2010ApJ...721..267A

We have conducted Very Long Baseline Array phase-referencing monitoring of H₂ O masers around the red supergiant, S Persei, for six years. We have fitted maser motions to a simple expanding-shell model with a common annual parallax and stellar proper motion, and obtained the annual parallax as 0.413 ±0.017 mas and the stellar proper motion as (-0.49±0.23 mas/yr, -1.19 ±0.20 mas/yr) in right ascension and declination, respectively. The obtained annual parallax corresponds to the trigonometric distance of 2.42^+0.11 _–0.09kpc. Assuming a Galactocentric distance of the Sun of 8.5 kpc, the circular rotational velocity of the local standard of rest at a distance of the Sun of 220 km/s, and a flat Galactic rotation curve, S Persei is suggested to have a non-circular motion deviating from the Galactic circular rotation for 15 km/s, which is mainly dominated by the anti-rotation direction component of 12.9±2.9 km/s. This red supergiant is thought to belong to the OB association, Per OB1, so that this non-circular motion is representative of a motion of the OB association in the Milky Way. This non-circular motion is somewhat larger than that explained by the standard density-wave theory for a spiral galaxy and is attributed to either a cluster shuffling of the OB association, or to non-linear interactions between non-stationary spiral arms and multi-phase interstellar media. The latter comes from a new view of a spiral arm formation in the Milky Way suggested by recent large N-body/smoothed particle hydrodynamics numerical simulations.