SiO masers in TX Cam: Simultaneous VLBA observations of two 43 GHz masers at four epochs

TL;DR

This study uses simultaneous VLBA observations of SiO masers in TX Cam at four epochs to analyze their spatial distribution, dynamics, and relation to stellar cycle, revealing ring disruptions, overlapping maser regions, and filamentary structures.

Contribution

It provides the first simultaneous high-resolution imaging of v=1 and v=2 SiO masers in TX Cam across multiple epochs, comparing observations with theoretical models.

Findings

Maser rings are disrupted near stellar minimum and more regular near maximum.

The v=2 maser ring generally has a smaller radius than the v=1 ring.

Filamentary structures show velocity gradients consistent with decelerating outflows.

Abstract

We present the results of simultaneous high resolution observations of v=1 and v=2, J=1-0 SiO masers toward TX Cam at four epochs covering a stellar cycle. Near maser maximum (Epochs III and IV), the individual components of both masers are distributed in ring-like structures but the ring is severely disrupted near stellar maser minimum (Epochs I and II). In Epochs III and IV there is a large overlap between the radii at which the two maser transitions occur. However in both epochs the average radius of the v=2 maser ring is smaller than for the v=1 maser ring, the difference being larger for Epoch IV. The observed relative ring radii in the two transitions, and the trends on the ring thickness, are close to those predicted by the model of Humphreys et al. (\cite{humphreys02}). In many individual features there is an almost exact overlap in space and velocity of emission from the two transitions, arguing against pure radiative pumping. At both Epochs III and IV in many spectral features only 50% of the flux density is recovered in our images, implying significant smooth maser structure. For both transitions we find that red- and blue-shifted masers occur in all parts of the rings, with relatively few masers at the systemic velocity. Thus there is no evidence for rotation, although the blue-shifted masers are somewhat more prominent to the west. At all four epochs red-shifted components are generally brighter than blue-shifted ones. At Epochs III and IV, we see many filamentary or spoke-like features in both v=1 and v=2 masers, especially in the red-shifted gas. These spokes show systematic velocity gradients consistent with a decelerating outward flow with increasing radius. We outline a possible model to explain why, given the presence of these spokes, there is a deficit of maser features at the systemic velocity.