Trigonometric Parallaxes of Massive Star Forming Regions: II. Cep A & NGC 7538

TL;DR

This study measures precise distances and motions of two star-forming regions, NGC 7538 and Cep A, revealing their true locations in the Galaxy and significant deviations from expected circular orbits.

Contribution

First direct parallax measurements for NGC 7538 and Cep A, refining their distances and Galactic positions, and analyzing their peculiar motions.

Findings

NGC 7538 is closer than previously thought, in the Perseus arm.

Cep A's distance is smaller, placing it in the Local arm or spur.

Both regions exhibit large peculiar motions counter to Galactic rotation.

Abstract

We report trigonometric parallaxes for the sources NGC 7538 and Cep A, corresponding to distances of 2.65 [+0.12/-0.11] kpc and 0.70 [+0.04/-0.04] kpc, respectively. The distance to NGC 7538 is considerably smaller than its kinematic distance and places it in the Perseus spiral arm. The distance to Cep A is also smaller than its kinematic distance and places it in the Local arm or spur. Combining the distance and proper motions with observed radial velocities gives the location and full space motion of the star forming regions. We find significant deviations from circular Galactic orbits for these sources: both sources show large peculiar motions (> 10 km/s) counter to Galactic rotation and NGC 7538 has a comparable peculiar motion toward the Galactic center.