Numerical simulations of stellar SiO maser variability. Investigation of the effect of shocks

TL;DR

This study combines stellar pulsation and SiO maser models to simulate the variability of SiO maser emissions in AGB star envelopes, focusing on shock effects and physical condition changes over the stellar cycle.

Contribution

It introduces a combined hydrodynamic and maser simulation approach to analyze SiO maser variability, emphasizing the role of shocks and physical condition changes.

Findings

Some variability aspects are inconsistent with shock-only collisional pumping.

Gas parcels with enhanced SiO are distributed and tracked over the stellar cycle.

Synthetic spectra and images are produced for different maser transitions.

Abstract

A stellar hydrodynamic pulsation model has been combined with a SiO maser model in an attempt to calculate the temporal variability of SiO maser emission in the circumstellar envelope (CE) of a model AGB star. This study investigates whether the variations in local physical conditions brought about by shocks are the predominant contributing factor to SiO maser variability because, in this work, the radiative part of the pump is constant. We find that some aspects of the variability are not consistent with a pump provided by shock-enhanced collisions alone. In these simulations, gas parcels of relatively enhanced SiO abundance are distributed in a model CE by a Monte Carlo method, at a single epoch of the stellar cycle. From this epoch on, Lagrangian motions of individual parcels are calculated according to the velocity fields encountered in the model CE during the stellar pulsation cycle. The potentially masing gas parcels therefore experience different densities and temperatures, and have varying line-of-sight velocity gradients throughout the stellar cycle, which may or may not be suitable to produce maser emission. At each epoch (separated by 16.6 days), emission lines from the parcels are combined to produce synthetic spectra and VLBI-type images. We report here the results for v=1, J=1-0 (43-GHz) and J=2-1 (86-GHz) masers.