Analysis of H2O Masers in Sharpless 269 using VERA Archival data ---
  Effect of maser structures on astrometric accuracy
  Astrometry using H2O maser sources in star forming regions is expected to be
a powerful tool to study the structures and dynamics of our Galaxy. Honma et
al. (2007) (hereafter H2007) claimed that the annual parallax of S269 is
determined within an error of 0.008 milliarcsec (mas), concluding that S269 is
located at 5.3 kpc +- 0.2 kpc from the sun, and R= 13.1 kpc. They claimed that
the rotational velocity of S269 is equal to that of the sun within a 3% error.
This small error, however, is hardly understood when taking into account the
results of other observations and theoretical studies of galactic dynamics. We
here reanalyzed the VERA archival data using the self-calibration method
(hybrid mapping), and found that clusters of maser features of S269 are
distributed in much wider area than that investigated in H2007. We confirmed
that, if we make a narrow region image without considering the presence of
multiple maser spots, and only the phase calibration is applied, we can
reproduce the same maser structures in H2007. The distribution extent of maser
spots in the feature differs 0.2 mas from east to west between our results and
H2007. Moreover, we found that change of relative positions of maser spots in
the cluster reaches 0.1 mas or larger between observational epochs. This
suggests that if one simply assumes the time-dependent, widely distributed
maser sources as a stable single point source, it could cause errors of up to
0.1 mas in the annual parallax of S269. Taking into account the internal
motions of maser spot clusters, the proper motion of S269 cannot be determined
precisely. We estimated that the peculiar motion of S269 with respect to a
Galactic circular rotation is ~20 km/s. These results imply that the observed
kinematics of maser emissions in S269 cannot give a strong constraint on
dynamics of the outer part of the Galaxy, in contrast to the claim by H2007.
