Proper Motions of Water Masers in W49 N Measured by KaVA

We report the proper motions of 22 GHz water masers toward W49 N that were observed by the KVN and VERA Array (KaVA) during 2017 February–May. We found 263, 268, and 310 features in three successive epochs; they were distributed in a region of size 4 × 4 arcsec ^2 . The strongest flux density was in the third epoch, and its averaged value was 18,090 Jy at V _LSR +0.47 km s ^−1 . For 102 H _2 O maser features, proper motion was detected across all three epochs. The average proper motions in R.A. and decl. offset were −0.352 and +0.890 mas yr ^−1 , respectively. The morphology of the distribution of the H _2 O maser features was found to be a bipolar outflow structure with an inclination angle of 37° ± 13° to the line of sight, and the features were expanding from a well-defined outflow center. A model of the source combining expansion and rotation yielded a distance to W49 of 11.12 ± 0.96 kpc that is consistent with the results from trigonometric parallax. A redshifted lobe was situated in the northeast direction and a blueshifted lobe in the southwest direction. We also discussed the location of the powerful flaring H _2 O maser feature at V _LSR = + 6 km s ^−1 and its possible mechanisms on the basis of spatial structures for the maser feature in VLBI maps observed with the KaVA, timed just before and during the rebrightening phase

Water masers map the complexity of mass loss from evolved stars

<p>Ten years of ALMA imaging of evolved star winds show that the rate, composition and dynamics of mass loss are changeable, inhomogenous and asymmetric. Water masers, in energy states from 200-6000 K,  trace the wind from the pulsation-dominated radio atmosphere, to acceleration through escape velocity, out to  regions where the molecules are photo-dissociated or freeze out. Dozens of transitions are accessible to ALMA, of which at least 7 have been imaged. Maser beaming allows the wind structure to be mapped from the radio atmosphere to hundreds of stellar radii (Rstar) at an order of magnitude higher resolution than is possible with thermal lines. Applying recently-developed models to the overlap or segregation of water (and OH and SiO)  maser transitions reveals the local density, temperature and other physical conditions on Rstar<i> </i>scales. Many water masers are concentrated in dense clumps whilst the less dense surrounding gas is traced by masers such as at 183-GHz and OH lines. These measurements provide input to constrain chemical and kinematic models of thermal lines and dust.</p><p>Only 22-GHz masers have been monitored long-term, showing variability linked to the stellar phase but too far from the star to be explained directly by  pulsation shocks, suggesting heating stimulates their collisional pumping. Large variations, sometimes localised, are  seen on other timescales, perhaps due to a companion, variable or episodic directed mass loss -- or simply cloud overlap.   High-resolution observations can reveal shocks, since shocked masers can appear almost as large as the emitting region, in contrast to the usual shrinking due to maser amplification. Shocks may explain the location of some high-energy transitions at tens Rstar, further than expected from the star.</p><p>We present studies of VY CMa (RSG) and R Hya (AGB). The former has been mapped in the most water transitions; the CSE of the latter bears the scars of a recent thermal pulse and suggested companion interaction.</p&gt

ATOMIUM: The astounding complexity of the near circumstellar environment of the M-type AGB star R Hydrae: I. Morpho-kinematical interpretation of CO and SiO emission

Evolved low- to intermediate-mass stars are known to shed their gaseous envelope into a large, dusty, molecule-rich circumstellar nebula which typically develops a high degree of structural complexity. Most of the large-scale, spatially correlated structures in the nebula are thought to originate from the interaction of the stellar wind with a companion. As part of the ATOMIUM large programme, we observed the M-type asymptotic giant branch (AGB) star R Hydrae with the Atacama Large Millimeter/submillimeter Array. The morphology of the inner wind of R Hya, which has a known companion at similar to 3500 au, was determined from maps of CO and SiO obtained at high angular resolution. A map of the CO emission reveals a multi-layered structure consisting of a large elliptical feature at an angular scale of similar to 10 '' that is oriented along the north-south axis. The wind morphology within the elliptical feature is dominated by two hollow bubbles. The bubbles are on opposite sides of the AGB star and lie along an axis with a position angle of similar to 115 degrees. Both bubbles are offset from the central star, and their appearance in the SiO channel maps indicates that they might be shock waves travelling through the AGB wind. An estimate of the dynamical age of the bubbles yields an age of the order of 100 yr, which is in agreement with the previously proposed elapsed time since the star last underwent a thermal pulse. When the CO and SiO emission is examined on subarcsecond angular scales, there is evidence for an inclined, differentially rotating equatorial density enhancement, strongly suggesting the presence of a second nearby companion. The position angle of the major axis of this disc is similar to 70 degrees in the plane of the sky. We tentatively estimate that a lower limit on the mass of the nearby companion is similar to 0.65 M-circle dot on the basis of the highest measured speeds in the disc and the location of its inner rim at similar to 6 au from the AGB star