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) cou
ld 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.
$eta$~Car is one of the most massive stars in the Galaxy. It underwent a massive eruption in the 19th century, which produced the impressive bipolar Homunculus nebula now surrounding it. The central star is an eccentric binary with a period of 5.54,y
ears. Although the companion has not been detected directly, it causes time-variable ionization and colliding-wind X-ray emission. By characterizing the complex structure and kinematics of the ejecta close to the star, we aim to constrain past and present mass loss of $eta$~Car. $eta$~Car is observed with the extreme adaptive optics instrument SPHERE at the Very Large Telescope, using its polarimetric mode in the optical with the ZIMPOL camera. A spatial resolution of 20,mas was achieved, i.e. very close to the presumed 13 mas apastron separation of the companion star. We detect new structures within the inner arcsecond to the star (2,300,au at a 2.3,kpc distance). We can relate these structures to the eruption near 1890 by tracking their proper motions derived from our new images and historical images over a 30,years time span. Besides, we find a fan-shaped structure in the inner 200~au to the star in the H$alpha$ line, that could potentially be associated with the wind collision zone of the two stars.
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 p
rocesses 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 $mu$~Ceps mass loss ($ge 25 %$). We cannot exclude that the unresolved central outflow may be made of smaller unresolved clumps.
Context. Red giant branch (RGB) stars are very bright objects in galaxies and are often used as standard candles. Interferometry is the ideal tool to characterize the dynamics and morphology of their atmospheres. Aims. We aim at precisely characteris
ing the surface dynamics of a sample of RGB stars. Methods. We obtained interferometric observations for three RGB stars with the MIRC instrument mounted at the CHARA interfer- ometer. We looked for asymmetries on the stellar surfaces using limb-darkening models. Results. We measured the apparent diameters of HD197989 (Epsilon Cyg) = 4.61+-0.02 mas, HD189276 (HR7633) = 2.95+-0.01 mas, and HD161096 (Beta Oph) = 4.43+-0.01 mas. We detected departures from the centrosymmetric case for all three stars with the tendency of a greater effect for lower logg of the sample. We explored the causes of this signal and conclude that a possible explanation to the interferometric signal is the convection-related and/or the magnetic-related surface activity. However, it is necessary to monitor these stars with new observations, possibly coupled with spectroscopy, in order to firmly establish the cause.
