Survey description

We have initiated the largest ever survey of red giant SiO maser sources in the Galactic bulge and inner Galaxy. At the 7mm and 3mm wavelengths (43 GHz and 86 GHz) of the SiO maser 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 observations of up to ∼34,000 red giant SiO maser sources with a now demonstrated detection rate of at least 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 velocity structure of these tracers 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.

Area covered on the sky:

Below is a plot showing the BAaDE targets, covering a region of ± 6 ° latitude along the full Galactic plane. The sources are selected to harbor dusty envelopes as indicated by their MSX color. Compared to optical surveys (some of which are indicated in the figure below), BAaDE covers the whole plane and in particular BAaDE reaches all latitudes.

Fig. 1: Black dots represents the MSX selected stars, and red dots are those observed to date. The bottom panel is a zoom of the central bar region, and displays areas covered by OGLEIII/IV, BRAVA and ARGOS.

Spectral coverage at the VLA:

The VLA observational setup, using two base bands, is shown in the figure below. Interesting transitions falling inside the bandwidth are marked in Figure 2, and their rest frequencies listed in the table below.

Fig. 2: The VLA 43 GHz frequency setup, consisting of two sets of 8 IF blocks each. The two sets are overlapping to ensure that each main SiO(1-0) line (blue lines) is situated close to the middle of an IF, avoiding the edges. This setup further allows the detection of transitions of 29SiO and 30SiO (red lines) in addition to other molecular species like HC5N and HC7N (red lines) which may be detectable in carbon-rich stars lacking the SiO lines.

Oxygen-rich transitions Rest frequency (MHz) Carbon-rich transitions
30SiO (1-0) v=0 42373.341
   SiO(1-0) v=3 42519.375
29SiO(1-0) v=1 42583.827
42602.153 HC5N (16-15)
42674.192 HCS+ (1-0)
   SiO (1-0) v=2 42820.570
42863.206 HC7N (38-37)
29SiO (1-0) v=0 42879.941
42944.988 SiC4 (14-13)
 SiO(1-0) v=1 43122.090
 SiO(1-0) v=0 43423.853

Spectral coverage at ALMA:

The ALMA observational setup uses four separate spectral windows, shown in Figure 3 below.

Fig. 3: The ALMA 86 GHz frequency setup, consisting of three spectral windows covering three main SiO(1-0) lines (blue lines) and one transition of 29SiO (red lines). The fourth spectral window covers the transition CS(2-1), possibly detectable in carbon-rich stars lacking oxygen lines.

Oxygen-rich transitions Rest frequency (MHz) Carbon-rich transitions
28SiO (2-1) v=2 85640.452
29SiO (2-1) v=0 85759.188
28SiO (2-1) v=1 86243.430
86339.921 H13CN (1-0) v=0
86754.288 H13CO+ (1-0)
28SiO(2-1) v=0 86846.995
97980.950 CS(2-1)