Elliptical distortion of the Milky Way's rotation traced by high-mass star-forming regions

The gravitational potential of the Milky Way is non-axisymmetric, caused by a bar or triaxial halo, which dominates elliptical rotation of the Milky Way. Employing a likelihood analysis, we exploit the astrometric data of masers thoroughly and constrain the elliptical rotation of the Galaxy. Masers in high-mass star-forming regions, observed by VLBA, are more distant tracers than stars observed in the optical bandpass, and thus are more appropriate for studying the global feature of the Milky Way's rotation. A clear elliptical potential of the Milky Way is detected, with an ellipticity of ${ϵ}_0\sim 0.09$ at the Sun, and the ellipticity increases towards the outer disk. The minor axis of the elliptical potential (the major axis of the rotation orbit) is found to be near the Sun with a displacement of $\sim {32}^{\circ }$. Based on the rotation model assumed for an elliptical potential, we also make a kinematical calibration of the Galactocentric distance of the Sun, which gives $R_0=7.63\pm 0.34kpc$. (paper)