Astrometry of 6.7 GHz Methanol Maser toward W 3(OH) with Japanese VLBI Network

Abstract

Abstract We present the results of multiepoch VLBI observations for a 6.7 GHz methanol maser toward the UC H II region of W 3(OH) with Japanese VLBI Network (JVN). Based on phase-referencing VLBI astrometry, we derived the trigonometric annual parallax to be 0.598$\ \pm\ $0.067 mas, corresponding to a distance of 1.67$^{+0.21}_{-0.17}$ kpc. This is the first detection of parallax for a 6.7 GHz methanol maser with JVN, and demonstrates that JVN/VERA is capable of conducting VLBI astrometry for 6.7 GHz methanol maser sources within few kpc from the Sun. Based on in-beam mapping of the W 3(OH) methanol maser, we also measured the internal proper motions of its 6.7 GHz methanol maser for the first time. The internal proper motions basically show north–south expansion with a velocity of few km s$^{-1}$, being similar to OH masers. The spatial distribution and the internal proper motions of 6.7 GHz methanol masers suggest a rotating and expanding torus structure surrounding the UC H II region.