The distribution of maser stars in the inner Milky Way: the effect of a weak, rotating bar

We derive the distribution of maser stars in the inner Milky Way based on an analysis of diagrams of longitude versus line-of-sight-velocity (= lV-diagrams) for two samples of maser stars: 771 OH/IR stars and 363 SiO-maser stars. The stars are all close to the plane of the Milky Way and have longitudes from -45 degrees to +45 degrees. The two lV-diagrams are compared qualitatively and found to be very similar. They also compare well with the lV-diagram of interstellar CO, but there are significant differences in detail between the stellar lV-diagrams and that of the ISM. Based on the qualitative discussion we divide the lV-diagrams into seven areas. In each area we count the number of stars as observed and compare these numbers with those predicted by an assumed set of orbits in a galactic potential. This potential is axially symmetric but a weak rotating bar has been added. We conclude that the maser stars move on almost circular orbits outside of about 3.5 kpc, but that the orbits become more and more elongated when one goes deep inside our MW. We find a strong effect of the Corotation (=CR) resonance at 3.3 kpc, we see a small but noticeable effect of the Outer Lindblad Resonance at 5 kpc and no effect of the Inner Lindblad Resonance (= ILR) at r = 0.8 kpc. We find a set of 6 groups of orbits that together predict counts in agreement with the counts of stars observed. We then calculate the trajectory of each orbit and so find the distribution of the maser stars in the plane of the MWG. This distribution has two new (but not unexpected) features. The first is a bar-like distribution within 2 kpc from the GC outlined. These orbits explain the high-velocity stars near l = 0 degrees in the forbidden and the permitted quadrants. The second feature are two croissant-like voids in the distribution close to the CR radius (3.3 kpc). These voids are the consequence of the presence of the co-rotation resonance. We find excellent agreement with an earlier reconstruction by Sevenster (1999) based on partially the same data but on a completely different analysis.