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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 new method can yield stringent constraints on the orbital parameters of long-period binaries containing AGB stars, and establishes a template for future studies.

Rate coefficients for the reaction of CN with CH2O were measured for the first time below room temperature in the range 32-103 K using a pulsed Laval nozzle apparatus together with the Pulsed Laser Photolysis-Laser-Induced Fluorescence technique. The rate coefficients exhibited a strong negative temperature dependence, reaching (4.62 ± 0.84) × 10-11 cm3 molecule-1 s-1 at 32 K, and no pressure dependence was observed at 70 K. The potential energy surface (PES) of the CN + CH2O reaction was calculated at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory, with the lowest energy channel to reaction characterized by the formation of a weakly-bound van der Waals complex, bound by 13.3 kJ mol-1, prior to two transition states with energies of -0.62 and 3.97 kJ mol-1, leading to the products HCN + HCO or HNC + HCO, respectively. For the formation of formyl cyanide, HCOCN, a large activation barrier of 32.9 kJ mol-1 was calculated. Reaction rate theory calculations were performed with the MESMER (Master Equation Solver for Multi Energy well Reactions) package on this PES to calculate rate coefficients. While this ab initio description provided good agreement with the low-temperature rate coefficients, it was not capable of describing the high-temperature experimental rate coefficients from the literature. However, increasing the energies and imaginary frequencies of both transition states allowed MESMER simulations of the rate coefficients to be in good agreement with data spanning 32-769 K. The mechanism for the reaction is the formation of a weakly-bound complex followed by quantum mechanical tunnelling through the small barrier to form HCN + HCO products. MESMER calculations showed that channel generating HNC is not important. MESMER simulated the rate coefficients from 4-1000 K which were used to recommend best-fit modified Arrhenius expressions for use in astrochemical modelling. The UMIST Rate12 (UDfa) model yielded no significant changes in the abundances of HCN, HNC, and HCO for a variety of environments upon inclusion of rate coefficients reported here. The main implication from this study is that the title reaction is not a primary formation route to the interstellar molecule formyl cyanide, HCOCN, as currently implemented in the KIDA astrochemical model. ispartof: PHYSICAL CHEMISTRY CHEMICAL PHYSICS vol:25 issue:11 pages:7719-7733 ispartof: location:England status: published

In recent years, many questions have arisen regarding the chemistry of photochemical products in the carbon-rich winds of evolved stars. To address them, it is imperative to constrain the distributions of such species through high angular resolution interferometric observations covering multiple rotational transitions. We used archival ALMA observations to map rotational lines involving high energy levels of cyanoacetylene (HC$_3$N) toward the inner envelope (radius, 17 pages, 9 figures, 2 tables. Accepted for publication in ApJ

Astrochemical models treat dust surfaces as ice covered. We investigate the effects of implementing increased bare dust binding energies of CO and S-bearing species on the chemistry in the outflows of asymptotic giant branch (AGB) stars. We demonstrate the potential for improving agreement with observations in the outflow of IK Tau.Increasing the binding energies to measured and computationally derived values in high mass-loss AGB outflows increased the production of daughter species. Switching from a high binding energy on bare dust to weaker binding to ice, the gas phase abundance increased at a radius in agreement with observations of IK Tau, suggesting that displacement of bound species could contribute to this observational puzzle. Using a strong binding to bare dust, a gas phase increase was not observed, however parent species concentrations had to be increased by around a factor of four to explain observed concentrations.

S-type AGB stars, with C/O ratios close to 1, are expected to have a mixed circumstellar chemistry as they transition from being oxygen-rich stars to carbon-rich stars. Recently, several different carbonaceous molecules, thought to be more characteristic of carbon stars, have been found in the circumstellar envelope of the S-type AGB star W Aql. We have obtained new high spatial resolution ALMA images of some of these molecules, specifically HC3N, SiC2 and SiC, and SiN, which we present here. We report diverse behaviour for these molecules, with SiC2 being seen with a symmetric spatial distribution around the star, SiN and SiC being asymmetrically distributed to the north-east of the star, and HC3N being seen in a broken shell to the south-west. These differing distributions point to complex dynamics in the circumstellar envelope of W Aql.

Evolved low- to intermediate-mass stars are known to shed their gaseous envelope into a large, dusty, molecule-rich circumstellar nebula which typically develops a high degree of structural complexity. Most of the large-scale, spatially correlated structures in the nebula are thought to originate from the interaction of the stellar wind with a companion. As part of the Atomium large programme, we observed the M-type asymptotic giant branch (AGB) star R Hydrae with ALMA. The morphology of the inner wind of R Hya, which has a known companion at ~3500 au, was determined from maps of CO and SiO obtained at high angular resolution. A map of the CO emission reveals a multi-layered structure consisting of a large elliptical feature at an angular scale of ~10'' that is oriented along the north-south axis. The wind morphology within the elliptical feature is dominated by two hollow bubbles. The bubbles are on opposite sides of the AGB star and lie along an axis with a position angle of ~115 deg. Both bubbles are offset from the central star, and their appearance in the SiO channel maps indicates that they might be shock waves travelling through the AGB wind. An estimate of the dynamical age of the bubbles yields an age of the order of 100 yr, which is in agreement with the previously proposed elapsed time since the star last underwent a thermal pulse. When the CO and SiO emission is examined on subarcsecond angular scales, there is evidence for an inclined, differentially rotating equatorial density enhancement, strongly suggesting the presence of a second nearby companion. The position angle of the major axis of this disc is ~70 deg in the plane of the sky. We tentatively estimate that a lower limit on the mass of the nearby companion is ~0.65 Msol on the basis of the highest measured speeds in the disc and the location of its inner rim at ~6 au from the AGB star., 21 pages, 23 figures

The nebular circumstellar environments of cool evolved stars are known to harbour a rich morphological complexity of gaseous structures on different length scales. A large part of these density structures are thought to be brought about by the interaction of the stellar wind with a close companion. The S-type asymptotic giant branch star Pi1 Gruis, which has a known companion at ~440 au and is thought to harbour a second, closer-by (, 19 pages, 20 figures

Rate coefficients have been measured for the reaction of CH radicals with formaldehyde, CH$_{2}$O, over the temperature range 31 - 133 K using a pulsed Laval nozzle apparatus combined with pulsed laser photolysis and laser induced fluorescence spectroscopy. The rate coefficients are very large and display a distinct decrease with decreasing temperature below 70 K, although classical collision rate theory fails to reproduce this temperature dependence. The measured rate coefficients have been parameterized and used as input for astrochemical models for both dark cloud and AGB stellar outflow scenarios. The models predict a distinct change (up to a factor of two) in the abundance of ketene, H$_{2}$CCO, which is the major expected molecular product of the CH + CH$_{2}$O reaction., 17 pages, 9 Figures, 5 Tables, Accepted for publication in The Astrophysical Journal

A spectral scan of the circumstellar environment of the asymptotic giant branch (AGB) star R~Doradus was taken with ALMA in cycle 2 at frequencies between 335 and 362 GHz and with a spatial resolution of $\sim$150 milliarcseconds. Many molecular lines show a spatial offset between the blue and red shifted emission in the innermost regions of the wind. The position-velocity diagrams of this feature, in combination with previous SPHERE data and theoretical work point towards the presence of a compact differentially rotating disk, orientated nearly edge-on. We model the $^{\rm 28}$SiO ($v=1,~J=8\to7$) emission with a disk model. We estimate the disk mass and angular momentum to be $3 \times 10^{-6}$ Solar masses and $5 \times 10^{40}\ {\rm m^2 kg/s}$. The latter presents an `angular momentum problem' that may be solved by assuming that the disk is the result of wind-companion interactions with a companion of at least 2.5 earth masses, located at 6 AU, the tentatively determined location of the disk's inner rim. An isolated clump of emission is detected to the south-east with a velocity that is high compared to the previously determined terminal velocity of the wind. Its position and mean velocity suggest that it may be associated with a companion planet, located at the disk's inner rim., Comment: 11 pages, 7 Figures

The modern era of highly sensitive telescopes is enabling the detection of more and more molecular species in various astronomical environments. Many of these are now being carefully examined for the first time. However, to move beyond detection to more detailed analysis such as radiative transfer modelling, certain molecular properties need to be properly measured and calculated. The importance of contributions from vibrationally excited states or collisional (de-)excitations can vary greatly, depending on the specific molecule and the environment being studied. Here, we discuss the present molecular data needs for detailed radiative transfer modelling of observations of molecular rotational transitions, primarily in the (sub-)millimetre and adjacent regimes, and with a focus on the stellar winds of AGB stars.