Trigonometric Parallaxes of Massive Star-Forming Regions. IX. The Outer Arm in the First Quadrant

TL;DR

This study measures the distance to a distant star-forming region in the Outer arm of the Milky Way using VLBA parallax, revealing its position, motion, and the arm's structure in the Galaxy.

Contribution

It provides precise parallax measurements of a high-mass star-forming region in the Outer arm, improving understanding of Galactic structure and spiral arm geometry.

Findings

Distance of 9.25 kpc to the region

Galactic warp at 10.7 kpc radius

Pitch angle of the Outer arm ~12 degrees

Abstract

We report a trigonometric parallax measurement with the Very Long Baseline Array for the water maser in the distant high-mass star-forming region G75.30+1.32. This source has a heliocentric distance of 9.25+-0.45 kpc, which places it in the Outer arm in the first Galactic quadrant. It lies 200 pc above the Galactic plane and is associated with a substantial HI enhancement at the border of a large molecular cloud. At a Galactocentric radius of 10.7 kpc, G75.30+1.32 is in a region of the Galaxy where the disk is significantly warped toward the North Galactic Pole. While the star-forming region has an instantaneous Galactic orbit that is nearly circular, it displays a significant motion of 18 km/s toward the Galactic plane. The present results, when combined with two previous maser studies in the Outer arm, yield a pitch angle of about 12 degrees for a large section of the arm extending from the first quadrant to the third.