Atmospheric molecular blobs shape up circumstellar envelopes of AGB stars

The authors report observations of recently formed dust and molecular gas in the atmosphere of IRC+10°216 and interpret HCN, SiS and SiC_2 lines as large convective cells in the photosphere, as seen in Betelgeuse. During their thermally pulsing phase, asymptotic giant branch (AGB) stars eject material that forms extended dusty envelopes^ 1 . Visible polarimetric imaging found clumpy dust clouds within two stellar radii of several oxygen-rich stars^ 2 – 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 Mira^ 7 – 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 π^1 Gru^ 11 , 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 structures^ 15 . 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 SiC_2 appear at different radii and in different clumps, which we interpret as large convective cells in the photosphere, as seen in Betelgeuse^ 16 . The convective cells coalesce with pulsation, causing anisotropies that, together with companions^ 17 , 18 , shape its circumstellar envelope.