Probing the inner winds of oxygen-rich evolved stars with ALMA observations of mm-wavelength SiO masers

SiO masers are useful tracers for studying the innermost regions of circumstellar envelopes (CSEs) of evolved stars where convection and pulsation shocks are damped and dust formation commences. We present results from ALMA Band 6 observations (213-270 GHz) of mm-wavelength (i.e. high-rotational transition number J) SiO masers towards 17 oxygen-rich AGB and red supergiant stars, carried out as part of the ATOMIUM Large Programme (PI: L. Decin) in 2018-2020. We found that most of the AGB targets exhibit high-J 28SiO, 29SiO, or 30SiO masers, with 28SiO v=1 J=5-4 being the most abundant line. 2D Gaussian component fitting of the ALMA extended-configuration data revealed that the distribution of high-J SiO maser components does not usually follow the ring structure seen in the best-studied 43 (v=0 J=1-0) and 86 (v=1 J=1-0) GHz SiO masers. Instead, they trace more complex irregularities in the inner wind e.g. streamers, infalls, and velocity gradients. A correlation between mass-loss rates and flux-weighted mean sizes of maser shells is suggested by linear regression models, but we cannot rule out the possibility of the correlation existing by chance without considering the contributions from the line-of-sight direction.