AAS 207th Meeting, 8-12 January 2006
Session 40 Galactic Structure with WIMPS, STARS and Gas
Oral, Monday, 10:00-11:30am, January 9, 2006, Salon 1

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[40.06] Trigonometric Parallax to W3OH in the Perseus Spiral Arm

M. J. Reid (Harvard-Smithsonian CfA), Y. Xu (MPIfR), X. W. Zheng (Nanjing University), K. M. Menten (MPIfR)

We have used the VLBA to determine a trigonometric parallax for W3OH, a massive star forming region in the Perseus spiral arm, with an accuracy of 10 micro-arcseconds. W3OH is at a distance of 1.953 ±0.04~kpc. This resolves a long-standing difference between kinematic distances (>4~kpc) and O-star luminosity distances (\approx2.2~kpc) to this region of the Perseus arm. The O-star luminosity distances are correct and this region is strongly kinematically anomalous. We have also measured the proper motion of W3OH, relative to extragalactic sources, and find a peculiar motion of 22~km/s directed inward and counter to Galactic rotation. This is in qualitative agreement with spiral density wave theory, but seems larger than expected for reasonably density enhancements. We are now conducting a large program with the VLBA to determine parallaxes and proper motions to a sample of a dozen massive star forming regions. The goals of this project are to determine the locations of spiral arms of the Galaxy and to test the spiral density wave paradigm.

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