Investigating Asymmetry Development from SiO to H2O Maser Regions in VX Sagittarii

Abstract

Simultaneous very-long-baseline interferometry monitoring observations of H2O and SiO masers toward VX Sagittarii were conducted from 2014 February to 2019 January. Thirty epochs of observations revealed that the H2O and SiO masers had asymmetric and ring-like structures, respectively. However, from 2017 September to 2018 March, the SiO maser transformed from a ring-like structure to a northeast–southwest (NE–SW) extension, and the 43.1 and 86.2 GHz SiO maser components had velocities of 39.48 and 10.65 km s−1 in the NE–SW direction, suggesting a possible localized strong shock wave. The H2O maser had a double-sided structure oriented in the NE–SW direction with near-stellar velocity components, which aligned with the extended direction of the SiO maser. The nonregular optical brightness and maser intensity variations were speculated to be related to the morphological evolution of the SiO maser. During the stable states attained by regular pulsations, the SiO maser region was presumed to experience radial acceleration, which reverted the SiO maser to a ring-like structure. However, the H2O maser region, where the acceleration almost terminates, retained its asymmetric morphology due to the prior influence of external forces. The results suggest that substantial energy transfer can alter the dynamics of the SiO maser and surrounding atmosphere, leading to an asymmetric distribution in the H2O maser region.