Distance and Proper Motion Measurement of the Red Supergiant, PZ Cas, in Very Long Baseline Interferometry H2O Maser Astrometry

Abstract

We present the very long baseline interferometry H2O maser monitoring observations of the red supergiant, PZ Cas, at 12 epochs from 2006 April to 2008 May. We fitted maser motions to a simple model composed of a common annual parallax and linear motions of the individual masers. The maser motions with the parallax subtracted were well modeled by a combination of a common stellar proper motion and a radial expansion motion of the circumstellar envelope. We obtained an annual parallax of 0.356+/-0.026 mas and a stellar proper motion of {\mu}*{\alpha}cos{\delta}=-3.7+/-0.2 and {\mu}*{\delta}=-2.0+/-0.3 mas/yr eastward and northward, respectively. The annual parallax corresponds to a trigonometric parallax of 2.81+0.22-0.19 kpc. By rescaling the luminosity of PZ Cas in any previous studies using our trigonometric parallax, we estimated the location of PZ Cas on a Hertzsprung-Russell diagram and found that it approaches a theoretically evolutionary track around an initial mass of ~25M(sun). The sky position and the distance to PZ Cas are consistent with the OB association, Cas OB5, which is located in a molecular gas super shell. The proper motion of PZ Cas is close to that of the OB stars and other red supergiants in Cas OB5 measured by the Hipparcos satellite. We derived the peculiar motion of PZ Cas of Us = 22.8+/-1.5, Vs = 7.1+/-4.4, and Ws = -5.7+/-4.4 km/s. This peculiar motion has rather a large Us component, unlike those of near high-mass star-forming regions with negatively large Vs motions. The uniform proper motions of the Cas OB5 member stars suggest random motions of giant molecular clouds moving into local potential minima in a time-dependent spiral arm, rather than a velocity field caused by the spiral arm density wave.