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.

中文翻译：

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薄壳富氧和富碳 AGB 星之间的红外颜色分离

我们展示了 51 个 AGB 源的 43 GHz VLA 光谱，目的是验证红外 (IR) 颜色切割，旨在将整个银河系中的富碳 (C) 和富氧 (O) AGB 源分开。颜色切割是基于 2MASS 和 MSX 光度法的 $[K_s]-[A]$ 与 $[A]-[E]$ 颜色-颜色图中的一条简单线，最初源自 Bulge 中的 SiO 检测率不对称和动态演化 (BAaDE) 示例。此处提供的光谱完全支持该划分，这表明 SiO 脉泽探测位于富含 O 的一侧，而在该划分的富含 C 的一侧发现 SiO 未探测和单个 HC$_3$N 探测。我们进一步将颜色切割与基于来自红外天文卫星 (IRAS) 的低分辨率光谱 (LRS) 的源分类进行比较，并发现良好的一致性，验证该划分是区分富含 O 和 C 的 AGB 源的可靠且有效的方法。这些观察结果还表明，在 BAaDE 调查中在 42.9 GHz 附近检测到的单线几乎可以肯定是 $^{29}$SiO v=0 线。这种罕见的同位素转变比主导的 $^{28}$SiO 线更亮的 SiO 脉泽源以前没有被报道过，我们的观察表明这些源可以逆转它们的行为，使得 $^{28}$ 的典型比率SiO和$^{29}$SiO在几年内恢复。