High-resolution VLBA observations of three 7mm SiO masers toward VXSgr at five epochs

Abstract

VXSgr is a red supergiant at an adopted distance of 1.6kpc with intense 43GHz SiO maser emission. In this paper, we present the high-resolution very long baseline interferometry (VLBI) observations of SiO masers toward VXSgr at five epochs. We used the Very Long Baseline Array to map the J = 1→0 (v = 1, 2) 28SiO masers and confirmed a ring-like structure. In the first two epochs, the v = 1masers form a ring, but v = 2maser spots residing only in the southern and northern regions do not form a complete ring. In the third epoch, the two masers are distributed in a ring structure and the v = 2masers are a bit closer to the central star. In the last two epochs, many new maser spots appear and overlap each other. These overlapping maser spots can be related to the shock waves and reflect the collisional pumping. We compare the observations with the pumping models and speculate that the real pumping mechanism may be complex in VXSgr and vary with time. The J = 1→0 (v = 0) 29SiO line emission is also detected, but is too weak to produce any VLBI map. © 2012. The American Astronomical Society. All rights reserved..