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 HC$_3$N 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 (LRS) from
the Infrared Astronomical Satellite (IRAS), 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
$^{29}$SiO v=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.
