Distance and proper motion measurement of water masers in Shapless 269 IRS 2w

TL;DR

This study refines the distance and proper motion measurements of water masers in S269 IRS 2w using VLBI data, providing more accurate astrometric parameters and insights into the region's kinematics.

Contribution

The paper presents a new, more reliable parallax measurement of S269 IRS 2w by selecting a better maser feature, improving distance accuracy and kinematic understanding.

Findings

New parallax estimate of 0.247 mas corresponding to 4.05 kpc

Proper motion of (0.39, -1.27) mas/yr determined

Peculiar motion of S269 IRS 2w characterized as (8, -21, 1) km/s

Abstract

We present astrometric analysis of archival data of water masers in the star-forming 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 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 (Us, Vs, Ws) of (8+/-6, -21+/-17, 1+/-18) km/s.