Trigonometric Parallax of W51 Main/South

TL;DR

This paper reports a precise measurement of the distance to W51 Main/South using VLBA parallax, providing insights into Galactic structure and the region's kinematics, including its orbit and outflows.

Contribution

It presents the first VLBA parallax measurement of W51 Main/South, refining its distance and the Galactic center distance, and analyzes the region's full-space motion and outflows.

Findings

Distance to W51 Main/South is 5.41 kpc.

Galactic center distance is estimated at 8.3 kpc.

W51 has a nearly circular orbit around the Galactic center.

Abstract

We report measurement of the trigonometric parallax of W51 Main/South using the Very Long Baseline Array (VLBA). We measure a value of 0.185 +/- 0.010 mas, corresponding to a distance of 5.41 (+0.31/-0.28) kpc. W51 Main/South is a well-known massive star-forming region near the tangent point of the Sagittarius spiral arm of the Milky Way. Our distance to W51 yields an estimate of the distance to the Galactic center of Ro = 8.3 +/- 0.46 (statistical) +/- 1.0 (systematic) kpc by simple geometry. Combining the parallax and proper motion measurements for W51, we obtained the full-space motion of this massive star forming region. We find W51 is in a nearly circular orbit about the Galactic center. The H2O masers used for our parallax measurements trace four powerful bipolar outflows within a 0.4 pc size region, some of which are associated with dusty molecular hot cores and/or hyper- or ultra-compact HII regions.