Abstract
We present the proper motions of water masers toward the Onsala 1 star-forming region, observed with the Japanese VLBI network at three epochs spanning 290 days. We found that there are two water-maser clusters (WMC 1 and WMC 2) separated from each other by 1.$^{\prime\prime}$6 (2900 AU at a distance of 1.8 kpc). A proper-motion measurement revealed that WMC1 is associated with a bipolar outflow elongated in the east–west direction with an expansion velocity of 69$\pm$11 km s$^{-1}$. WMC1 and WMC2 are associated with two 345 GHz continuum dust emission sources, which are located 2$^{\prime\prime}$ (3600 AU) east from the core of an ultracompact H ii region traced by 8.4 GHz radio continuum emission. This indicates that the star-formation activity of Onsala 1 could move from the west side of the ultracompact H ii region to the east side of two young stellar objects associated with the water masers. We have also found that the WMC1 and UC H ii regions could be gravitationally bound. Their relative velocity along the line of sight is $\sim$3 km s$^{-1}$, and the total mass is $\sim 37 M_\odot$. Onsala 1 seems to harbor a binary star at a different evolutionary stage.
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