SiO Maser Observations toward Orion-KL with VERA

Abstract

Abstract We present results of phase-referencing VLBI observations of SiO masers in the Orion-KL region made with VERA. Using a strong maser spot in the 43GHz $v =$ 2 $J=$ 1-0 emission, we derived the trigonometric parallax of Orion-KL to be 2.39$\pm$0.03mas, corresponding to a distance of 418$\pm$6pc, with the highest accuracy among existing parallax measurements of the source. We made a superimposed image of $v =$ 1 $J=$ 1-0 and $v =$ 2 $J=$ 1-0 maser features in Orion-KL based on absolute positions obtained from the phase-referencing astrometry with a common reference source. The maser features of both transitions show similar X-shaped distributions centered at Source I. However, in each of the four arms of the X-shape, the SiO $v =$ 2 features tend to lie closer to Source I than the SiO $v =$ 1 features. The radial velocities of the maser emission decrease with the distance from Source I. The spatial and radial velocity distributions of the SiO masers suggest that the SiO masers lie in the rotating materials associated with a disk around Source I, rather than a decelerating outflow.