2009A&A...499..473H

The spiral structure of our Milky Way Galaxy is not yet known. HII regions and giant molecular clouds are the most prominent spiral tracers. Models with 2-4 arms have been proposed to outline the structure of our Galaxy. Recently, new data of spiral tracers covering a larger region of the Galactic disk have been published. We wish to outline the spiral structure of the Milky way using all tracer data. We collected the spiral tracer data of our Milky Way from the literature, namely, HII regions and giant molecular clouds (GMCs). With weighting factors based on the excitation parameters of HII regions or the masses of GMCs, we fitted the distribution of these tracers with models of two, three, four spiral-arms or polynomial spiral arms. The distances of tracers, if not available from stellar or direct measurements, were estimated kinetically from the standard rotation curve of Brand & Blitz (1993A&A...275...67B) with R₀=8.5kpc, and Θ₀=220km/s or the newly fitted rotation curves with R₀=8.0kpc and Θ₀=220km/s or R₀=8.4kpc and Θ₀=254km/s. We found that the two-arm logarithmic model cannot fit the data in many regions. The three- and the four-arm logarithmic models are able to connect most tracers. However, at least two observed tangential directions cannot be matched by the three- or four-arm model. We composed a polynomial spiral arm model, which can not only fit the tracer distribution but also match observed tangential directions. Using new rotation curves with R₀=8.0kpc and Θ₀=220km/s and R₀=8.4kpc and Θ₀=254km/s for the estimation of kinematic distances, we found that the distribution of HII regions and GMCs can fit the models well, although the results do not change significantly compared to the parameters with the standard R₀ and Θ₀.