DISTANCE AND PROPER MOTION MEASUREMENT OF THE RED SUPERGIANT, S PERSEI, WITH VLBI H₂O MASER ASTROMETRY

Abstract

We have conducted Very Long Baseline Array phase-referencing monitoring of H2O 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(-0.09)(+0.11) 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.