Astrometrically registered maps of H2O and SiO masers toward VX Sagittarii

TL;DR

This study presents simultaneous, astrometrically registered maps of H2O and SiO masers around VX Sagittarii, revealing asymmetric outflows and the spatial distribution of masers, which enhances understanding of stellar wind structures.

Contribution

First simultaneous, astrometrically registered mapping of multiple maser transitions around VX Sagittarii, enabling detailed analysis of outflow asymmetry and maser spatial distribution.

Findings

H2O masers show asymmetric distribution above dust layers.

SiO masers form nearly circular rings indicating spherical wind.

Outermost 129.3 GHz maser size varies with optical phase, suggesting radiative pumping.

Abstract

The supergiant VX Sagittarii is a strong emitter of both H2O and SiO masers. However, previous VLBI observations have been performed separately, which makes it difficult to spatially trace the outward transfer of the material consecutively. Here we present the astrometrically registered, simultaneous maps of 22.2 GHz H2O and 43.1/42.8/86.2/129.3 GHz SiO masers toward VX Sagittarii. The H2O masers detected above the dust-forming layers have an asymmetric distribution. The multi-transition SiO masers are nearly circular ring, suggesting spherically symmetric wind within a few stellar radii. These results provide the clear evidence that the asymmetry in the outflow is enhanced after the smaller molecular gas clump transform into the inhomogeneous dust layers. The 129.3 GHz maser arises from the outermost region compared to that of 43.1/42.8/86.2 GHz SiO masers. The ring size of the 129.3 GHz maser is maximized around the optical maximum, suggesting that radiative pumping is dominant.