(Sub)stellar companions shape the winds of evolved stars

Disentangling physics and chemistry in AGB outflows: revealing degeneracies when adding complexity

Observations of the outflows of asymptotic giant branch (AGB) stars continue to reveal their chemical and dynamical complexity. Spherical asymmetries, such as spirals and disks, are prevalent and thought to be caused by binary interaction with a (sub)stellar companion. Furthermore, high density outflows show evidence of dust-gas interactions. The classical chemical model of these outflows - a gas-phase only, spherically symmetric chemical kinetics model - is hence not appropriate for the majority of observed outflows. We have included several physical and chemical advancements step-by-step: a porous density distribution, dust-gas chemistry, and internal UV photons originating from a close-by stellar companion. Now, we combine these layers of complexity into the most chemically and physically advanced chemical kinetics model of AGB outflows to date. By varying over all model parameters, we obtain a holistic view of the outflow's composition and how it (inter)depends on the different complexities. A stellar companion has the largest influence, especially when combined with a porous outflow. We compile sets of gas-phase molecules that trace the importance of dust-gas chemistry and allow us to infer the presence of a companion and porosity of the outflow. This shows that our new chemical model can be used to infer physical and chemical properties of specific outflows, as long as a suitable range of molecules is observed.

Atmospheric molecular blobs shape up circumstellar envelopes of AGB stars

During their thermally pulsing phase, asymptotic giant branch (AGB) stars eject material that forms extended dusty envelopes1. Visible polarimetric imaging found clumpy dust clouds within two stellar radii of several oxygen-rich stars2-6. Inhomogeneous molecular gas has also been observed in multiple emission lines within several stellar radii of different oxygen-rich stars, including W Hya and Mira7-10. At the stellar surface level, infrared images have shown intricate structures around the carbon semiregular variable R Scl and in the S-type star π1 Gru11,12. Infrared images have also shown clumpy dust structures within a few stellar radii of the prototypical carbon AGB star IRC+10°216 (refs. 13,14), and studies of molecular gas distribution beyond the dust formation zone have also shown complex circumstellar structures15. Because of the lack of sufficient spatial resolution, however, the distribution of molecular gas in the stellar atmosphere and the dust formation zone of AGB carbon stars is not known, nor is how it is subsequently expelled. Here we report observations with a resolution of one stellar radius of the recently formed dust and molecular gas in the atmosphere of IRC+10°216. Lines of HCN, SiS and SiC2 appear at different radii and in different clumps, which we interpret as large convective cells in the photosphere, as seen in Betelgeuse16. The convective cells coalesce with pulsation, causing anisotropies that, together with companions17,18, shape its circumstellar envelope.

DEATHSTAR—CO Envelope Size and Asymmetry of Nearby AGB Stars

Low- and intermediate-mass stars evolve into asymptotic giant branch (AGB) stars near the end of their lives, losing mass through slow and massive winds. The ejected material creates a chemically-rich expanding envelope around the star, namely the circumstellar envelope (CSE). Investigating the anisotropy of the mass-loss phenomenon on the AGB is crucial in gaining a better understanding of the shaping of the CSE during the transition from AGB star to planetary nebula (PN). We investigate possible signs of deviation from spherical symmetry in the CO-emitting CSEs of 70 AGB stars by analysing their emission maps in CO J=2−1 and 3−2 observed with the Atacama Compact Array, as part of the DEATHSTAR project. We find that about one third of the sources are likely aspherical, as they exhibit large-scale asymmetries that are unlikely to have been created by a smooth wind. Further high-resolution observations would be necessary to investigate the nature of, and the physical processes behind, these asymmetrical structures.

Lessons from the Ionised and Molecular Mass of Post-CE PNe

Close binary evolution is widely invoked to explain the formation of axisymmetric planetary nebulae after a brief common envelope phase. The evolution of the primary would be interrupted abruptly, its still quite massive envelope being fully ejected to form the PN, which should be more massive than a planetary nebula coming from the same star, were it single. We test this hypothesis by investigating the ionised and molecular masses of a sample consisting of 21 post-common-envelope planetary nebulae, roughly one-fifth of their known total population, and comparing them to a large sample of regular planetary nebulae (not known to host close-binaries). We find that post-common-envelope planetary nebulae arising from single-degenerate systems are, on average, neither more nor less massive than regular planetary nebulae, whereas post-common-envelope planetary nebulae arising from double-degenerate systems are considerably more massive and show substantially larger linear momenta and kinetic energy than the rest. The reconstruction of the common envelope of four objects further suggests that the mass of single-degenerate nebulae actually amounts to a very small fraction of the envelope of their progenitor stars. This leads to the uncomfortable question of where the rest of the envelope is, raising serious doubts on our understanding of these intriguing objects.

AGB Stars and Their Circumstellar Envelopes. I. the VULCAN Code

The interplay between AGB interiors and their outermost layers, where molecules and dust form, is a problem of high complexity. As a consequence, physical processes like mass loss, which depend on the chemistry of the circumstellar envelope, are often oversimplified. The best candidates to drive mass-loss in AGB stars are dust grains, which trap the outgoing radiation and drag the surrounding gas. Grains build up, however, is far from being completely understood. Our aim is to model both the physics and the chemistry of the cool expanding layers around AGB stars in order to characterize the on-going chemistry, from atoms to dust grains. This has been our rationale to develop ab initio VULCAN, a FORTRAN hydro code able to follow the propagation of shocks in the circumstellar envelopes of AGB stars. The version presented in this paper adopts a perfect gas law and a very simplified treatment of the radiative transfer effects and dust nucleation. In this paper, we present preliminary results obtained with our code.