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Astrometry using H2O maser sources in star forming regions is expected to bea powerful tool to study the structures and dynamics of our Galaxy. Honma etal. (2007) (hereafter H2007) claimed that the annual parallax of S269 isdetermined within an error of 0.008 milliarcsec (mas), concluding that S269 islocated at 5.3 kpc +- 0.2 kpc from the sun, and R= 13.1 kpc. They claimed thatthe 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 theresults of other observations and theoretical studies of galactic dynamics. Wehere reanalyzed the VERA archival data using the self-calibration method(hybrid mapping), and found that clusters of maser features of S269 aredistributed in much wider area than that investigated in H2007. We confirmedthat, if we make a narrow region image without considering the presence ofmultiple maser spots, and only the phase calibration is applied, we canreproduce the same maser structures in H2007. The distribution extent of maserspots in the feature differs 0.2 mas from east to west between our results andH2007. Moreover, we found that change of relative positions of maser spots inthe cluster reaches 0.1 mas or larger between observational epochs. Thissuggests that if one simply assumes the time-dependent, widely distributedmaser sources as a stable single point source, it could cause errors of up to0.1 mas in the annual parallax of S269. Taking into account the internalmotions of maser spot clusters, the proper motion of S269 cannot be determinedprecisely. We estimated that the peculiar motion of S269 with respect to aGalactic circular rotation is ~20 km/s. These results imply that the observedkinematics of maser emissions in S269 cannot give a strong constraint ondynamics of the outer part of the Galaxy, in contrast to the claim by H2007.

Analysis of H2O Masers in Sharpless 269 using VERA Archival data --- Effect of maser structures on astrometric accuracy