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

Abstract

We derive the distribution of maser stars in the inner Milky Way (MW) based on an analysis of lV-diagrams (lVd) for two samples of maser stars: 771 OH/IR stars and 363 SiO-maser stars. They are all close to the plane of the MW and have long. from -45 to +45deg. The two lVds are compared and found to be very similar. They also compare well with the lVd of interstellar CO, but there are significant differences in detail between the stellar lVds and that of the ISM. Based on the qualitative discussion we divide the lVds into seven areas. In each area we compare the number of stars observed 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 (res) at 3.3 kpc, we see a small but noticeable effect of the Outer Lindblad res at 5 kpc and no effect of the Inner Lindblad res 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-vel stars near l=0deg 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), which are the consequence of the presence of the CR res. We find excellent agreement with an earlier reconstruction by Sevenster (1999).