Astrometry of 6.7 GHz Methanol Maser Sources and the Bar Structure of the Milky Way Galaxy

The goal of this thesis is to kinematically establish the existence of the bar structure in the Milky Way Galaxy based on VLBI astrometry of masers in star-forming regions. To achieve this goal, the thesis consists of three parts described below. First, we measured annual parallax and absolute proper motions of the 6.7 GHz methanol maser source W3(OH) with the JVN/VERA (Japanese VLBI Network / VLBI Exploration of Radio Astrometry). We derived the trigonometric annual parallax to be 0.633 ± 0.112 mas, corresponding to a distance of 1.58 −0.24 kpc. This is the first detection of a parallax for 6.7 GHz methanol maser with the JVN/VERA, and demonstrates that the JVN/VERA is capable of conducting VLBI astrometry for 6.7 GHz methanol maser sources within a few kpc from the Sun. We also measured the internal proper motions of 6.7 GHz methanol maser of W3(OH) for the first time. The internal proper motions basically show north-south expansion with velocities of few km s−1, being similar to OH masers. The global distributions and the internal proper motions of 6.7 GHz methanol masers suggest a rotating and expanding torus structure surrounding ultracompact H II region. We also obtained averaged absolute proper motions for all six spots of W3(OH), which is μα cos δ = −1.10±0.30 mas yr−1 and μδ = −0.16 ± 0.38 mas yr−1. The accuracy of the absolute proper motion obtained here indicates that we will be able to detect the non-circular motion caused by the Galactic bar, which is expected to be an order of sub-mas yr−1. Next, we conducted the fringe check observations to select observable target sources toward the Galactic bar. We selected sources from the 6.7 GHz methanol maser catalog of Pestalozzi et al. (2005). In our sources selection, we used four criteria as follows: First, sources must have the galactic longitude |l| < 40◦ and Dec> −37 deg. Second, the sources must have a galacto-centric radius less than 5 kpc, which is estimated from the kinematic distance. Third, we chose strong sources with the flux are 15 Jy or more at single dish observations except some sources that were specially included to investigate the detection limit for the flux under the 15 Jy. Fourth, the maser sources must have one or more detectable reference sources with the JVN or the VERA within 4 degree separations from the maser sources. In this ii selection, we chose the reference sources with the peak flux about 100 mJy beam−1 or more at 8 GHz (X-band) in the VLBA Calibrator Survey. The fringe check observations were conducted with six sessions from October 2008 to June 2009 with the JVN/VERA. In total, 75 maser sources were observed. 26 maser sources were detected with signal to noise ratio of more than 5σ at the baseline of about 1,000 km or longer in the 75 sources, corresponding to the detection rate of about 35%. For astrometric observations, out of the 26 sources detected in the fringe check observations, we chose ten maser sources, which are easy to observe with phase referencing because of high flux at longer than 1,000 km VERA baselines and suitable reference sources. These ten sources were G 353.4−0.36, G 351.41+0.64, G 30.76−0.05, G 29.95−0.02, G 28.14+0.00, G 25.70−0.04, G 25.65+1.04, G 24.78+0.08, G 23.01−0.41 and G 9.98−0.02. We obtained the three dimensional velocities with VLBI phasereference observations. With these data sets for 2, 3 or 4 epochs, we fitted linear proper motions with least-square fittings without a parameter of annual parallax. Finally, we obtained the suitable proper motions for nine sources. By combining our data with previous results from the VERA/VLBA, we compared the 3-D data of star forming regions with the flat/non-flat circular rotation models and a dynamical model proposed by Wada (1994) and Sakamoto et al. (1999). We find clearly a deviation from the flat/non-flat circular rotation models in the absolute proper motions at the three dimension of (l, μl cos b, VLSR). Around the maximum region of VLSR, the absolute proper motions are smaller than the flat rotation model for the five sources out of eight. From the dynamical models and VLBI data, an acceptable parameter of the inclination angle of the Galactic bar are derived as ∼ 30◦ 50◦, which is consistent with previous studies. Thus, with the existence of the Galactic bar, observed proper motions with VLBI can be explained better.