We report the first Very Long Baseline Array ( VLBA ) observations of 43 GHz v =1 , J =1–0 SiO masers in the circumstellar envelope of the M-type semi-regular supergiant variable star AH Sco at 2 epochs separated by 12 days in March 2004 .
These high-resolution VLBA images reveal that the distribution of SiO masers is roughly on a persistent elliptical ring with the lengths of the major and minor axes of about 18.5 and 15.8 mas , respectively , along a position angle of 150 ^ { \circ } .
And the red-shifted masers are found to be slightly closer to the central star than the blue-shifted masers .
The line-of-sight velocity structure of the SiO masers shows that with respect to the systemic velocity of -6.8 km s ^ { -1 } the higher velocity features are closer to the star , which can be well explained by the simple outflow or infall without rotation kinematics of SiO masers around AH Sco .
Study of proper motions of 59 matched features between two epochs clearly indicates that the SiO maser shell around AH Sco was undergoing an overall contraction to the star at a velocity of \approx 13 km s ^ { -1 } at a distance of 2.26 kpc to AH Sco .
Our 3-dimensional maser kinematics model further suggests that such an inward motion is very likely due to the gravitation of the central star .
The distance to AH Sco of 2.26 \pm 0.19 kpc obtained from the 3-dimensional kinematics model fitting is consistent with its kinematic distance of 2.0 kpc .