2008ApJ...681.1574C

We report the first Very Long Baseline Array (VLBA) observations of v=1, J=1-0 SiO masers at 43 GHz in the circumstellar envelope of the M-type semiregular supergiant variable star AH Sco at two epochs separated by 12 days in 2004 March. These high-resolution VLBA images reveal that the distribution of the SiO masers is roughly on a persistent elliptical ring, with the lengths of the major and minor axes being about 18.5 and 15.8 mas, respectively, along a position angle of 150°. The redshifted masers are found to be slightly closer to the central star than are the blueshifted masers. The line-of-sight velocity structure of the SiO masers shows that with respect to the systemic velocity of -6.8 km/s, 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. A study of the proper motions of 59 matched features between our two epochs clearly indicates that the SiO maser shell around AH Sco was undergoing an overall contraction toward the star at a velocity of ~13 km/s at a distance of 2.26 kpc to AH Sco. Our three-dimensional maser kinematics model further suggests that such an inward motion is very likely to be due to the gravitation of the central star. The distance to AH Sco of 2.26±0.19 kpc that we obtained from our three-dimensional kinematics model fitting is consistent with its kinematic distance of 2.0 kpc.