Analysis of H2O masers in Sharpless 269 using VERA archival data—Effect of maser structures on astrometric accuracy

Abstract

Astrometry using H 2 O 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 Sharpless 269 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 its galactrocetnric distance is R = 13.1 kpc. From the proper motion, they claimed that the galacto-centric 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 a maser feature investigated 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 −1 . 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.

Resolving the distance controversy for Sharpless 269

“…The strongest maser spot was detected in all sixteen epochs with a flux density maximum of 95.8 Jy beam −1 at epoch H. This spot has a distinctive elongated shape at all epochs (see Fig. 1 as was firstly reported by Miyoshi et al (2012) and highlighted by Asaki et al (2014). The spot varies somewhat over time but retains its basic shape throughout all our observations.…”

“…2 at about the same location in the source makes it remarkable. Its position and shape seem to correspond to the maser spot reported by Honma et al (2007) for the VERA observations made between 2004 and 2005, and the reanalysis of VERA data made by Miyoshi et al (2012) and Asaki et al (2014). Its longevity may be related to its complex structure, maser spots typically being much more compact.…”

“…This corresponds to an image of 1.64 × 1.64 using a cellsize of 0.05 mas pixel −1 within a radial velocity range between −6.4 and 25.7 km s −1 . The range values for the data cube were calculated to include all the maser spots reported in Miyoshi et al (2012) and Asaki et al (2014). We detected nine maser spots that were persistent for at least three epochs.…”

“…Later Miyoshi et al (2012) and Asaki et al (2014) disputed the distance to S 269 reported by Honma et al (2007) , firstly pointing out that kinematic and optical photometric distance estimates reported shorter values (3.7-3.8 kpc, see Moffat et al 1979;Wouterloot & Brand 1989;Xu et al 2009). Moreover, they reanalyzed the VERA data specifically using more compact maser spots than those used by Honma et al (2007), and reported a parallax value 0.247 ± 0.034 mas, which corresponds to a distance of 4.05 +0.65 −0.49 kpc (Asaki et al 2014).…”

DISTANCE AND PROPER MOTION MEASUREMENT OF WATER MASERS IN SHARPLESS 269 IRS 2w

“…They suggested that S269 has a very small peculiar motion with respect to a Milky Way flat rotation curve. On the other hand, Miyoshi et al (2012 showed, from the same data as H2007, that the water maser emissions of S269 IRS 2w are widely distributed in space and radial velocity, and have various proper motions. A systemic proper motion of S269 can be different from the absolute proper motion of the single maser emission because it may be a part of a complicated internal motion such as an outflow with a typical speed of a few tens of km s −1 from a massive protostar.…”