Distance and Proper Motion Measurement of the Red Supergiant, S Persei, With Vlbi H2o Maser Astrometry

We have conducted Very Long Baseline Array phase-referencing monitoring of H 2 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 , −1.19 ± 0.20 mas yr −1 ) in right ascension and declination, respectively. The obtained annual parallax corresponds to the trigonometric distance of 2.42 +0.11 −0.09 kpc. 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 −1 , 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 −1 , which is mainly dominated by the anti-rotation direction component of 12.9 ± 2.9 km s −1 . 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.