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The material lost through stellar winds of Asymptotic Giant Branch (AGB) stars is one of the main contributors to the chemical enrichment of galaxies. The general hypothesis of the mass loss mechanism of AGB winds is a combination of stellar pulsations and radiative pressure on dust grains, yet current models still suffer from limitations. Among others, they assume chemical equilibrium of the gas, which may not be justified due to rapid local dynamical changes in the wind. This is important as it is the chemical composition that regulates the thermal structure of the wind, the creation of dust grains in the wind, and ultimately the mass loss by the wind. Using a self-consistent hydrochemical model, we investigated how non-equilibrium chemistry affects the dynamics of the wind. This paper compares a hydrodynamical and a hydrochemical dust-free wind, with focus on the chemical heating and cooling processes. No sustainable wind arises in a purely hydrodynamical model with physically reasonable pulsations. Moreover, temperatures are too high for dust formation to happen, rendering radiative pressure on grains impossible. A hydrochemical wind is even harder to initiate due to efficient chemical cooling. However, temperatures are sufficiently low in dense regions for dust formation to take place. These regions occur close to the star, which is needed for radiation pressure on dust to sufficiently aid in creating a wind. Extending this model self-consistently with dust formation and evolution, and including radiation pressure, will help to understand the mass loss by AGB winds., Accepted for publication in MNRAS. 30 pages (incl. Appendix), 19 figures

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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 processes 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 $\mu$~Cep's mass loss ($\ge 25 \%$). We cannot exclude that the unresolved central outflow may be made of smaller unresolved clumps., Comment: 15 pages, 4 tables, 9 figures. 2nd version : one co-author removed and acknowledgement updated (consistent with erratum https://doi.org/10.1093/mnras/stz1006)