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

Abstract

We have conducted VLBA phase-referencing monitoring of H2O masers around the\nred supergiant, S Persei, for six years. We have fitted maser motions to a\nsimple expanding-shell model with a common annual parallax and stellar proper\nmotion, and obtained the annual parallax as 0.413+/-0.017 mas, and the stellar\nproper motion as (-0.49+/-0.23 mas/yr, -1.19+/-0.20 mas/yr) in right ascension\nand declination, respectively. The obtained annual parallax corresponds to the\ntrigonometric distance of 2.42+0.11-0.09 kpc. Assuming the Galactocentric\ndistance of the Sun of 8.5 kpc, the circular rotational velocity of the LSR at\nthe distance of the Sun of 220 km/s, and a flat Galactic rotation curve, S\nPersei is suggested to have a non-circular motion deviating from the Galactic\ncircular rotation for 15 km/s, which is mainly dominated by the anti rotation\ndirection component of 12.9+/-2.9 km/s. This red supergiant is thought to\nbelong to the OB association, Per OB1, so that this non-circular motion is\nrepresentative of a motion of the OB association in the Milky Way. This\nnon-circular motion is somewhat larger than that explained by the standard\ndensity-wave theory for a spiral galaxy, and is attributed to either a cluster\nshuffling of the OB association, or to non-linear interactions between\nnon-stationary spiral arms and multi-phase interstellar media. The latter comes\nfrom a new view of a spiral arm formation in the Milky Way suggested by recent\nlarge N-body/smoothed particle hydrodynamics numerical simulations.\n