Program: GN-2008B-Q-92

We plan to measure the spectrophotometric parallax of stars in the W3 HII region by using K-band spectra. We continue our survey of giant HII regions, aimed to study the nature of massive star formation and to trace the spiral structure of our Galaxy. These results will be a critical test for our distance determination method. We and other authors have found distances systematically lower by a factor of 2 as compared to the kinematic distances (accounting for the near/far ambiguity of the radio technique). This particular HII region has a very accurate distance measured by trigonometric parallax (1.95 kpc) which is also in disagreement with the kinematic method (4 kpc). The emerging scenario is that the current picture we have from the Galactic spiral arms, which is based on the rotation model, has serious problems. Is the spectrophotometric parallax method in the near-infrared a reliable alternative ?