Distance to G14.33-0.64 in the Sagittarius Spiral Arm: H2O Maser Trigonometric Parallax with VERA

TL;DR

This study precisely measures the distance to the star-forming region G14.33-0.64 using VLBI parallax, revealing the Sagittarius arm is closer than previously thought and refining our understanding of Galactic structure.

Contribution

First direct parallax measurement of G14.33-0.64, clarifying its position in the Sagittarius arm and improving Galactic spiral arm models.

Findings

Distance to G14.33-0.64 is 1.12 kpc, less than half the kinematic estimate.

Sagittarius arm is closer (~1 kpc) than previously assumed (~2-3 kpc).

G14.33-0.64 shows no significant peculiar motion, aligning with Galactic rotation.

Abstract

We report on trigonometric parallax measurements for the Galactic star forming region G14.33-0.64 toward the Sagittarius spiral arm. We conducted multi-epoch phase-referencing observations of an H2O maser source in G14.33-0.64 with the Japanese VLBI array VERA. We successfully detected a parallax of 0.893+/-0.101 mas, corresponding to a source distance of 1.12+/-0.13 kpc, which is less than half of the kinematic distance for G14.33-0.64. Our new distance measurement demonstrates that the Sagittarius arm lies at a closer distance of ~1 kpc, instead of previously assumed ~2-3 kpc from kinematic distances. The previously suggested deviation of the Sagittarius arm toward the Galactic center from the symmetrically fitted model (Taylor & Cordes 1993) is likely due to large errors of kinematic distances at low galactic longitudes. G14.33-0.64 most likely traces the near side of the Sagittarius arm. We attempted fitting the pitch angle of the arm with other parallax measurements along the arm, which yielded two possible pitch angles of i=34.7+/-2.7 degrees and i=11.2+/-10.5 degrees. Our proper motion measurements suggest G14.33-0.64 has no significant peculiar motion relative to the differential rotation of the Galaxy (assumed to be in a circular orbit), indicating that the source motion is in good agreement with the Galactic rotation.