The distance to the center of the Galaxy - H2O maser proper motions in Sagittarius B2(N)

The authors measured the proper motions of many H₂O maser spots associated with the star-forming region, Sgr B2(N), which lies close to the Galactic Center. They used five VLBI observations spanning 1.5 yr and achieved relative positional accuracies of about 20 microarcseconds (μas) across a 4arcsec field. The proper motion information shows that the dominant motions of the Sgr B2(N) masers are expansion, with evidence for some rotation. The expansion velocity is ≡45 km s⁻¹. The rotation velocity at a radial distance of 1arcsec is ≡20 km s⁻¹, indicating a mass enclosed by the 2arcsec masing region of ≡2×10⁴M_sun;. The authors find the distance to the Sgr B2(N) H₂O masers, and hence the distance to the Galactic Center (R₀), to be 7.1 kpc with a one standard deviation uncertainty, including systematic sources of error, of 1.5 kpc.