Eight new astrometry results of 6.7 GHz CH3OH and 22 GHz H2O masers in the Perseus arm

Abstract

AbstractWe report astrometric results for seven 6.7 GHz CH3OH and one 22 GHz H2O masers in the Perseus arm with VLBA and VERA observations. Among the eight sources, we succeeded in obtaining trigonometric parallaxes for all sources, except G098.03+1.44 at 6.7 GHz band. By combining our results with previous astrometry results (Choi et al. 2014), we determined an arm width of 0.41 kpc and a pitch angle of 8.2 ± 2.5 deg for the Perseus arm. By using a large sample of the Perseus arm (26 sources), we examined the three-dimensional, non-circular motions (defined as U, V and W) of sources in the Perseus arm as a function of the distance (D) perpendicular to the arm. Interestingly, we found a weighted mean of <U > = 12.7 ± 1.2 km s−1 for 14 sources with D < 0 kpc (i.e. sources on the interior side of the arm) and <U > = −0.3 ± 1.5 km s−1 for 12 sources with D > 0 kpc (i.e. sources exterior to the arm). These findings might be the first observational indication of the ”damping phase of a spiral arm” suggested by the non-steady spiral arm model of Baba et al. (2013). The small pitch angle of the Perseus arm (< 10 deg) also supports the damping phase, based on ”pitch angle vs. arm amplitude” relation shown in Grøsbol et al. (2004).