Infrared Color Separation between Thin-shelled Oxygen-rich and Carbon-rich AGB Stars

We present 43 GHz VLA spectra for 51 AGB sources with the goal of verifying an infrared (IR) color cut intended to separate carbon-rich (C) and oxygen-rich (O) AGB sources throughout the Galaxy. The color cut is a simple line in the [K-s] - [A] versus [A] - [E] color-color diagram based on 2MASS and MSX photometry, and was originally derived from SiO detection rates in the Bulge Asymmetries and Dynamical Evolution (BAaDE) sample. The division is fully supported by the spectra presented here, which show that SiO maser detections lie on the O-rich side, and SiO nondetections and a single HC3N detection are found on the C-rich side of the division. We further compare the color cut with classifications of the sources based on Low-Resolution Spectra from the Infrared Astronomical Satellite, and find good agreement, verifying that the division is a reliable and efficient method for differentiating O- and C-rich AGB sources. These observations also demonstrate that single lines detected in the BAaDE survey around 42.9 GHz are almost certainly the(29)SiOv = 0 line. SiO maser sources where this rare isotopologue transition is brighter than the dominant(28)SiO lines have not been reported before, and our observations show that these sources can reverse their behavior such that the typical ratios of(28)SiO and(29)SiO are restored within a few years.