Trigonometric parallaxes of star-forming regions in the Sagittarius spiral arm ^⋆

¹ Purple Mountain Observatory, Chinese Academy of Sciences, 210008 Nanjing, PR China
e-mail: ywwu@pmo.ac.cn
² Max-Planck-Institut für Radioastronomie, auf dem Hügel 69, 53121 Bonn, Germany
³ Graduate University of Chinese Academy of Sciences, 100049 Beijing, PR China
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge 02138 MA, USA
⁵ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁶ School of Astronomy and Space Sciences of Nanjing University, 210093 Nanjing, PR China

Received: 27 September 2013
Accepted: 10 February 2014

Abstract

We report measurements of parallaxes and proper motions of ten high-mass star-forming regions in the Sagittarius spiral arm of the Milky Way as part of the BeSSeL Survey with the VLBA. Combining these results with eight others from the literature, we investigated the structure and kinematics of the arm between Galactocentric azimuths β ≈ −2° and 65°. We found that the spiral pitch angle is 7°.3 ± 1°.5; the arm’s half-width, defined as the rms deviation from the fitted spiral, is ≈0.2 kpc; and the nearest portion of the Sagittarius arm is 1.4 ± 0.2 kpc from the Sun. Unlike for adjacent spiral arms, we found no evidence for significant peculiar motions of sources in the Sagittarius arm opposite to Galactic rotation.