An Improved Model of SiO Maser Emission in Miras

TL;DR

This paper presents an improved dynamic atmosphere and maser propagation model for SiO maser emission in Mira variables, aligning well with observations and highlighting dust effects and phase-dependent brightness.

Contribution

The model advances previous work by accurately simulating maser ring formation, IR radiation effects, and the size of the radio photosphere in Mira variables.

Findings

Maser rings form at radii consistent with VLBI observations.

A radio photosphere approximately twice the stellar radius explains observed maser site visibility.

Maser brightness peaks at optical phases 0.1 to 0.25.

Abstract

We describe a combined dynamic atmosphere and maser propagation model of SiO maser emission in Mira variables. This model rectifies many of the defects of an earlier model of this type, particularly in relation to the infra-red (IR) radiation field generated by dust and various wavelength-dependent, optically thick layers. Modelled masers form in rings with radii consistent with those found in VLBI observations and with earlier models. This agreement requires the adoption of a radio photosphere of radius approximately twice that of the stellar photosphere, in agreement with observations. A radio photosphere of this size renders invisible certain maser sites with high amplification at low radii, and conceals high-velocity shocks, which are absent in radio continuum observations. The SiO masers are brightest at an optical phase of 0.1 to 0.25, which is consistent with observed phase-lags. Dust can have both mild and profound effects on the maser emission. Maser rings, a shock and the optically thick layer in the SiO pumping band at 8.13\micron appear to be closely associated in three out of four phase samples.