Trigonometric parallax --- Galactic structure Y.Xu M.J.Reid X.W.Zheng K.M.Menten

Trigonometric parallax ---Galactic structureY.Xu M.J.Reid X.W.Zheng K.M.Menten Observe > 10 massive star forming regions Trigonometric parallax---absolute distance Distance-scale calibration

What we can do: Structure and dynamics of the Galaxy Distribution of Dark Matter ----estimate of our galaxy mass Disk and outflows in star forming regions

Physical mechanism of masers Precise calibration of other methods --- absolute magnitude ages of globular Milky Way as a meter stick  Extragalactic distance Hubble constant, age of Universe

Reason of using SFR is SFR maser is enough to bright, Can observe all region of galaxy, and origin of maser is interstellar gases ----which is almost on galactic plane.

Reason of using methanol maser For SiO • The bright source is near the sun. • Far source from sun is too weak to observe. • For OH • not archive enough accuracy and • high special resolution. • Each spot is too broad to • determine the position of maser system. For H2O Short lifetime

Background sources 12 GHz methanol maser Phase referencing JVAS (8 GHz) VLBI  ~ 0.01 mas NVSS (1.4 GHz) within 0.7 VLA  C- & U- band

HII regions spiral arms of galaxies. Rotation curve  Measure distances to HII regions Method used: Kinematic distances

Construct a “plan view” of the Milky Way

Problems: • Kinematic distance ambiguities • Fish et al. (2003), APJ, 587, 701 • Non-circular rotation

Pilot study Perseus Arm----W3OH kinematic distance ~ 4.5 kpc luminosity distance ~ 2.2 kpc kinematically anomalous  Discrepancy or inaccurate for luminosity distance

First epoch Pilot study---(Program BX005) Accuracy better than 0.1 mas Five 8-hr tracks When finished ~ 0.05 mas ~ 10 % at a distance of 2 kpc

Masers in one spectral channel

Trigonometric parallax --- Galactic structure Y.Xu M.J.Reid X.W.Zheng K.M.Menten