Atmospheric molecular blobs shape up circumstellar envelopes of AGB stars

TL;DR

This study uses high-resolution imaging to reveal how molecular blobs and convective cells in AGB stars influence the shaping of their circumstellar envelopes, providing new insights into stellar mass loss processes.

Contribution

It presents the first high-resolution observations of molecular gas and dust in the atmosphere of IRC+10216, linking convective cells to envelope shaping.

Findings

Different molecular lines appear at various radii and in distinct clumps.

Convective cells and pulsations cause anisotropies in the circumstellar environment.

Large convective cells influence the structure and evolution of the star's envelope.

Abstract

During their thermally pulsing phase, Asymptotic Giant Branch (AGB) stars eject material that forms extended dusty envelopes. Visible polarimetric imaging found clumpy dust clouds within two stellar radii of several oxygen-rich stars. Inhomogeneous molecular gas has also been observed in multiple emission lines within several stellar radii of different oxygen rich stars, including W Hya and Mira. At the stellar surface level, infrared images have revealed intricate structures around the carbon semi-regular variable R Scl and in the S-type star $\pi^{\mathrm{1}}$ Gru. Infrared images have also shown clumpy dust structures within a few stellar radii of the prototypical carbon AGB star IRC+10216, and studies of the molecular gas distribution beyond the dust formation zone have also shown complex circumstellar structures. 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+10216. Lines of HCN, SiS, and SiC$_2$ appear at different radii and in different clumps, which we interpret as large convective cells in the photosphere, as seen in Betelgeuse. The convective cells coalesce with pulsation causing anisotropies that, together with companions, shape its circumstellar envelope.