Atmospheric molecular blobs shape up circumstellar envelopes of AGB stars

Abstract

The manuscript presents high angular resolution millimeter imaging of the well-studied carbon-rich AGB star IRC+10216 with ALMA. Thanks to the proximity of the target and the high angular resolution of ALMA, the data show inhomogeneous, clumpy structures in the innermost circumstellar envelope. Observational studies of where and how dust forms are crucial for clarifying the mass-loss mechanism in AGB stars. The result presented in this manuscript that dust formation and the atmospheric structure are intrinsically clumpy is therefore important for the solution of this long-standing problem. Furthermore, the images obtained in different molecular lines reveal distinct morphology, which the authors interpret in terms of inhomogeneities in density and temperature. These data are valuable for further understanding chemistry in the atmosphere shocked by pulsation and/or convection.However, although the authors claim that their results are the first to spatially resolve anisotropic formation of dust and molecule in the atmosphere of AGB stars, non-spherical structures in the atmosphere and dust formation very close to the star have been detected in the recent years. For example, visible polarimetric imaging has revealed clumpy dust clouds very close to the star, within 2 stellar radii, with spatial resolutions comparable to or better than the present paper (

Chemical tracers of a highly eccentric AGB–main-sequence star binary

“…These broadly agree with 3D hydrodynamical simulations of AGB atmospheres, which predict the formation of large convective shells in the low-gravity environment of the AGB star's extended atmosphere, resulting in a clumpy and non-spherical atmospheric structure and asymmetric dust formation [117,118,119]. Similar asymmetric features have also been observed in the millimetre range with ALMA, in both the continuum emission and for molecular lines that originate in, or close to, the stellar atmosphere [120,121,122,123]. A recent study of the nearby carbon star CW Leo determined that the asymmetries in the stellar atmosphere and inner wind are unlikely to have been formed as a result of binary interactions, but rather as a result of varying temperature and density conditions caused by convection cells [123].…”

“…Similar asymmetric features have also been observed in the millimetre range with ALMA, in both the continuum emission and for molecular lines that originate in, or close to, the stellar atmosphere [120,121,122,123]. A recent study of the nearby carbon star CW Leo determined that the asymmetries in the stellar atmosphere and inner wind are unlikely to have been formed as a result of binary interactions, but rather as a result of varying temperature and density conditions caused by convection cells [123]. This is despite the larger-scale shells observed around this star being thought to have formed as a result of binary interactions [71].…”

“…Similarly, the PV diagram is not perfectly symmetric across the LSR velocity axis and shows clumps of brighter emission. These clumpy asymmetries are more likely to be caused by variations in density and temperature driven by chaotic motions of convection cells, similar to the clumpy emission seen in the inner wind of CW Leo [123]. It is also possible that some of the enhancements were caused by the interaction between the shock created by the companion's passage and the pulsation of the AGB star, as modelled in the simulations of Aydi et al [125].…”