Project Summary

The BAaDE survey aims to map the positions and velocities of up to ~28,000 SiO maser stars along the full Galactic plane, with the largest concentration in the Galactic bulge and inner Galaxy. The SiO maser stars are detectable both near the otherwise obscured plane and Center as well as in regions with less optical extinction, expanding by a large number the sample of currently known stellar tracers in the inner Galaxy. With this sample, models of the dynamics of the Milky Way bar and bulge region can be tested.

An example BAaDE SiO maser spectra with detection of the 28SiO(1-0) v=1 and v=2 transitions, and a weaker detection of the 29SiO(1-0) v=0 line.


We survey red giant sources in the Galactic bulge and inner Galaxy for SiO maser emission at 7mm and 3mm wavelengths (43 GHz and 86 GHz). At these wavelengths we are not hindered by extinction, and extremely accurate stellar velocities (< 1km/s) and positions are determined in one minute of integration time per source. The targeted observations of up to ∼28,000 red giant SiO maser sources with a now demonstrated detection rate of 60-70% will yield numbers comparable to optical surveys, but with the additional strength of a much more thorough coverage of the highly obscured inner Galaxy. The number of sources will be large enough to trace complex structures and minority populations.

The observed velocity structure is to be compared with the kinematic structures seen in molecular gas in the Galactic plane, including the bulge, disk and center, and thereby highlight kinematically coherent (possibly younger) stellar systems, complex orbit structure in the bar, or stellar streams resulting from recently infallen systems. Modeling of the bar and bulge dynamics will be done using the new kinematic information in the inner Galaxy region. Our survey will also yield sufficiently luminous SiO masers suitable for follow-up orbit and parallax determination using VLBI. A specific aim is to determine in detail orbits of stars supporting the stellar bar. As the SiO maser stars are detectable both near the obscured plane and Center as well as in regions with less optical extinction, there will be a solid connection to optical studies.