Proper Motions of Water Masers in W49 N Measured by KaVA

Abstract

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.

Keywords

• Astrophysical masers
• Water masers
• Star formation
• Interstellar medium
• Interstellar molecules
• Stellar jets