2013MNRAS.436.1708R

This paper presents a milliarcsecond-scale comparison of the polarized component-level v = 1 J = 1-0, v = 2 J = 1-0 and v = 1 J = 2-1 SiO maser emission towards the supergiant star VY CMa. These observations used the VLBA at λ = 7mm and λ = 3mm over two epochs. The goal is to use the relative characteristics and spatial distribution of the transitions in individual resolved maser components to provide observational constraints on SiO maser excitation and pumping models. We find that many v = 1 J = 1-0 and v = 2 J = 1-0 features overlap in the second observing epoch, at comparable sensitivity. The v = 2 J = 1-0 emission is primarily located within several v = 1 J = 1-0 regions, and has a high degree of morphological similarity in the northeastern envelope, supportive of local collisional pumping. We find significantly higher spatial overlap between v = 1 J = 1-0 and J = 2-1 features than generally previously reported. However, the overlapping v = 1 J = 2-1 emission is usually weaker than predicted by hydrodynamical models, possibly due to low-density collisional pumping. The overall maser morphology contains several large-scale features that are persistent over multiple years and are likely associated with intrinsic physical circumstellar conditions. Our data cannot distinguish between competing near-circumstellar bulk kinematic models but do provide evidence for localized inhomogeneous mass-loss.