H. Olofsson

Aims. We mapped the dust and molecular gas emission distributions in the vicinity (a few stellar radii) of the asymptotic giant branch (AGB) star EP Aquarii, which has a previously reported stellar companion at ∼50 au, to search for binary-induced asymmetries. Methods. We present an analysis of new high-angular-resolution (≳4 mas) images obtained with MATISSE (in the wavelength range ∼3–8 μm) as well as archival high-angular-resolution (≳20 mas) ALMA observations, including ¹² CO ( J = 2-1) line and continuum emission at ∼230 GHz. We describe the circumstellar morphology and kinematics of the molecular gas on these spatial scales. Results. The MATISSE images display a bright central source and two slightly extended emission features (‘blobs’) located on opposite sides of the AGB star (at radial distances of ∼4–10 au). These structures are consistent with previous mid-infrared interferometric observations, suggesting stability over decades. The central source is elongated along the direction defined by the blobs, in both the MATISSE and ALMA continuum images. The ALMA ¹² CO ( J = 2-1) maps display an asymmetric inner wind with a velocity gradient. Conclusions. The observations are consistent with an edge-on dusty equatorial overdensity (disc-like) and thus hint at the presence of a close (≲4 au) companion. The inferred symmetry axis is perpendicular to the CO velocity gradient and the orbital plane proposed for the distant (∼50 au) companion. EP Aquarii may represent the first misaligned hierarchical triple system with a central AGB star.

Stars of low and intermediate initial mass lose most of their stellar mass at the end of their lives during the asymptotic giant branch (AGB) phase. Determining their gas and dust mass-loss rates (MLRs) is crucial for quantifying the contribution of evolved stars to the life cycle of dust and gas in the Universe. The Atacama Large Millimeter/submillimeter Array was used to observe 38 carbon stars (C stars) in the large Magellanic cloud (LMC) and three C stars in the small Magellanic cloud (SMC) in the CO J= 2–1 line. Line profiles were fitted to derive stellar velocities and wind-expansion velocities ( V _exp ). CO emission is detected in two C stars in the SMC and 33 C stars in the LMC, extending earlier work from 2016 that detected CO emission from a total of four C stars observed in the LMC. This is the first detection of carbon monoxide around an AGB star in the SMC. One object in the LMC shows emission in ¹³ CO. The windexpansion velocity ranges from ∼7.5 to ∼30 km s ⁻¹ . Archival data were used to determine the pulsation periods as well as construct and model the spectral energy distributions using two dust radiative transfer codes. Mass-loss rates were independently derived from these two codes as well as from the intensity of the CO line, using a simple formula. On average, the dust-based MLRs higher than the MLRs based on the CO line by a factor of 1.6. Additional CO data in other transitions, combined with proper modelling, is required to further investigate this possible discrepancy. Mass-loss rates, pulsation periods, and expansion velocities were compared to a sample of Galactic C stars. There is a strong bias, as the Magellanic Cloud targets sample the highest MLRs and luminosities, yet they represent only a minority of stars in a Galactic sample. Comparing this sample with a similarly extreme set of Galactic stars with periods longer than 500 days, we identify no correlation between metallicity and either the MLR or V _exp .

Context. Post-asymptotic giant branch (post-AGB) binaries are surrounded by stable dusty discs. Despite extensive evidence for disc-binary interactions and complex morphologies, systematic studies of the inner disc rim morphology and dynamics are lacking. Aims. We image the detailed inner rim morphology for a sample of ‘full’ post-AGB circumbinary discs observed using near-infrared interferometry. At resolutions down to ∼1–2 mas – corresponding to ∼1–10 AU – we aim to reveal potential substructures that may trace perturbations from the binary or embedded substellar companions, or that arise from hydrodynamical instabilities. Methods. We developed a systematic image reconstruction workflow using a combination of the SPARCO and ORGANIC algorithms. This was applied to VLTI/PIONIER data of eight diverse post-AGB binaries obtained within the INSPIRING ESO large programme, providing high-fidelity images of dust continuum emission at the inner rim. Extensive tests were applied to assess the robustness of the recovered image features. Results. The dusty disc rim is well-resolved for all targets. Only one of the images can be fully accounted for by simple radiative transfer effects due to disc inclination, while in several others indications of potential substructures are detected. Strikingly, four exhibit robust azimuthal brightness enhancements at locations not expected from inclination effects alone. These can indicate strong radiative or dynamical responses to the binary, or vortices formed via hydrodynamical instabilities. One target – IW Car – displays an even more puzzling morphology, showing a single large-scale outer flux arc and several small-scale arcs closer to the binary, possibly revealing accretion streams onto the binary, a misaligned innermost disc, or a spiral feature located in the disc or in an outflow. Conclusions. This work presents the first homogeneous interferometric imaging survey of the inner regions of post-AGB circumbinary discs, enabling direct comparison of inner rim morphologies across a representative sample. The inner disc regions are highly diverse and dynamic, harbouring numerous candidate substructures. Multi-wavelength and time-series imaging will be essential in constraining the extent, motion and wavelength-dependence of these features and uncovering their physical origins.

Context . The circumstellar envelopes of AGB stars are sites of rich molecular chemistry. The present understanding of C-rich AGB chemistry largely relies on observations of the archetypal carbon star IRC+10 216. Current molecular abundance estimates for carbon stars are based either on single-dish spectra sampling a range of excitation conditions, or on interferometric mapping of a few lines. Aims . We aim to estimate the circumstellar abundances of SiO, SiS, and their most abundant isotopologues ( ²⁹ SiO, ³⁰ SiO, ²⁹ SiS, ³⁰ SiS, and Si ³⁴ S) for a sample of five carbon stars. This study compares molecular abundances across the sources, tests chemical modelling predictions, and examines whether IRC+10 216 is representative of the broader carbon star population. Methods . We derived molecular abundances using detailed 1D non-local thermodynamic equilibrium (non-LTE) radiative transfer (RT) modelling, constrained by both morphological and excitation information obtained from spatially resolved ALMA maps and single-dish observations. We further compared the derived abundances to chemical modelling results. Results . We obtain good fits to the SiO and SiS line profiles, and derived well-constrained abundance profiles and reliable isotopic ratios for all sources except AFGL 3068. While the SiS peak abundances are very similar across the sample (2.0 × 10 ⁻⁶ –4.7 × 10 ⁻⁶ ), we find that the SiO peak abundances of the rest of the stars are a factor of ~5 larger than that of IRC +10 216. The e -folding radii ( R _e ) are in the range 1.3 × 10 ¹⁶ − 7.0×10 ¹⁶ cm for SiO and 6.0 × 10 ¹⁵ − 1.0×10 ¹⁷ cm for SiS. The R _e increases with gas density for both SiO and SiS. Our RT models cannot simultaneously fit the low- and high- J SiO lines of IRC+10216. Chemical models reproduce the derived SiO abundance profiles well, while over-predicting the SiS R _e values. Conclusions . Our models highlight the necessity of having spatially resolved observations across a broad range of excitation conditions to robustly constrain molecular abundance profiles, while also making evident the limitations inherent in 1D RT modelling using simplified (circum)stellar models. We find that the currently assumed SiS photodissociation rate in chemical models is underestimated.

Context . Stars with masses between roughly 1 and 8 M _⊙ end their lives on the asymptotic giant branch (AGB), when intense mass loss takes place, with major consequences for the chemical evolution of the universe. The mechanism responsible for the outflows is generally accepted to be radiation pressure acting on dust grains that form in the dense extended atmospheres of AGB stars. Dust formation is enabled, or at least dramatically enhanced, by the action of convection and stellar pulsations. The complex physics underlying convection, stellar pulsations, and dust nucleation precludes predicting AGB mass loss from first principles. Aims . Our aim was to characterize the recent mass ejections of the AGB star Mira A using observations of the inner envelope. In particular, we studied two lobes observed to be expanding away from Mira A to obtain empirical insights into the mass-ejection process. Methods . We investigated the evolution of the lobes using images of polarized light obtained at six epochs using the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) on the Very Large Telescope and of molecular emission at two epochs obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). Six lines of SO ₂ were used to investigate the excitation temperature and column density of SO ₂ in the lobes. We used the ¹³ CO J = 3-2 line and radiative transfer models to constrain the column density of the gas, which allowed us to infer the abundances of SO, SO ₂ , AlO, AlF, and PO in the lobes. Results . While dust seems confined almost exclusively to the edges of the lobes, gas fills the lobes and displays higher densities than expected at the observed radial distances based on the large-scale mass-loss rate of Mira A, with a total gas mass in the lobes of ∼2 × 10 ⁻⁵ M _⊙ . We find the expansion of the lobes to be consistent with both a constant velocity (ejection time in 2010 or 2011) or a decelerating expansion (ejection time in 2012). If ejection events with a similar magnitude happen periodically, we derive periods between 50 and 200 years to account for the mass-loss rate of Mira A. This periodicity is very uncertain because of the complexity of the circumstellar environment that hampers accurate determinations of the mass-loss rate. We find abundances in the lobes of ∼1.5 × 10 ⁻⁶ and ∼2.5 × 10 ⁻⁶ for SO and SO ₂ , respectively, when accounting for radiative transfer effects and of 2 × 10 ⁻¹⁰ , 6.5 × 10 ⁻¹⁰ , and 4 × 10 ⁻⁷ for AlO, AlF, and PO assuming LTE and optically thin emission. The strong variation in brightness of the different features identified in the polarized-light images is puzzling. We suggest that an asymmetric stellar radiation field preferentially illuminates specific regions of the circumstellar envelope at a given time, producing a lighthouse-like effect.

Aims. We investigate how the presence of a binary companion appears to affect dust and molecule formation in the circumstellar environment of a star on the asymptotic giant branch (AGB). Methods.L - and N -band observations obtained over the course of one month with the multi aperTure mid-infrared spectroScopic experiment instrument (MATISSE) at the very large telescope interferometer (VLTI) were used to constrain the distribution of dust and molecules in the close environment of π ¹ Gru through image reconstruction. The reconstructed images were fit to the interferometric observables, that is, the visibilities and closure phases, using the two Python-based tools Python for MiRA ( PYRA ) and mean astrophysical images with PYRA ( MYTHRA ) built around the multi-aperture image reconstruction algorithm ( MiRA ). Results. Our observations support (i) a wind Roche-lobe overflow (WRLOF) scenario, where mass transfer from the AGB star to its companion produces a possible circumcompanion disk in the L band around π ¹ Gru C, with a central cavity that likely traces dust sublimation and a circumcompanion envelope in the N band. (ii) A main-sequence nature for the companion: Because both Atacama large millimeter/submillimeter array (ALMA) and the VLTI/MATISSE N -band observations show emission, we favor a thermal infrared emission from the main-sequence star over free-free emission from a with dwarf companion. Finally, (iii) a plume-like structure extending from π ¹ Gru C, likely marking the onset of the spiral observed at larger scales. Together, this provides direct evidence that links small-scale mass transfer and disk formation through WRLOF to the global circumstellar morphology. Conclusions. These results highlight the atmospheric deformation induced by the companion, reveal ongoing mass transfer between the evolved star and its companion, and indicate a circumcompanion disk-like structure.

Aims . The goal of this study is to further the understanding of the wind formation mechanism in asymptotic giant branch (AGB) stars through the analysis of the close environment (within a few stellar radii) of the carbon star X TrA. Methods . X TrA was observed for the first time with the Mid-Infrared SpectroScopic Experiment instrument (MATISSE) in the L and N bands in low spectral resolution mode (R=30), and its close surroundings were mapped in specific wavelength ranges corresponding to specific molecules (C ₂ H ₂ and HCN, at 3.1 and 3.8 μm) and dust (amorphous carbon and, for example, SiC at 11.3 μm), via image reconstruction techniques. Results . The angular diameter of the star ranges from 10 mas in the L band pseudo-continuum (3.5 μm) to 20 mas at 3.1 and 11.3 μm. The reconstructed images show some mild elongated features (along the east-west direction) and asymmetric protrusions, which are most evident around 3.1 μm. Imaging results highlight the clumpy nature of the circumstellar environment, starting from the photospheric region up to more distant layers. Conclusions . The angular diameters found for X TrA in the image data are in agreement with previous photospheric diameter estimates (following VLTI/MIDI 8–13 μm observations), and their wavelength dependence is similar to values found for other carbon stars observed with MATISSE (R Scl and V Hya). The 3.1 μm images presented here show highly asymmetric features, another case of a C-rich star with irregular morphologies close to the stellar disk; this supports the notion that the C ₂ H ₂ + HCN abundance distribution usually originates from a clumpy layer around carbon stars.

Context. The circumstellar envelopes (CSEs) of asymptotic giant branch (AGB) stars harbour a rich variety of molecules and are sites of complex chemistry. Our current understanding of the circumstellar chemical processes of carbon-rich AGB stars is predominantly based on observations of a single star, IRC +10 216, often regarded as an archetypical carbon star. Aims. We aim to estimate stellar and circumstellar properties for five carbon stars, and constrain their circumstellar CS abundances. This study compares the CS abundances among the sources, informs circumstellar chemical models, and helps to assess if IRC+10 216 is a good representative of the physics and chemistry of carbon star CSEs. Methods. We modelled the spectral energy distributions (SEDs) and CO line emission to derive the stellar and outflow properties. Using these, we then retrieved CS abundance profiles with detailed radiative transfer modelling, imposing spatial and excitation constraints from ALMA and single-dish observations. Results. We obtain good fits to the SEDs and CO lines for all sources and reproduce the CS line emission across various transitions and apertures, yielding robust estimates of the CS abundance profiles. Peak CS fractional abundances range from 1×10⁻⁶−4×10⁻⁶, with e-folding radii of 1.8×10¹⁶−6.8×10¹⁶ cm. We also derive reliable ¹²C/¹³C and ³²S/³⁴S ratios from CS isotopologue modelling. Conclusions. Our results refine previous single-dish CS abundance estimates and improve the relative uncertainty on the CS e-folding radius for IRAS 07454-7112 by a factor of ~2.5. Chemical models reproduce our estimates of the CS radial extent, corroborating the CS photodissociation framework used therein. We find no significant differences between the derived CS abundance profiles for IRC +10 216 and the rest of the sample, apart from the expected density-driven variations.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Context. Low- and intermediate-mass stars evolve through the asymptotic giant branch (AGB) when an efficient mass-loss process removes a significant fraction of their initial mass. For most sources, this mass-loss process relies on the interplay between convection, stellar pulsations, and dust formation. However, predicting the mass-loss history of a given star from first principles is complex and not yet feasible at present. At the end of the AGB, at least some stars experience a substantial increase in their mass-loss rate for unknown reasons, leading to the creation of post-AGB objects that are completely enshrouded in thick dusty envelopes. Recent studies have suggested that some of these sources may be the product of interactions between an evolved star with a close companion. Aims. We observed six obscured post-AGB stars (four C-rich and two O-rich sources) to constrain the properties of their circumstellar envelopes, recent mass-loss histories, and initial masses of the central stars. Methods. We used observations of the J = 2 − 1 line of ¹³CO, C¹⁷O, and C¹⁸O with the Atacama Large Millimeter/submillimeter Array (ALMA) to determine the circumstellar gas masses and the ¹⁷O/¹⁸O isotopic ratios, the latter of which can be used to infer initial stellar masses. These results were interpreted in the context of comparisons with stellar evolution models in the literature and existing observations of other post-AGB stars. Results. Based on the inferred ¹⁷O/¹⁸O isotopic ratios, we find all stars to have relatively low initial masses (< 2 M_⊙), contrary to literature indications of higher masses for some sources. One of the C-rich sources, HD 187885, has a low ¹⁷O/¹⁸O ratio; coupled with a low metallicity, this would imply a relatively low mass (∼1.15 M_⊙) for a carbon star. For all but one source (GLMP 950), we observe kinematic components with velocities of ≳30 km s⁻¹, which are higher than typical AGB wind expansion velocities. For most sources, these higher velocity outflows display point-symmetric morphologies. The case of Hen 3-1475 is especially spectacular, with the high-velocity molecular outflow appearing to be interleaved with the high-velocity outflow of ionised gas observed at optical wavelengths. Based on the size of the emission regions of the slow components of the outflows, we derived typical kinematic ages associated with the C¹⁸O J = 2 − 1 emission of ≲1500 years and obtained relatively high associated mass-loss rates (≳10⁻⁴ M_⊙ yr⁻¹). The sources with known spectral types are found to have evolved faster than expected, compared to stellar evolutionary models.