ALMA observations and 3D modelling of mm-wavelength SiO masers in oxygen-rich evolved stars

Abstract

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. In this thesis, we present results from ALMA Band 6 observations (213--270 GHz) of mm-wavelength (i.e. high-rotational transition number J; J=5-4, 6-5) 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 targets exhibit high-J 28SiO, 29SiO or 30SiO masers, with 28SiO v=1 J=5-4 being the most abundant line. 2D Gaussian maser component fitting of the ALMA extended configuration data revealed that the distribution of high-J SiO maser components does not usually follow the well-studied ring structure seen in the low-J 43 (v=0 J=1-0) and 86 (v= 1 J=1-0) GHz SiO masers but instead trace more complex irregularities in the inner wind e.g. elongated structures, streamers, infalls, velocity-position gradients and partial ring structures. Although a correlation between mass-loss rates and flux-weighted angular separations from the star seems likely based on linear regression models, it is impossible to rule out the possibility of the correlation existing by chance without multi-epoch observations. We also present the improvements made to the 3D modelling code for VLBI-scale maser flares (maser3D) and its first results for masers in AGB stars. The modifications provide the user with more control over stellar radii, wind velocity profiles, number densities, maser cloud geometry and orientation, as well as faster run-time via parallelisation. Three increasingly complicated maser models in AGB stars were constructed and tested. The simulated light curves and VLBI images show that either collisional or radiative pumping is dominant depending on the viewpoint and a random observer is likely to detect an unremarkable blue- or red-shifted maser. We also tested a first 3D model of high-J SiO masers towards $\pi^1$ Gru based on SPH simulation and ATOMIUM observations. The initial comparison between the model and ALMA observations is satisfactory, given the lack of accurate temperature and chemistry modelling inputs.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Albert Zijlstra (Supervisor) & Anita Richards (Supervisor)