Interferometric observations of SiO thermal emission in the inner wind of M-type AGB stars IK Tauri and IRC+10011^★,★★

¹ Molecular Astrophysics Group, Instituto de Física Fundamental (IFF-CSIC), C/Serrano 123, 28006 Madrid, Spain
e-mail: jl.verbena@csic.es
² Observatorio Astronómico Nacional (OAN-IGN), C/Alfonso XII 3, 28014 Madrid, Spain
³ Observatorio Astronómico Nacional (OAN-IGN), Apdo 112, 28803 Alcalá de Henares, Madrid, Spain
⁴ Instituto Geográfico Nacional, Centro de Desarrollos Tecnológicos, Observatorio de Yebes, Apdo 148, 19080 Yebes, Spain
⁵ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d’Hères, France

Received: 14 December 2018
Accepted: 25 February 2019

Abstract

Context. Asymptotic giant branch (AGB) stars go through a process of strong mass loss that involves pulsations of the atmosphere, which extends to a region in which the conditions are adequate for dust grains to form. Radiation pressure acts on these grains which, coupled to the gas, drive a massive outflow. The details of this process are not clear, including which molecules are involved in the condensation of dust grains.

Aims. We seek to study the role of the SiO molecule in the process of dust formation and mass loss in M-type AGB stars.

Methods. Using the IRAM NOEMA interferometer we observed the ²⁸SiO and ²⁹SiO J = 3−2, v = 0 emission from the inner circumstellar envelope of the evolved stars IK Tau and IRC+10011. We computed azimuthally averaged emission profiles to compare the observations to models using a molecular excitation and ray-tracing code for SiO thermal emission.

Results. We observe circular symmetry in the emission distribution. We also find that the source diameter varies only marginally with radial velocity, which is not the expected behaviour 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, give us information concerning the chemical behaviour of the SiO molecule and its role in the dust formation process.

Conclusions. The results indicate that there is a strong coupling between the depletion of gas-phase SiO and gas acceleration in the inner envelope. This could be explained by the condensation of SiO into dust grains.