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

Abstract

We present astrometric analysis of archival data of water masers in the starforming region Sharpless 269 (S269) IRS 2w, observed with the VLBI Exploration of Radio Astrometry. An annual parallax of one of the bright maser features in this region was previously reported to be 0.189 ± 0.008 milliarcsecond (mas) using part of the same archival data as we used. However, we found that this maser feature is not the best to represent the annual parallax to S269 IRS 2w because the morphology is remarkably elongated in the east-west direction. For this study we have selected another maser feature showing simpler morphology.This makes the new annual parallax estimate more credible. Our newly obtained annual parallax is 0.247 ± 0.034 mas, corresponding to 4.05 +0.65 −0.49 kpc. This value is well consistent with the 3.7-3.8 kpc obtained using the kinematic distance estimates and photometric distance modulus. We considered two hypotheses for the water maser spatial distribution, a bipolar outflow and an expanding ring, in a kinematic model fitting analysis with a radially expanding flow. At this stage, any conclusions about the systemic proper motion could not be drawn from the kinematic analysis. Alternatively, we evaluated the mean proper motion to be (0.39 ± 0.92, −1.27 ± 0.90) mas yr −1 eastward and northward, respectively, from the obtained proper motions of the detected water maser features. The newly obtained annual parallax and mean proper motion give the peculiar motion of S269 IRS 2w to be (U s , V s , W s ) of (8 ± 6, −21 ± 17, 1 ± 18) km s −1 .

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.…”

“…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). The tension between the two parallax distances is crucial for two reasons.…”

Gas kinematics in high-mass star-forming regions from the Perseus spiral arm

“…Anyhow, it is possible to use maser spots for measuring distances by the trigonometric parallax method (see observational review in Reid & Honma (2014) and theoretical considerations in Sobolev & Gray (2012); Sobolev et al (2008)), though in the normally avoided cases of the complicated spot structure this method can bring incorrect results, Asaki et al (2014). This precise method is successfully used in the studies of the spiral structure of the Galaxy when the number of sources is large enough (error is less than 10% for determination of parameters with 500 sources, Honma et al (2015)).…”

VLBI Monitoring of the Sub-parsec-scale Jet in the Radio Galaxy 3C 66B at 22 GHz

“…The images were made with DIFMAP using CLEAN and a self-calibration technique. The above process using phase-referencing data greatly improve the image quality for 3C 66B, making it possible to image very weak extended jets, as reported by Asaki et al (2014) for water maser mapping. We cannot find a significant counterjet component at any epoch, in contrast with previous lower frequency results (Giovannini et al 2001;Kharb et al 2009).…”