During the AGB, stars go through processes of strong mass-loss, caused by the pulsating levitation of the atmosphere to a region where the conditions are right for gas to condense into dust. The dust grains are then accelerated by radiation pressure from the central star, and due to the efficient coupling of the dust to the gas, drive a massive outflow which will become part of the interstellar medium, after forming a circumstellar envelope and a planetary nebula. How this dust forms and accelerates in O-rich stellar envelopes is still not well-known, including which molecules are involved in the formation of such dust grains.
In this work we examine the role of the SiO molecule in the dust-formation and mass-loss processes. Using the IRAM NOEMA interferometer we observed the ^29SiO and ^28SiO J=3-2 v=0 emission from the O-rich evolved stars IK Tau and IRC+10011. We then computed azimuthally averaged flux density profiles from the observations and performed fits using a well-tested radiative transfer and ray-tracing code for SiO thermal emission.
We observed circular symmetry in the very compact flux distribution. We also found that the source diameter almost doesn't vary with radial velocity, which is not the expected behavior for envelopes expanding at an almost constant velocity. The adopted density, velocity and abundance laws, together with the mass loss-rate, which best fit the observations, will give us information on the behavior of the SiO molecule and its role in the dust formation process. We conclude that gas acceleration is still present in the SiO-rich inner layers and suggest that there is a strong coupling between the depletion of silicon and the grain formation.