow densities, drift velocities between the dust and gas, and dust types. Dust-gas chemistry can significantly affect the gas-phase composition, depleting parent and daughter species and increasing the abundance of certain daughter species via grain-surface formation followed by desorption/sputtering. Its influence depends on four factors: outflow density, dust temperature, initial composition, and drift velocity. The largest effects are for higher density outflows with cold dust and O-rich parent species, as these species generally have a larger binding energy. At drift velocities larger than $\sim 10$ km s$^{-1}$, ice mantles undergo sputtering; however, they are not fully destroyed. Models with dust-gas chemistry can better reproduce the observed depletion of species in O-rich outflows. When including colder dust in the C-rich outflows and adjusting the binding energy of CS, the depletion in C-rich outflows is also better reproduced. To best interpret high-resolution molecular line observations from AGB outflows, dust-gas interactions are needed in chemical kinetics models., Comment: 20 pages, 14 figures, accepted for for publication in the Monthly Notices of the Royal Astronomical Society

Models of the chemical evolution of the interstellar medium, galaxies, and the Universe rely on our understanding of the amounts and chemical composition of the material returned by stars and supernovae. Stellar yields are obtained from stellar-evolution models, which currently lack predictive prescriptions of stellar mass loss, although it significantly affects stellar lifetimes, nucleosynthesis, and chemical ejecta. Galaxy properties are derived from observations of the integrated light of bright member stars. Stars in the late stages of their evolution are among the infrared-brightest objects in galaxies. An unrealistic treatment of the mass-loss process introduces significant uncertainties in galaxy properties derived from their integrated light. We describe current efforts and future needs and opportunities to characterize AGB outflows: driving mechanisms, outflow rates, underlying fundamental physical and chemical processes such as dust grain formation, and dependency of these on metallicity., Comment: Submitted to Astro2020 decadal survey; 8 pages, 2 figures

Red supergiant stars are surrounded by a gaseous and dusty circumstellar environment created by their mass loss which spreads heavy elements into the interstellar medium. The structure and the dynamics of this envelope are crucial to understand the processes driving the red supergiant mass loss and the shaping of the pre-supernova ejecta. We have observed the emission from the CO $J = 2-1$ line from the red supergiant star $\mu$~Cep with the NOEMA interferometer. In the line the synthesized beam was $0.92 \times 0.72$~arcsec ($590 \times 462$~au at 641~pc). The continuum map shows only the unresolved contribution of the free-free emission of the star chromosphere. The continuum-subtracted channel maps reveal a very inhomogeneous and clumpy circumstellar environment. In particular, we detected a bright CO clump, as bright as the central source in the line, at 1.80~arcsec south-west from the star, in the blue channel maps. After a deprojection of the radial velocity assuming two different constant wind velocities, the observations were modelled using the 3D radiative transfer code \textsc{lime} to derive the characteristics of the different structures. We determine that the gaseous clumps observed around $\mu$~Cep are responsible for a mass loss rate of $(4.9 \pm 1.0) \times 10^{-7}~{\rm M}_\odot\,{\rm yr}^{-1}$, in addition to a spatially unresolved wind component with an estimated mass-loss rate of $2.0 \times 10^{-6}~{\rm M}_\odot\,{\rm yr}^{-1}$. Therefore, the clumps have a significant role in $\mu$~Cep's mass loss ($\ge 25 \%$). We cannot exclude that the unresolved central outflow may be made of smaller unresolved clumps., Comment: 15 pages, 4 tables, 9 figures. 2nd version : one co-author removed and acknowledgement updated (consistent with erratum https://doi.org/10.1093/mnras/stz1006)

In 1981, the idea of a superwind that ends the life of cool giant stars was proposed. Extreme OH/IR-stars develop superwinds with the highest mass-loss rates known so far, up to a few 10^(-4) Msun/yr, informing our understanding of the maximum mass-loss rate achieved during the Asymptotic Giant Branch (AGB) phase. A condundrum arises whereby the observationally determined duration of the superwind phase is too short for these stars to become white dwarfs. Here, we report on the detection of spiral structures around two cornerstone extreme OH/IR-stars, OH26.5+0.6 and OH30.1-0.7, identifying them as wide binary systems. Hydrodynamical simulations show that the companion's gravitational attraction creates an equatorial density enhancement mimicking a short extreme superwind phase, thereby solving the decades-old conundrum. This discovery restricts the maximum mass-loss rate of AGB stars around the single-scattering radiation-pressure limit of a few 10^(-5) Msun/yr. This brings about crucial implications for nucleosynthetic yields, planet survival, and the wind-driving mechanism., Comment: Publication date: 25 February 2019 at 16:00h GMT

We aim to determine the distributions of molecular SiS and CS in the circumstellar envelopes of oxygen-rich asymptotic giant branch stars and how these distributions differ between stars that lose mass at different rates. In this study we analyse ALMA observations of SiS and CS emission lines for three oxygen-rich galactic AGB stars: IK Tau, with a moderately high mass-loss rate of $5\times10^{-6}$M$_\odot$ yr$^{-1}$, and W Hya and R Dor with low mass loss rates of $\sim1\times10^{-7}$M$_\odot$ yr$^{-1}$. These molecules are usually more abundant in carbon stars but the high sensitivity of ALMA allows us to detect their faint emission in the low mass-loss rate AGB stars. The high spatial resolution of ALMA also allows us to precisely determine the spatial distribution of these molecules in the circumstellar envelopes. We run radiative transfer models to calculate the molecular abundances and abundance distributions for each star. We find a spread of peak SiS abundances with $\sim10^{-8}$ for R Dor, $\sim10^{-7}$ for W Hya, and $\sim3\times10^{-6}$ for IK Tau relative to H$_2$. We find lower peak CS abundances of $\sim7\times10^{-9}$ for R Dor, $\sim7\times10^{-8}$ for W Hya and $\sim4\times10^{-7}$ for IK Tau, with some stratifications in the abundance distributions. For IK Tau we also calculate abundances for the detected isotopologues: C$^{34}$S, $^{29}$SiS, $^{30}$SiS, Si$^{33}$S, Si$^{34}$S, $^{29}$Si$^{34}$S, and $^{30}$Si$^{34}$S. Overall the isotopic ratios we derive for IK Tau suggest a lower metallicity than solar., Comment: 16 pages

Context: At the end of their lives AGB stars are prolific producers of dust and gas. The details of this mass-loss process are still not understood very well. Herschel PACS and SPIRE spectra offer a unique way of investigating properties of AGB stars in general and the mass-loss process in particular. Methods: The HIPE software with the latest calibration is used to process the available PACS and SPIRE spectra of 40 evolved stars. The spectra are convolved with the response curves of the PACS and SPIRE bolometers and compared to the fluxes measured in imaging data of these sources. Custom software is used to identify lines in the spectra, and to determine the central wavelengths and line intensities. Standard molecular line databases are used to associate the observed lines. Because of the limited spectral resolution of the spectrometers several known lines are typically potential counterparts to any observed line. To help identifications the relative contributions in line intensity of the potential counterpart lines are listed for three characteristic temperatures based on LTE calculations and assuming optically thin emission. Result: The following data products are released: the reduced spectra, the lines that are measured in the spectra with wavelength, intensity, potential identifications, and the continuum spectra, i.e. the full spectra with all identified lines removed. As simple examples of how this data can be used in future studies we have fitted the continuum spectra with three power laws and find that the few OH/IR stars seem to have significantly steeper slopes than the other oxygen- and carbon-rich objects in the sample. As another example we constructed rotational diagrams for CO and fitted a two-component model to derive rotational temperatures., Comment: A&A accepted

We surveyed 20 AGB stars of different chemical types using the APEX telescope, and combined this with an IRAM 30 m and APEX survey of CS and SiS emission towards over 30 S-type stars. For those stars with detections, we performed radiative transfer modelling to determine abundances and abundance distributions. We detect CS towards all the surveyed carbon stars, some S-type stars, and the highest mass-loss rate oxygen-rich stars ($> 5\times 10^{-6}$ Msol yr$^{-1}$). SiS is detected towards the highest mass-loss rate sources of all chemical types ($> 8\times 10^{-7}$ Msol yr$^{-1}$). We find CS peak fractional abundances ranging from ~ $ 4\times 10^{-7}$ to ~ $2\times 10^{-5}$ for the carbon stars, from ~ $ 3\times 10^{-8}$ to ~ $1\times 10^{-7}$ for the oxygen-rich stars and from ~ $ 1\times 10^{-7}$ to ~ $8\times 10^{-6}$ for the S-type stars. We find SiS peak fractional abundances ranging from ~ $ 9\times 10^{-6}$ to ~ $ 2\times 10^{-5}$ for the carbon stars, from ~ $ 5\times 10^{-7}$ to ~ $ 2\times 10^{-6}$ for the oxygen-rich stars, and from ~ $ 2\times 10^{-7}$ to ~ $ 2\times 10^{-6}$ for the S-type stars. We derived Si$^{32}$S/Si$^{34}$S = 11.4 for AI Vol, the only star for which we had a reliable isotopologue detection. Overall, we find that wind density plays an important role in determining the chemical composition of AGB CSEs. It is seen that for oxygen-rich AGB stars both CS and SiS are detected only in the highest density circumstellar envelopes and their abundances are generally lower than for carbon-rich AGB stars by around an order of magnitude. For carbon-rich and S-type stars SiS was also only detected in the highest density circumstellar envelopes, while CS was detected consistently in all surveyed carbon stars and sporadically among the S-type stars.

(abridged) In the inner regions of AGB outflows, several molecules have been detected with abundances much higher than those predicted from thermodynamic equilibrium (TE) chemical models. The presence of the majority of these species can be explained by shock-induced non-TE chemical models, where shocks caused by the pulsating star take the chemistry out of TE in the inner region. Moreover, a non-uniform density structure has been detected in several AGB outflows. A detailed parameter study on the quantitative effects of a non-homogeneous outflow has so far not been performed. We implement a porosity formalism for treating the increased leakage of light associated with radiation transport through a clumpy, porous medium. The effects from the altered UV radiation field penetration on the chemistry, accounting also for the increased reaction rates of two-body processes in the overdense clumps, are examined. We present a parameter study of the effect of clumping and porosity on the chemistry throughout the outflow. Both the higher density within the clumps and the increased UV radiation field penetration have an important impact on the chemistry, as they both alter the chemical pathways. The increased amount of UV radiation in the inner region leads to photodissociation of parent species, releasing the otherwise deficient elements. We find an increased abundance in the inner region of all species not expected to be present assuming TE chemistry, such as HCN in O-rich outflows, H$_2$O in C-rich outflows, and NH$_3$ in both. Outflows whose clumps have a large overdensity and that are very porous to the interstellar UV radiation field yield abundances comparable to those observed in O- and C-rich outflows for most of the unexpected species investigated. The inner wind abundances of H$_2$O in C-rich outflows and of NH$_3$ in O- and C-rich outflows are however underpredicted., Comment: 33 pages, 20 figures, 15 tables, accepted for publication in Astronomy & Astrophysics

Using ALMA, we observed the stellar wind of two oxygen-rich Asymptotic Giant Branch (AGB) stars, IK Tau and R Dor, between 335 and 362 GHz. One aim was to detect metal oxides and metal hydroxides (AlO, AlOH, FeO, MgO, MgOH), some of which are thought to be direct precursors of dust nucleation and growth. We report on the potential first detection of FeO (v=0, Omega=4, J=11-10) in RDor (mass-loss rate, Mdot, ~1e-7 Msun/yr). The presence of FeO in IK Tau (Mdot~5e-6 Msun/yr) cannot be confirmed due to a blend with 29SiS, a molecule that is absent in R Dor. The detection of AlO in R Dor and of AlOH in IK Tau was reported earlier by Decin et al. (2017). All other metal oxides and hydroxides, as well as MgS, remain undetected. We derive a column density N(FeO) of 1.1+/-0.9e15 cm^{-2} in R Dor, or a fractional abundance [FeO/H]~1.5e-8 accounting for non-LTE effects. The derived fractional abundance [FeO/H] is a factor ~20 larger than conventional gas-phase chemical kinetic predictions. This discrepancy may be partly accounted for by the role of vibrationally excited OH in oxidizing Fe, or may be evidence for other currently unrecognised chemical pathways producing FeO. Assuming a constant fractional abundance w.r.t. H_2, the upper limits for the other metals are [MgO/H_2] <5.5e-10 (R Dor) and <7e-11 (IK Tau), [MgOH/H_2] <9e-9 (R Dor) and <1e-9 (IK Tau), [CaO/H_2] <2.5e-9 (R Dor) and <1e-10 (IK Tau), [CaOH/H_2] <6.5e-9 (R Dor) and <9e-10 (IK Tau), and [MgS/H_$] <4.5e-10 (R Dor) and <6e-11 (IK Tau). The retrieved upper limit abundances for these latter molecules are in accord with the chemical model predictions., Comment: 14 pages (including appendices), accepted for publication by Astrophysical Journal

A spectral line and imaging survey of the low mass-loss rate AGB star R Dor (Mdot ~ 1e-7 Msun/yr) and the high mass-loss rate AGB star IK Tau (Mdot ~5e-6 Msun/yr) was made with ALMA between 335 and 362 GHz at a spatial resolution of ~150 mas, corresponding to the locus of the main dust formation region of both targets. Some 200 spectral features from 15 molecules (and their isotopologues) were observed, including rotational lines in both the ground and vibrationally excited states. 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_inf, previously derived for both objects (36 km/s versus v_inf ~17.7 km/s for IK Tau and 23 km/s versus v_inf ~5.5 km/s 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 ~3.7e-7 Msun for IK Tau and ~2e-8 Msun for R Dor., Comment: 39 pages, Astronomy & Astrophysics, in press

(abridged) 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. We have analysed molecular transitions of CO, SiO, and HCN measured with the APEX telescope and all three instruments on the Herschel Space Observatory, together with literature data. 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 have performed an analysis of the SiO and HCN molecular transitions in order to calculate their abundances. 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 outflow probed by our molecular data. For SiO, we find that the initial abundance lies between $5.5 \times 10^{-5}$ and $6.0 \times 10^{-5}$ w.r.t. H$_2$. The abundance profile is constant up to $60\ \pm 10\ R_*$, after which it declines following a Gaussian profile with an $e$-folding radius of $3.5 \pm 0.5 \times 10^{13}$ cm. For HCN, we find an initial abundance of $5.0 \times 10^{-7}$ w.r.t. H$_2$. The Gaussian profile that describes the decline starts at the stellar surface and has an $e$-folding radius $r_e$ of $1.85 \pm 0.05 \times 10^{15}$ cm. We cannot to unambiguously identify the mechanism by which SiO is destroyed at $60\ \pm 10\ R_*$. The initial abundances found are larger 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., Comment: 16 pages, 13 figures, 7 tables, accepted for publication in Astronomy & Astrophysics