Astrometrically registered maps of H2O and SiO masers toward VX Sagittarii

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.

A Very Long Baseline Array Trigonometric Parallax for RR Aql and the Mira Period–Luminosity Relation

“…This has previously been noted for H 2 O maser emission (e.g., from W Hya by Reid & Menten 1991) and SiO maser emission (e.g., from TX Cam by Kemball & Diamond 1997). We fit the central position (assumed to be the stellar position) and the radius of a ring to the maser positions using a least-square method described by Yoon et al (2018), which assumes the distribution of SiO masers lie at a common radius from the central star. The fitted results are shown in Table 4.…”

Source-Frequency Phase-Referencing Observation of AGNs with KaVA Using Simultaneous Dual-Frequency Receiving

“…For an AGN jet, the frequency-dependent shift in the position of the VLBI core is known as the core-shift (Blandford & Königl 1979). KVN is doing regular astrometric observations with SFPR up to 130 GHz (e.g., Rioja et al 2014Rioja et al , 2015Dodson et al 2014Dodson et al , 2018Yoon et al 2018), and consistent results were found between KVN and the Very Long Baseline Array (VLBA) (Rioja et al 2014).…”

Precise radio astrometry and new developments for the next generation of instruments

“…This has enabled astrometry for asymptotic giant branch (AGB) stars at frequencies up to a factor of three higher than any previous work. First results from these observations are now being published (e.g., VX Sagittarii (Yoon et al 2018), R Crateris (Kim et al 2018) and V627 Cas (Yang et al 2019)). Thus, the unbiased measure of overlap or offset between masers at different transitions, ring sizes and temporal evolution provides a wealth of information to: i) provide the basis to investigate the spatial structures and dynamics from the SiO to H 2 O maser regions associated with a mass loss process and to trace the development of asymmetry in CSEs; ii) probe the astrochemistry of the matter thrown off into the ISM, as it is excited by the stellar pulsation of the host star; and iii) discriminate between competing proposed maser pumping mechanisms (i.e., radiative or collisional) and to deduce the physical conditions in the CSE.…”