VLBI Observations of Water Masers in Onsala 1: Massive Binary Star Forming Site?

TL;DR

This study uses VLBI observations to analyze water masers in Onsala 1, revealing two maser clusters associated with star formation activity, a bipolar outflow, and evidence suggesting a binary star system at different evolutionary stages.

Contribution

First detailed VLBI proper motion measurements of water masers in Onsala 1, uncovering a potential binary star system and star formation activity moving across the region.

Findings

Two water maser clusters separated by 2900 AU.

WMC1 associated with bipolar outflow at 69 km/s.

Evidence of a gravitationally bound binary star system.

Abstract

We present 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 (WMC1 and WMC2) separated from each other by 1".6 (2900 AU at a distance of 1.8 kpc). The proper motion measurement reveals that WMC1 is associated with a bipolar outflow elongated in the east-west direction with an expansion velocity of 69+-11 km/s. WMC1 and WMC2 are associated with two 345 GHz continuum dust emission sources, and located 2" (3600 AU) east from the core of an ultracompact HII region traced by 8.4 GHz radio continuum emission. This indicates that star formation activity of Onsala 1 could move from the west side of ultracompact HII region to the east side of two young stellar objects associated with the water masers. We also find that WMC1 and UC HII region could be gravitationally bound. Their relative velocity along the line of sight is 3 km/s, and total mass is 37 Mo. Onsala 1 seems to harbor a binary star at different evolutionary stage.