Annual Parallax of the K-Type Star System IRAS 22480+6002 Measured with VERA

We present astrometric VLBI observations of H₂O masers associated with IRAS 22480+ 6002 (= IRC +60370, hereafter I22480) with the VLBI Exploration of Radio Astrometry (VERA). The stellar type of I22480 looks unusual in stellar maser sources, and has been debated since the 1970's. We successfully determined that an annual parallax of a group of H₂O maser spots is π = 0.400±0.025 mas, corresponding to a distance to I22480 of D = 2.50^+0.17_-0.15 kpc. This suggests that an estimated bolometric luminosity of I22480 should be revised to 35000 L_⊙, preferring a K-type supergiant to an RV Tau-type variable star, previously suggested. Although its spectral type is unusual in stellar maser sources, internal motions of H₂O maser features suggest that the H₂O masers are associated with the circumstellar envelope of this star. We derived a secular proper motion of I22480, (μ_α, μ_δ) = (-2.58±0.33, -1.91±0.17) [mas yr^-1], from a possible stellar motion relative to the maser feature motion. The derived motion of I22480 in the Milky Way has a deviation of ∼30 km s^-1 in the Galactic azimuthal direction from the circular motion, estimated from a galactocentric distance to I22480 and an assumption of a flat Galactic rotation curve. This peculiar motion is still comparable to those typically seen in H₂O maser sources located in the Perseus Arm. Taking into account its peculiar motion and proximity to the Galactic midplane (z ≃ 60 pc), I22480 may be a member of the Galactic thin disk.