2018ApJ...862..153S

We observed the 43 GHz v = 1, 2, and 3 and 86 GHz v = 1 SiO maser transitions quasi-simultaneously for a Mira-variable-dominated sample of over 80 sources from the Bulge Asymmetries and Dynamical Evolution (BAaDE) project, using ATCA, and statistically compared the relative line strengths. On average, the 43 GHz v = 1 line is brighter than the 86 GHz v = 1 line by a factor of 1.36_–0.14^+0.15. As a result, an 86 GHz v = 1 observed sample can be observed to 85.9% of the distance of a 43 GHz v = 1 observed sample using the same sensitivity. We discuss what impact this may have on the BAaDE Galactic plane survey using the VLA and ALMA. Despite fewer v = 3 detections, specific trends are discerned or strengthened when the 43 GHz v = 3 line is detected. In particular the 43 and 86 GHz v = 1 lines are on average equal for sources with no detectable 43 GHz v = 3 emission, but the 43 GHz v = 1 line strength is on average about twice as bright as the 86 GHz v = 1 line for sources with detectable 43 GHz v = 3 emission. Some weak correlations are found between line strengths and Midcourse Space Experiment flux densities and colors, which are tightened when considering only sources with detectable 43 GHz v = 3 emission. We discuss these trends in the context of a radiative pumping model to highlight how the 43 GHz v = 3 line, when coupled with the v = 1 and v = 2 lines, can further our understanding of variable conditions like density in the circumstellar envelopes.